fix: Evict from a CLOCK ring of strong keys instead of walking buckets

# Conflicts:
#	src/xrpld/core/detail/Config.cpp

API-CHANGELOG.md

@@ -28,6 +28,7 @@ Version 3.4.0 is not yet released. These changes are available in the 3.4.0 beta
 
 ### Additions in 3.4.0
 
+- `server_info` (admin): The `node_size` field has been removed; it reported the deprecated `[node_size]` config setting, which is still accepted as an alias for `[memory_limit]` and still emits a startup warning. Admin responses now include `memory_limit`, the cache memory budget in gigabytes rounded up (0 when enforcement is disabled).
 - `ledger`: `nftoken_id`, `nftoken_ids`, and `offer_id` are now included in transaction metadata when transactions are expanded (`expand`, or admin-only `full`), matching the `tx`, `account_tx`, and `subscribe` (`transactions` stream) responses. ([#5706](https://github.com/XRPLF/rippled/pull/5706))
 
 ### Bugfixes in 3.4.0

cfg/xrpld-example.cfg

@@ -1368,23 +1368,41 @@
 #
 # [node_size]
 #
-#   Tunes the servers based on the expected load and available memory. Legal
-#   sizes are "tiny", "small", "medium", "large", and "huge". We recommend
-#   you start at the default and raise the setting if you have extra memory.
+#   DEPRECATED. Each size is now an alias for a [memory_limit] value:
+#   tiny = 4, small = 8, medium = 32, large = 64, huge = 128. Set
+#   [memory_limit] instead; setting this logs a warning at startup.
 #
-#   The code attempts to automatically determine the appropriate size for
-#   this parameter based on the amount of RAM and the number of execution
-#   cores available to the server. The current decision matrix is:
+# [memory_limit]
 #
-#   |         |         Cores          |
-#   |---------|------------------------|
-#   |   RAM   |   1  | 2 or 3 |   ≥ 4  |
-#   |---------|------|--------|--------|
-#   |  < ~8GB | tiny |   tiny |   tiny |
-#   | < ~12GB | tiny |  small |  small |
-#   | < ~16GB | tiny |  small | medium |
-#   | < ~24GB | tiny |  small |  large |
-#   | < ~32GB | tiny |  small |   huge |
+#   The memory budget, in gigabytes, that the server sizes its caches
+#   within. Cache sizes scale with the budget; the SHAMap tree node cache
+#   is capped to fit within half of it, enforced as it grows. Defaults to
+#   detected physical RAM (capped by the container limit when one is set);
+#   0 selects minimal sizes with no enforcement. Values above 1024 are
+#   rejected, and a value above detected RAM logs a warning.
+#   Set this when xrpld shares the machine with other services or runs in
+#   a container with a memory limit below the host's RAM. Thread counts
+#   are unrelated: they come from the core count and the [workers] /
+#   [io_workers] overrides.
+#
+#   Example:
+#       [memory_limit]
+#       16
+#
+# [tree_cache_age]
+#
+#   Seconds a SHAMap tree node stays cached after its last use. The default
+#   is 300. Accepted values are 10 to 3600.
+#
+# [ledger_cache_age]
+#
+#   Seconds a full ledger stays in the ledger cache after its last use. The
+#   default is 180. Accepted values are 10 to 3600.
+#
+# [ledger_fetch_size]
+#
+#   How many historical ledgers to acquire per fetch pass while backfilling.
+#   The default is 4. Accepted values are 1 to 16.
 #
 # [signing_support]
 #

include/xrpl/basics/TaggedCache.h

@@ -15,9 +15,11 @@
 #include <chrono>
 #include <cstddef>
 #include <cstdint>
+#include <deque>
 #include <functional>
 #include <memory>
 #include <mutex>
+#include <optional>
 #include <string>
 #include <thread>
 #include <type_traits>
@@ -74,13 +76,24 @@ public:
     using SharedPointerType = SharedPointer;
 
 public:
+    /**
+     * @param cacheHardCap When positive, a hard upper bound on the number of
+     *     strongly-cached entries, enforced by demoting the approximately
+     *     oldest entry whenever growth would exceed it. 0 disables the cap
+     *     (the periodic sweep alone bounds the cache).
+     * @param partitions Number of partitions the underlying map is split
+     *     into; defaults to the hardware concurrency. Exposed so tests can
+     *     pin a small, known partition count.
+     */
     TaggedCache(
         std::string const& name,
         int size,
         ClockType::duration expiration,
         ClockType& clock,
         beast::Journal journal,
-        beast::insight::Collector::Ptr const& collector = beast::insight::NullCollector::make());
+        beast::insight::Collector::Ptr const& collector = beast::insight::NullCollector::make(),
+        int cacheHardCap = 0,
+        std::optional<std::size_t> partitions = std::nullopt);
 
 public:
     /**
@@ -101,6 +114,13 @@ public:
     int
     getTrackSize() const;
 
+    /**
+     * Returns the number of strong-key slots evictForHardCap has examined
+     * since construction.
+     */
+    std::uint64_t
+    getEvictVisits() const;
+
     float
     getHitRate();
 
@@ -314,6 +334,9 @@ private:
     public:
         SharedWeakComboPointerType ptr;
         ClockType::time_point lastAccess;
+        // Stamped by queueStrong each time the entry becomes strong; the
+        // strong-key slot carrying the same seq is the entry's live slot.
+        std::uint64_t strongSeq{0};
 
         ValueEntry(ClockType::time_point const& lastAccess, SharedPointerType const& ptr)
             : ptr(ptr), lastAccess(lastAccess)
@@ -357,6 +380,26 @@ private:
 
     using CacheType = HardenedPartitionedHashMap<key_type, Entry, Hash, KeyEqual>;
 
+    // Counts an entry that just became strong and, under a hard cap, queues
+    // its key for eviction and evicts down to the cap. Caller holds mutex_.
+    void
+    addStrong(CacheType::Iterator const& it);
+
+    // Stamps the entry with a new strong seq and appends its key to
+    // strongRing_, dropping stale slots once they outnumber the live ones.
+    // Caller holds mutex_.
+    void
+    queueStrong(key_type const& key, Entry& entry);
+
+    // CLOCK eviction over strongRing_: pops slots from the front, discards
+    // stale ones, gives `keep` and entries used since they became strong a
+    // second chance at the back, and demotes the first other strong entry,
+    // repeating until the count is back under cacheHardCap_ or the demotion
+    // budget is spent. Amortized O(1) per insert. No-op for key caches;
+    // caller holds mutex_.
+    void
+    evictForHardCap(CacheType::Iterator const& keep);
+
     [[nodiscard]] std::thread
     sweepHelper(
         ClockType::time_point const& whenExpire,
@@ -390,8 +433,38 @@ private:
     // Desired maximum cache age
     ClockType::duration const targetAge_;
 
+    // Hard upper bound on strongly-cached entries, enforced by
+    // evictForHardCap whenever the strong count grows (fresh inserts and
+    // weak-to-strong revivals). 0 disables it (sweep-only sizing).
+    int const cacheHardCap_;
+
+    // Total hard-cap evictions (under mutex_); the first marks saturation
+    // onset for logging.
+    std::uint64_t hardCapEvictions_{0};
+
     // Number of items cached
     int cacheCount_{0};
+
+    // A key, the strong seq its entry had when the slot was queued, and the
+    // time it was queued.
+    struct StrongSlot
+    {
+        key_type key;
+        std::uint64_t seq;
+        ClockType::time_point since;
+    };
+
+    // Keys in the order their entries became strong, consumed front-first
+    // by evictForHardCap. Filled only when cacheHardCap_ > 0. A slot is
+    // stale once its entry is gone, weak, or strong again under a newer seq.
+    std::deque<StrongSlot> strongRing_;
+
+    // Source of strong seqs; advanced under mutex_.
+    std::uint64_t strongSeqNext_{0};
+
+    // Slots examined by evictForHardCap; read by getEvictVisits.
+    std::uint64_t evictVisits_{0};
+
     CacheType cache_;  // Hold strong reference to recent objects
     std::uint64_t hits_{0};
     std::uint64_t misses_{0};

include/xrpl/basics/TaggedCache.ipp

@@ -57,7 +57,9 @@ inline TaggedCache<
         ClockType::duration expiration,
         ClockType& clock,
         beast::Journal journal,
-        beast::insight::Collector::Ptr const& collector)
+        beast::insight::Collector::Ptr const& collector,
+        int cacheHardCap,
+        std::optional<std::size_t> partitions)
     : journal_(journal)
     , clock_(clock)
     , stats_(
@@ -67,6 +69,8 @@ inline TaggedCache<
     , name_(name)
     , targetSize_(size)
     , targetAge_(expiration)
+    , cacheHardCap_(cacheHardCap)
+    , cache_(partitions)
 {
 }
 
@@ -137,6 +141,23 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
     return cache_.size();
 }
 
+template <
+    class Key,
+    class T,
+    bool IsKeyCache,
+    class SharedWeakUnionPointer,
+    class SharedPointerType,
+    class Hash,
+    class KeyEqual,
+    class Mutex>
+inline std::uint64_t
+TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash, KeyEqual, Mutex>::
+    getEvictVisits() const
+{
+    std::scoped_lock const lock(mutex_);
+    return evictVisits_;
+}
+
 template <
     class Key,
     class T,
@@ -170,6 +191,7 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
 {
     std::scoped_lock const lock(mutex_);
     cache_.clear();
+    strongRing_.clear();
     cacheCount_ = 0;
 }
 
@@ -188,6 +210,7 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
 {
     std::scoped_lock const lock(mutex_);
     cache_.clear();
+    strongRing_.clear();
     cacheCount_ = 0;
     hits_ = 0;
     misses_ = 0;
@@ -219,6 +242,141 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
     return true;
 }
 
+template <
+    class Key,
+    class T,
+    bool IsKeyCache,
+    class SharedWeakUnionPointer,
+    class SharedPointerType,
+    class Hash,
+    class KeyEqual,
+    class Mutex>
+inline void
+TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash, KeyEqual, Mutex>::
+    addStrong(CacheType::Iterator const& it)
+{
+    ++cacheCount_;
+    if constexpr (!IsKeyCache)
+    {
+        if (cacheHardCap_ > 0)
+        {
+            queueStrong(it->first, it->second);
+            if (cacheCount_ > cacheHardCap_)
+                evictForHardCap(it);
+        }
+    }
+}
+
+template <
+    class Key,
+    class T,
+    bool IsKeyCache,
+    class SharedWeakUnionPointer,
+    class SharedPointerType,
+    class Hash,
+    class KeyEqual,
+    class Mutex>
+inline void
+TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash, KeyEqual, Mutex>::
+    queueStrong(key_type const& key, Entry& entry)
+{
+    entry.strongSeq = ++strongSeqNext_;
+    strongRing_.push_back({key, entry.strongSeq, clock_.now()});
+
+    // Stale slots accumulate from sweep and del; drop them once
+    // they outnumber the live ones so the ring stays O(strong entries).
+    constexpr std::size_t kRingSlack = 1024;
+    if (strongRing_.size() > 2 * static_cast<std::size_t>(cacheCount_) + kRingSlack)
+    {
+        std::erase_if(strongRing_, [this](StrongSlot const& s) {
+            auto const it = cache_.find(s.key);
+            return it == cache_.end() || it->second.isWeak() || it->second.strongSeq != s.seq;
+        });
+    }
+}
+
+template <
+    class Key,
+    class T,
+    bool IsKeyCache,
+    class SharedWeakUnionPointer,
+    class SharedPointerType,
+    class Hash,
+    class KeyEqual,
+    class Mutex>
+inline void
+TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash, KeyEqual, Mutex>::
+    evictForHardCap(CacheType::Iterator const& keep)
+{
+    if constexpr (!IsKeyCache)
+    {
+        key_type const keepKey = keep->first;
+
+        // Growth paths raise the count by one at a time, so a few demotions
+        // per call let eviction catch up without stalling them.
+        constexpr int kMaxDemotionsPerCall = 8;
+
+        for (int demotions = 0; cacheCount_ > cacheHardCap_ && demotions < kMaxDemotionsPerCall;
+             ++demotions)
+        {
+            // Every strong entry owns one live slot, and only `keep` can be
+            // re-queued twice in one call, so two passes over the ring reach
+            // a victim whenever one exists.
+            bool demoted = false;
+            for (std::size_t budget = 2 * strongRing_.size();
+                 !demoted && budget > 0 && !strongRing_.empty();
+                 --budget)
+            {
+                StrongSlot const slot = strongRing_.front();
+                strongRing_.pop_front();
+                ++evictVisits_;
+
+                auto it = cache_.find(slot.key);
+                if (it == cache_.end() || it->second.isWeak() || it->second.strongSeq != slot.seq)
+                    continue;  // stale: the entry is gone, weak, or re-queued since
+
+                if (slot.key == keepKey || it->second.lastAccess > slot.since)
+                {
+                    // The newest entry stays; one used since it became
+                    // strong gets a second chance at the back.
+                    queueStrong(slot.key, it->second);
+                    continue;
+                }
+
+                if (it->second.ptr.useCount() == 1)
+                {
+                    // Sole owner: release entirely.
+                    cache_.erase(it);
+                }
+                else
+                {
+                    // Others hold it: keep it weakly tracked.
+                    it->second.ptr.convertToWeak();
+                }
+                --cacheCount_;
+                demoted = true;
+
+                // First eviction marks saturation onset; then a heartbeat
+                // every 100k to avoid flooding.
+                ++hardCapEvictions_;
+                if (hardCapEvictions_ == 1 || hardCapEvictions_ % 100000 == 0)
+                {
+                    JLOG(journal_.warn()) << name_ << ": hard-cap eviction #" << hardCapEvictions_
+                                          << " (cap " << cacheHardCap_ << ", strong " << cacheCount_
+                                          << ") - cache saturated, growth now evicts";
+                }
+            }
+
+            if (!demoted)
+            {
+                JLOG(journal_.debug()) << name_ << ": over hard cap " << cacheHardCap_
+                                       << " but no strong entry other than the newest to demote";
+                return;
+            }
+        }
+    }
+}
+
 template <
     class Key,
     class T,
@@ -360,11 +518,14 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
 
     if (cit == cache_.end())
     {
-        cache_.emplace(
-            std::piecewise_construct,
-            std::forward_as_tuple(key),
-            std::forward_as_tuple(clock_.now(), data));
-        ++cacheCount_;
+        auto const emplacedIt = cache_
+                                    .emplace(
+                                        std::piecewise_construct,
+                                        std::forward_as_tuple(key),
+                                        std::forward_as_tuple(clock_.now(), data))
+                                    .first;
+        // The just-inserted entry is the newest; evictForHardCap keeps it.
+        addStrong(emplacedIt);
         return false;
     }
 
@@ -414,12 +575,12 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
             data = cachedData;
         }
 
-        ++cacheCount_;
+        addStrong(cit);
         return true;
     }
 
     entry.ptr = data;
-    ++cacheCount_;
+    addStrong(cit);
 
     return false;
 }
@@ -695,7 +856,13 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
     ++misses_;
     auto const [it, inserted] = cache_.emplace(digest, Entry(clock_.now(), std::move(sle)));
     if (!inserted)
+    {
         it->second.touch(clock_.now());
+    }
+    else
+    {
+        addStrong(it);
+    }
     return it->second.ptr.getStrong();
 }
 // End CachedSLEs functions.
@@ -728,7 +895,7 @@ TaggedCache<Key, T, IsKeyCache, SharedWeakUnionPointer, SharedPointerType, Hash,
     if (entry.isCached())
     {
         // independent of cache size, so not counted as a hit
-        ++cacheCount_;
+        addStrong(cit);
         entry.touch(clock_.now());
         return entry.ptr.getStrong();
     }

include/xrpl/config/Constants.h

@@ -22,11 +22,14 @@ struct Sections
     static constexpr auto kIoWorkers = "io_workers";
     static constexpr auto kIps = "ips";
     static constexpr auto kIpsFixed = "ips_fixed";
+    static constexpr auto kLedgerCacheAge = "ledger_cache_age";
+    static constexpr auto kLedgerFetchSize = "ledger_fetch_size";
     static constexpr auto kLedgerHistory = "ledger_history";
     static constexpr auto kLedgerReplay = "ledger_replay";
     static constexpr auto kLedgerTxTables = "ledger_tx_tables";
     static constexpr auto kMaxSubscriptionsPerConnection = "max_subscriptions_per_connection";
     static constexpr auto kMaxTransactions = "max_transactions";
+    static constexpr auto kMemoryLimit = "memory_limit";
     static constexpr auto kNetworkId = "network_id";
     static constexpr auto kNetworkQuorum = "network_quorum";
     static constexpr auto kNodeDatabase = "node_db";
@@ -64,6 +67,7 @@ struct Sections
     static constexpr auto kSslVerifyFile = "ssl_verify_file";
     static constexpr auto kSweepInterval = "sweep_interval";
     static constexpr auto kTransactionQueue = "transaction_queue";
+    static constexpr auto kTreeCacheAge = "tree_cache_age";
     static constexpr auto kValidationSeed = "validation_seed";
     static constexpr auto kValidatorKeys = "validator_keys";
     static constexpr auto kValidatorKeyRevocation = "validator_key_revocation";

include/xrpl/protocol/jss.h

@@ -396,6 +396,7 @@ JSS(mean);                        // out: get_aggregate_price
 JSS(median);                      // out: get_aggregate_price
 JSS(median_fee);                  // out: TxQ
 JSS(median_level);                // out: TxQ
+JSS(memory_limit);                // out: server_info
 JSS(message);                     // error.
 JSS(meta);                        // out: NetworkOPs, AccountTx*, Tx
 JSS(meta_blob);                   // out: NetworkOPs, AccountTx*, Tx
@@ -436,7 +437,6 @@ JSS(node_read_retries);           // out: GetCounts
 JSS(node_reads_hit);              // out: GetCounts
 JSS(node_reads_total);            // out: GetCounts
 JSS(node_reads_duration_us);      // out: GetCounts
-JSS(node_size);                   // out: server_info
 JSS(nodes);                       // out: VaultInfo
 JSS(nodestore);                   // out: GetCounts
 JSS(node_writes);                 // out: GetCounts

src/test/app/SHAMapStore_test.cpp

@@ -1207,7 +1207,7 @@ public:
 
         auto backend{node_store::Manager::instance().makeBackend(
             section,
-            megabytes(env.app().config().getValueFor(SizedItem::BurstSize, std::nullopt)),
+            megabytes(env.app().config().getValueFor(SizedItem::BurstSize)),
             scheduler,
             env.app().getJournal("NodeStoreTest"))};
         backend->open();
@@ -1229,22 +1229,12 @@ public:
         // Normally, SHAMapStoreImp handles all these details.
         auto nscfg = env.app().config().section(Sections::kNodeDatabase);
 
-        // Provide default values.
+        // Provide default values (mirrors SHAMapStoreImp::makeNodeStore).
         if (!nscfg.exists(Keys::kCacheSize))
-        {
-            nscfg.set(
-                Keys::kCacheSize,
-                std::to_string(
-                    env.app().config().getValueFor(SizedItem::TreeCacheSize, std::nullopt)));
-        }
+            nscfg.set(Keys::kCacheSize, "16384");
 
         if (!nscfg.exists(Keys::kCacheAge))
-        {
-            nscfg.set(
-                Keys::kCacheAge,
-                std::to_string(
-                    env.app().config().getValueFor(SizedItem::TreeCacheAge, std::nullopt)));
-        }
+            nscfg.set(Keys::kCacheAge, "5");
 
         NodeStoreScheduler scheduler(env.app().getJobQueue());
 

src/test/core/Config_test.cpp

@@ -595,6 +595,108 @@ main
         BEAST_EXPECT(c.networkId == 10000);
     }
 
+    void
+    testMemoryLimit()
+    {
+        testcase("memory limit");
+
+        {
+            Config c;
+            c.loadFromString("");
+            BEAST_EXPECT(!c.memoryLimit);
+        }
+
+        auto const parse = [](std::string const& value) {
+            Config c;
+            c.loadFromString("[memory_limit]\n" + value + "\n");
+            return c;
+        };
+
+        BEAST_EXPECT(parse("16").memoryLimit == std::uint64_t{16} << 30);
+        BEAST_EXPECT(parse("0").memoryLimit == std::uint64_t{0});
+        BEAST_EXPECT(parse("0").cacheMemoryBudget() == 0);
+
+        // Garbage and out-of-range values are rejected.
+        expectException([&parse] { parse("banana"); });
+        expectException([&parse] { parse("2000"); });
+
+        // Standalone mode does not change the budget: detected RAM unless
+        // a limit is configured.
+        {
+            Config c;
+            c.setupControl(true, true, true);
+            c.loadFromString("[memory_limit]\n8\n");
+            BEAST_EXPECT(c.cacheMemoryBudget() == std::uint64_t{8} << 30);
+        }
+
+        // Values derive from the budget: half of it at 8 KiB per entry for
+        // the tree cache; 0 yields the floors.
+        BEAST_EXPECT(parse("16").getValueFor(SizedItem::TreeCacheSize) == 1048576);
+        BEAST_EXPECT(parse("64").getValueFor(SizedItem::TreeCacheSize) == 4194304);
+        BEAST_EXPECT(parse("0").getValueFor(SizedItem::TreeCacheSize) == 16384);
+        BEAST_EXPECT(parse("16").getValueFor(SizedItem::TxnDbCache) == 32);
+        BEAST_EXPECT(parse("0").getValueFor(SizedItem::TxnDbCache) == 4);
+        BEAST_EXPECT(parse("16").getValueFor(SizedItem::SweepInterval) == 30);
+        BEAST_EXPECT(parse("16").getValueFor(SizedItem::LedgerSize) == 96);
+        BEAST_EXPECT(parse("16").getValueFor(SizedItem::BurstSize) == 16);
+        BEAST_EXPECT(parse("1024").getValueFor(SizedItem::BurstSize) == 48);
+
+        // Deprecated [node_size] tiers are aliases for budgets (by name,
+        // case-insensitively, or legacy 0-4 index); an explicit
+        // [memory_limit] wins.
+        auto const alias = [](std::string const& value) {
+            Config c;
+            c.loadFromString("[node_size]\n" + value + "\n");
+            return c;
+        };
+
+        BEAST_EXPECT(alias("large").cacheMemoryBudget() == std::uint64_t{64} << 30);
+        BEAST_EXPECT(alias("large").getValueFor(SizedItem::TreeCacheSize) == 4194304);
+        BEAST_EXPECT(alias("HUGE").cacheMemoryBudget() == std::uint64_t{128} << 30);
+        BEAST_EXPECT(alias("3").cacheMemoryBudget() == std::uint64_t{64} << 30);
+        BEAST_EXPECT(alias("9").cacheMemoryBudget() == std::uint64_t{128} << 30);
+
+        {
+            Config c;
+            c.loadFromString("[node_size]\nsmall\n\n[memory_limit]\n100\n");
+            BEAST_EXPECT(c.cacheMemoryBudget() == std::uint64_t{100} << 30);
+        }
+
+        // Policy values are fixed but individually overridable.
+        {
+            Config c;
+            c.loadFromString("[tree_cache_age]\n900\n\n[ledger_fetch_size]\n8\n");
+            BEAST_EXPECT(c.getValueFor(SizedItem::TreeCacheAge) == 900);
+            BEAST_EXPECT(c.getValueFor(SizedItem::LedgerFetch) == 8);
+            BEAST_EXPECT(c.getValueFor(SizedItem::LedgerAge) == 180);
+        }
+        expectException([] {
+            Config c;
+            c.loadFromString("[ledger_fetch_size]\n100\n");
+        });
+    }
+
+    void
+    testMountinfoPath()
+    {
+        testcase("mountinfo path");
+
+        using detail::decodeMountinfoPath;
+
+        // The kernel escapes space, tab, newline and backslash as octal.
+        BEAST_EXPECT(decodeMountinfoPath("/sys/fs/cgroup") == "/sys/fs/cgroup");
+        BEAST_EXPECT(decodeMountinfoPath("/var/lib/cgroup\\040mounts") == "/var/lib/cgroup mounts");
+        BEAST_EXPECT(decodeMountinfoPath("a\\011b\\012c\\134d") == "a\tb\nc\\d");
+        BEAST_EXPECT(decodeMountinfoPath("\\040\\040") == "  ");
+
+        // Anything that is not a backslash and three octal digits is kept.
+        BEAST_EXPECT(decodeMountinfoPath("\\04") == "\\04");
+        BEAST_EXPECT(decodeMountinfoPath("\\") == "\\");
+        BEAST_EXPECT(decodeMountinfoPath("\\0x0") == "\\0x0");
+        BEAST_EXPECT(decodeMountinfoPath("\\400") == "\\400");
+        BEAST_EXPECT(decodeMountinfoPath("") == "");
+    }
+
     void
     testValidatorsFile()
     {
@@ -1676,6 +1778,8 @@ r.ripple.com:51235
         testAmendment();
         testOverlay();
         testNetworkID();
+        testMemoryLimit();
+        testMountinfoPath();
     }
 };
 

src/test/rpc/LedgerRequest_test.cpp

@@ -301,7 +301,6 @@ public:
         using namespace std::chrono_literals;
         Env env{*this, envconfig([](std::unique_ptr<Config> cfg) {
                     cfg->fees.referenceFee = 10;
-                    cfg->nodeSize = 0;
                     return cfg;
                 })};
         Account const gw{"gateway"};

src/test/rpc/ServerInfo_test.cpp

@@ -80,6 +80,8 @@ admin = 127.0.0.1
             BEAST_EXPECT(result.isMember(jss::info));
             auto const& info = result[jss::info];
             BEAST_EXPECT(info.isMember(jss::build_version));
+            // Admin request: reports the cache memory budget in GB.
+            BEAST_EXPECT(info.isMember(jss::memory_limit));
             // Git info is not guaranteed to be present
             if (info.isMember(jss::git))
             {

src/tests/libxrpl/basics/TaggedCache.cpp

@@ -4,15 +4,19 @@
 #include <xrpl/basics/IntrusiveRefCounts.h>
 #include <xrpl/basics/TaggedCache.ipp>  // IWYU pragma: keep
 #include <xrpl/basics/chrono.h>
+#include <xrpl/beast/insight/NullCollector.h>
 #include <xrpl/beast/utility/Journal.h>
 #include <xrpl/protocol/Protocol.h>
 
 #include <gtest/gtest.h>
 #include <helpers/TestSink.h>
 
+#include <cstddef>
 #include <memory>
 #include <string>
+#include <type_traits>
 #include <utility>
+#include <vector>
 
 namespace xrpl {
 
@@ -243,4 +247,290 @@ TEST(TaggedCacheTest, tagged_cache)
     }
 }
 
+TEST(TaggedCacheTest, hard_cap_enforced_on_insert)
+{
+    using namespace std::chrono_literals;
+    beast::Journal const journal{TestSink::instance()};
+
+    TestStopwatch clock;
+    clock.set(0);
+
+    using Key = LedgerIndex;
+    using Value = std::string;
+    using Cache = TaggedCache<Key, Value>;
+
+    // A cap-enabled cache must never let the strong-cache count exceed the
+    // cap, enforced on the insert path alone (no sweep). The large targetSize
+    // and long age make the periodic sweep irrelevant here, so only
+    // evictForHardCap can be bounding it.
+    int const cap = 100;
+    Cache capped(
+        "capped", 1'000'000, 3600s, clock, journal, beast::insight::NullCollector::make(), cap);
+
+    bool everExceeded = false;
+    for (Key k = 1; k <= 1000; ++k)
+    {
+        capped.insert(k, "v");
+        if (capped.getCacheSize() > cap)
+            everExceeded = true;
+    }
+    EXPECT_FALSE(everExceeded);
+    EXPECT_LE(capped.getCacheSize(), cap);
+    EXPECT_GT(capped.getCacheSize(), 0);
+}
+
+TEST(TaggedCacheTest, hard_cap_enforced_across_partitions)
+{
+    using namespace std::chrono_literals;
+    beast::Journal const journal{TestSink::instance()};
+
+    TestStopwatch clock;
+    clock.set(0);
+
+    using Key = LedgerIndex;
+    using Value = std::string;
+    using Cache = TaggedCache<Key, Value>;
+
+    // The cap is well below the partition count, so with sequential keys
+    // (partitioned round-robin) the partition a given insert lands in holds
+    // only that one entry when the count first crosses the cap: eviction
+    // must fall through to another partition rather than stop at an empty
+    // home partition and leave cacheCount_ stuck above cacheHardCap_.
+    int const cap = 4;
+    std::size_t const partitions = 16;
+    Cache capped(
+        "capped-partitions",
+        1'000'000,
+        3600s,
+        clock,
+        journal,
+        beast::insight::NullCollector::make(),
+        cap,
+        partitions);
+
+    bool everExceeded = false;
+    for (Key k = 1; k <= 2000; ++k)
+    {
+        capped.insert(k, "v");
+        if (capped.getCacheSize() > cap)
+            everExceeded = true;
+    }
+    EXPECT_FALSE(everExceeded);
+    EXPECT_LE(capped.getCacheSize(), cap);
+    EXPECT_GT(capped.getCacheSize(), 0);
+}
+
+TEST(TaggedCacheTest, hard_cap_enforced_in_sparse_partition)
+{
+    using namespace std::chrono_literals;
+    beast::Journal const journal{TestSink::instance()};
+
+    TestStopwatch clock;
+    clock.set(0);
+
+    using Key = LedgerIndex;
+    using Value = std::string;
+    using Cache = TaggedCache<Key, Value>;
+
+    // A single partition whose bucket array grows into the thousands while
+    // an external owner (held in `heldRefs`, the common case for a value
+    // also referenced from elsewhere) keeps every demoted entry in the
+    // table as weakly-tracked rather than erased: the handful of strong
+    // survivors end up sparse among thousands of weak entries, and
+    // evictForHardCap must keep finding one.
+    int const cap = 5;
+    std::size_t const partitions = 1;
+    Cache capped(
+        "capped-sparse",
+        1'000'000,
+        3600s,
+        clock,
+        journal,
+        beast::insight::NullCollector::make(),
+        cap,
+        partitions);
+
+    std::vector<std::shared_ptr<Value>> heldRefs;
+    bool everExceeded = false;
+    for (Key k = 1; k <= 2000; ++k)
+    {
+        capped.insert(k, "v");
+        heldRefs.push_back(capped.fetch(k));
+        if (capped.getCacheSize() > cap)
+            everExceeded = true;
+    }
+    EXPECT_FALSE(everExceeded);
+    EXPECT_LE(capped.getCacheSize(), cap);
+    EXPECT_GT(capped.getCacheSize(), 0);
+}
+
+TEST(TaggedCacheTest, hard_cap_bounded_work_with_weak_entries)
+{
+    using namespace std::chrono_literals;
+    beast::Journal const journal{TestSink::instance()};
+
+    TestStopwatch clock;
+    clock.set(0);
+
+    using Key = LedgerIndex;
+    using Value = std::string;
+    using Cache = TaggedCache<Key, Value>;
+
+    // Every demoted entry stays weakly tracked because `heldRefs` owns it,
+    // so weak entries come to outnumber strong ones 100 to 1. Eviction work
+    // per insert must stay constant rather than grow with the weak count.
+    int const cap = 100;
+    Key const inserts = 10'000;
+    Cache capped(
+        "capped-weak-heavy",
+        1'000'000,
+        3600s,
+        clock,
+        journal,
+        beast::insight::NullCollector::make(),
+        cap);
+
+    std::vector<std::shared_ptr<Value>> heldRefs;
+    bool everExceeded = false;
+    for (Key k = 1; k <= inserts; ++k)
+    {
+        capped.insert(k, "v");
+        heldRefs.push_back(capped.fetch(k));
+        if (capped.getCacheSize() > cap)
+            everExceeded = true;
+    }
+    EXPECT_FALSE(everExceeded);
+    EXPECT_EQ(capped.getCacheSize(), cap);
+    EXPECT_EQ(capped.getTrackSize(), inserts);
+    EXPECT_LE(capped.getEvictVisits(), 4u * inserts);
+}
+
+TEST(TaggedCacheTest, hard_cap_enforced_via_fetch_handler)
+{
+    using namespace std::chrono_literals;
+    beast::Journal const journal{TestSink::instance()};
+
+    TestStopwatch clock;
+    clock.set(0);
+
+    using Key = LedgerIndex;
+    using Value = std::string;
+    using Cache = TaggedCache<Key, Value>;
+
+    // fetch(key, handler) is the miss path a NodeStore- or database-backed
+    // lookup takes; it must count and cap an inserted entry the same as the
+    // canonicalize path uses, not insert one uncounted and uncapped.
+    int const cap = 100;
+    Cache capped(
+        "capped-handler",
+        1'000'000,
+        3600s,
+        clock,
+        journal,
+        beast::insight::NullCollector::make(),
+        cap);
+
+    bool everExceeded = false;
+    for (Key k = 1; k <= 1000; ++k)
+    {
+        auto const ptr = capped.fetch(k, [] { return std::make_shared<Value>("v"); });
+        EXPECT_NE(ptr, nullptr);
+        if (capped.getCacheSize() > cap)
+            everExceeded = true;
+    }
+    EXPECT_FALSE(everExceeded);
+    EXPECT_LE(capped.getCacheSize(), cap);
+    EXPECT_GT(capped.getCacheSize(), 0);
+}
+
+TEST(TaggedCacheTest, hard_cap_disabled)
+{
+    using namespace std::chrono_literals;
+    beast::Journal const journal{TestSink::instance()};
+
+    TestStopwatch clock;
+    clock.set(0);
+
+    using Key = LedgerIndex;
+    using Value = std::string;
+    using Cache = TaggedCache<Key, Value>;
+
+    // cacheHardCap = 0: growth is bounded only by the periodic sweep.
+    Cache uncapped(
+        "uncapped", 1'000'000, 3600s, clock, journal, beast::insight::NullCollector::make(), 0);
+
+    for (Key k = 1; k <= 1000; ++k)
+        uncapped.insert(k, "v");
+    EXPECT_EQ(uncapped.getCacheSize(), 1000);
+}
+
+// A key the cache can hash, compare and partition on but not default
+// construct. The requirements TaggedCache places on key_type are those
+// three; the eviction path must not add a fourth.
+struct ExplicitKey
+{
+    std::size_t value;
+
+    explicit ExplicitKey(std::size_t v) : value(v)
+    {
+    }
+
+    operator std::size_t() const
+    {
+        return value;
+    }
+
+    bool
+    operator==(ExplicitKey const&) const = default;
+};
+static_assert(!std::is_default_constructible_v<ExplicitKey>);
+
+struct ExplicitKeyHash
+{
+    std::size_t
+    operator()(ExplicitKey const& key) const
+    {
+        return key.value;
+    }
+};
+
+TEST(TaggedCacheTest, hard_cap_with_non_default_constructible_key)
+{
+    using namespace std::chrono_literals;
+    beast::Journal const journal{TestSink::instance()};
+
+    TestStopwatch clock;
+    clock.set(0);
+
+    using Value = std::string;
+    using Cache = TaggedCache<
+        ExplicitKey,
+        Value,
+        /*IsKeyCache*/ false,
+        SharedWeakCachePointer<Value>,
+        std::shared_ptr<Value>,
+        ExplicitKeyHash>;
+
+    int const cap = 100;
+    Cache capped(
+        "capped-explicit-key",
+        1'000'000,
+        3600s,
+        clock,
+        journal,
+        beast::insight::NullCollector::make(),
+        cap);
+
+    bool everExceeded = false;
+    for (std::size_t k = 1; k <= 1000; ++k)
+    {
+        capped.insert(ExplicitKey{k}, "v");
+        if (capped.getCacheSize() > cap)
+            everExceeded = true;
+    }
+    EXPECT_FALSE(everExceeded);
+    EXPECT_LE(capped.getCacheSize(), cap);
+    EXPECT_GT(capped.getCacheSize(), 0);
+}
+
 }  // namespace xrpl

src/xrpld/app/main/Application.cpp

@@ -294,9 +294,8 @@ public:
 
         auto const cores = std::thread::hardware_concurrency();
 
-        // Use a single thread when running on under-provisioned systems
-        // or if we are configured to use minimal resources.
-        if ((cores == 1) || ((config.nodeSize == 0) && (cores == 2)))
+        // Use a single thread on under-provisioned systems.
+        if (cores <= 2)
             return 1;
 
         // Otherwise, prefer six threads.
@@ -339,14 +338,12 @@ public:
 
                       auto count = static_cast<int>(std::thread::hardware_concurrency());
 
-                      // Be more aggressive about the number of threads to use
-                      // for the job queue if the server is configured as
-                      // "large" or "huge" if there are enough cores.
-                      if (config->nodeSize >= 4 && count >= 16)
+                      // Scale the job queue with the available cores.
+                      if (count >= 16)
                       {
                           count = 6 + std::min(count, 8);
                       }
-                      else if (config->nodeSize >= 3 && count >= 8)
+                      else if (count >= 8)
                       {
                           count = 4 + std::min(count, 6);
                       }
@@ -872,7 +869,7 @@ public:
             node_store::DummyScheduler dummyScheduler;
             std::unique_ptr<node_store::Database> source =
                 node_store::Manager::instance().makeDatabase(
-                    megabytes(config_->getValueFor(SizedItem::BurstSize, std::nullopt)),
+                    megabytes(config_->getValueFor(SizedItem::BurstSize)),
                     dummyScheduler,
                     0,
                     config_->section(Sections::kImportNodeDatabase),

src/xrpld/app/misc/NetworkOPs.cpp

@@ -2882,27 +2882,13 @@ NetworkOPsImp::getServerInfo(bool human, bool admin, bool counters)
 
     if (admin)
     {
-        // Note: By default the node size is "tiny". When parsing it's an error if the final
-        // NODE_SIZE is over 4 so below code should be safe.
-        // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
-        switch (registry_.get().getApp().config().nodeSize)
-        {
-            case 0:
-                info[jss::node_size] = "tiny";
-                break;
-            case 1:
-                info[jss::node_size] = "small";
-                break;
-            case 2:
-                info[jss::node_size] = "medium";
-                break;
-            case 3:
-                info[jss::node_size] = "large";
-                break;
-            case 4:
-                info[jss::node_size] = "huge";
-                break;
-        }
+        // The cache memory budget in GB, rounded up: a nonzero sub-GiB
+        // budget (a cgroup limit under 1 GiB, kept nonzero so enforcement
+        // stays on) would otherwise truncate to 0, the same value reported
+        // when enforcement is disabled.
+        constexpr std::uint64_t oneGiB = std::uint64_t{1} << 30;
+        auto const budget = registry_.get().getApp().config().cacheMemoryBudget();
+        info[jss::memory_limit] = static_cast<json::UInt>((budget + oneGiB - 1) >> 30);
 
         auto when = registry_.get().getValidators().expires();
 

src/xrpld/app/misc/SHAMapStoreImp.cpp

@@ -122,7 +122,9 @@ SHAMapStoreImp::SHAMapStoreImp(
                 Keys::kCacheMb, std::to_string(config.getValueFor(SizedItem::HashNodeDbCache)));
         }
 
-        if (!section.exists(Keys::kFilterBits) && (config.nodeSize >= 2))
+        // 32 GB is the budget the deprecated medium node_size tier maps to,
+        // preserving the old medium-and-up gate for the bloom filter.
+        if (!section.exists(Keys::kFilterBits) && config.cacheMemoryBudget() >= (32ull << 30))
             section.set(Keys::kFilterBits, "10");
     }
 
@@ -190,20 +192,13 @@ SHAMapStoreImp::makeNodeStore(int readThreads)
 {
     auto nscfg = app_.config().section(Sections::kNodeDatabase);
 
-    // Provide default values.
+    // Documented defaults: 16384 records, 5 minutes (DatabaseNodeImp reads
+    // cache_age in minutes).
     if (!nscfg.exists(Keys::kCacheSize))
-    {
-        nscfg.set(
-            Keys::kCacheSize,
-            std::to_string(app_.config().getValueFor(SizedItem::TreeCacheSize, std::nullopt)));
-    }
+        nscfg.set(Keys::kCacheSize, "16384");
 
     if (!nscfg.exists(Keys::kCacheAge))
-    {
-        nscfg.set(
-            Keys::kCacheAge,
-            std::to_string(app_.config().getValueFor(SizedItem::TreeCacheAge, std::nullopt)));
-    }
+        nscfg.set(Keys::kCacheAge, "5");
 
     std::unique_ptr<node_store::Database> db;
 
@@ -235,7 +230,7 @@ SHAMapStoreImp::makeNodeStore(int readThreads)
     else
     {
         db = node_store::Manager::instance().makeDatabase(
-            megabytes(app_.config().getValueFor(SizedItem::BurstSize, std::nullopt)),
+            megabytes(app_.config().getValueFor(SizedItem::BurstSize)),
             scheduler_,
             readThreads,
             nscfg,
@@ -619,7 +614,7 @@ SHAMapStoreImp::makeBackendRotating(std::string path)
 
     auto backend{node_store::Manager::instance().makeBackend(
         section,
-        megabytes(app_.config().getValueFor(SizedItem::BurstSize, std::nullopt)),
+        megabytes(app_.config().getValueFor(SizedItem::BurstSize)),
         scheduler_,
         app_.getJournal(kNodeStoreName))};
     backend->open();

src/xrpld/core/Config.h

@@ -38,9 +38,7 @@ enum class SizedItem : std::size_t {
     HashNodeDbCache,
     TxnDbCache,
     LgrDbCache,
-    OpenFinalLimit,
     BurstSize,
-    RamSizeGb,
     AccountIdCacheSize,
 };
 
@@ -138,7 +136,11 @@ private:
      */
     bool signingEnabled_ = false;
 
-    // The amount of RAM, in bytes, that we detected on this system.
+    // The amount of RAM, in bytes, that we detected on this system. Kept at
+    // byte granularity (not rounded to GiB) so a cgroup limit under 1 GiB
+    // still reads as a nonzero budget instead of collapsing into the
+    // detection-failed case below.
+    // 0 when detection failed.
     std::uint64_t const ramSize_;
 
 public:
@@ -209,10 +211,10 @@ public:
     std::uint32_t ledgerHistory = 256;
     std::uint32_t fetchDepth = 1000000000;
 
-    // Tunable that adjusts various parameters, typically associated
-    // with hardware parameters (RAM size and CPU cores). The default
-    // is 'tiny'.
-    std::size_t nodeSize = 0;
+    // Cache memory budget in bytes, from [memory_limit] (gigabytes). Unset
+    // defaults to detected physical RAM; 0 disables enforcement. The
+    // deprecated [node_size] tiers map onto this budget.
+    std::optional<std::uint64_t> memoryLimit;
 
     bool sslVerify = true;
     std::string sslVerifyFile;
@@ -250,6 +252,11 @@ public:
     // size, but we allow admins to explicitly set it in the config.
     std::optional<int> sweepInterval;
 
+    // Optional overrides for the fixed cache policy values.
+    std::optional<int> treeCacheAge;     // [tree_cache_age], seconds
+    std::optional<int> ledgerCacheAge;   // [ledger_cache_age], seconds
+    std::optional<int> ledgerFetchSize;  // [ledger_fetch_size], ledgers per fetch pass
+
     // Reduce-relay - Experimental parameters to control p2p routing algorithms
 
     // Enable base squelching of duplicate validation/proposal messages
@@ -375,25 +382,22 @@ public:
     }
 
     /**
-     * Retrieve the default value for the item at the specified node size
-     *
-     * @param item The item for which the default value is needed
-     * @param node Optional value, used to adjust the result to match the
-     *             size of a node (0: tiny, ..., 4: huge). If unseated,
-     *             uses the configured size (NODE_SIZE).
-     *
-     * @throws This method can throw std::out_of_range if you ask for values
-     *         that it does not recognize or request a non-default node-size.
+     * Retrieve the value for the item, derived from the memory budget.
      *
+     * @param item The item for which the value is needed
      * @return The value for the requested item.
-     *
-     * @note The defaults are selected so as to be reasonable, but the node
-     *       size is an imprecise metric that combines multiple aspects of
-     *       the underlying system; this means that we can't provide optimal
-     *       defaults in the code for every case.
      */
     [[nodiscard]] int
-    getValueFor(SizedItem item, std::optional<std::size_t> node = std::nullopt) const;
+    getValueFor(SizedItem item) const;
+
+    /**
+     * The effective cache memory budget in bytes.
+     *
+     * [memory_limit] if set, otherwise detected physical RAM. 0 means
+     * enforcement is disabled (explicit 0, or RAM detection failed).
+     */
+    [[nodiscard]] std::uint64_t
+    cacheMemoryBudget() const;
 
     [[nodiscard]] beast::Journal
     journal() const
@@ -408,4 +412,20 @@ setupFeeVote(Section const& section);
 DatabaseCon::Setup
 setupDatabaseCon(Config const& c, std::optional<beast::Journal> j = std::nullopt);
 
+namespace detail {
+
+/**
+ * A /proc/self/mountinfo path field with its octal escapes decoded.
+ *
+ * The kernel writes space, tab, newline and backslash in the root and
+ * mount point fields as `\040`, `\011`, `\012` and `\134`.
+ *
+ * @param escaped The field as read from the file.
+ * @return The path as the kernel names it.
+ */
+[[nodiscard]] std::string
+decodeMountinfoPath(std::string_view escaped);
+
+}  // namespace detail
+
 }  // namespace xrpl

src/xrpld/core/detail/Config.cpp

@@ -35,7 +35,6 @@
 #include <filesystem>
 #include <format>
 #include <iostream>
-#include <iterator>
 #include <limits>
 #include <memory>
 #include <optional>
@@ -43,12 +42,41 @@
 #include <sstream>
 #include <stdexcept>
 #include <string>
+#include <string_view>
 #include <system_error>
-#include <thread>
-#include <type_traits>
 #include <utility>
 #include <vector>
 
+namespace xrpl::detail {
+
+std::string
+decodeMountinfoPath(std::string_view escaped)
+{
+    // A backslash and three octal digits, the first at most 3, encode one byte.
+    auto const octal = [](char c, char max = '7') { return c >= '0' && c <= max; };
+
+    std::string path;
+    path.reserve(escaped.size());
+    for (std::size_t i = 0; i < escaped.size(); ++i)
+    {
+        if (escaped[i] == '\\' && i + 3 < escaped.size() && octal(escaped[i + 1], '3') &&
+            octal(escaped[i + 2]) && octal(escaped[i + 3]))
+        {
+            int const byte = ((escaped[i + 1] - '0') << 6) | ((escaped[i + 2] - '0') << 3) |
+                (escaped[i + 3] - '0');
+            path.push_back(static_cast<char>(byte));
+            i += 3;
+        }
+        else
+        {
+            path.push_back(escaped[i]);
+        }
+    }
+    return path;
+}
+
+}  // namespace xrpl::detail
+
 #if BOOST_OS_WINDOWS
 #include <sysinfoapi.h>
 
@@ -69,16 +97,204 @@ getMemorySize()
 
 #if BOOST_OS_LINUX
 #include <sys/sysinfo.h>  // IWYU pragma: keep
+#include <sys/types.h>
+
+#include <unistd.h>
+
+#include <fstream>
 
 namespace xrpl::detail {
 
+// This process's cgroup path from /proc/self/cgroup: the v2 line is
+// "0::<path>"; a v1 line is "<id>:<controllers>:<path>". Empty when absent.
+[[nodiscard]] std::string
+getOwnCgroupPath(std::string_view controller)
+{
+    std::ifstream in("/proc/self/cgroup");
+    std::string line;
+
+    while (std::getline(in, line))
+    {
+        auto const first = line.find(':');
+        auto const second = line.find(':', first + 1);
+        if (first == std::string::npos || second == std::string::npos)
+            continue;
+
+        std::string_view const controllers(line.data() + first + 1, second - first - 1);
+        if (controller.empty())
+        {
+            // The v2 entry is exactly "0::<path>".
+            if (first == 1 && line[0] == '0' && controllers.empty())
+                return line.substr(second + 1);
+        }
+        else if (controllers.contains(controller))
+        {
+            return line.substr(second + 1);
+        }
+    }
+
+    return {};
+}
+
+// The value in a cgroup limit file; 0 when absent or unlimited. "max" (v2)
+// fails the read, and the page-counter maximum (v1) both mean unlimited.
+[[nodiscard]] std::uint64_t
+readCgroupLimit(std::string const& path)
+{
+    std::ifstream in(path);
+    std::uint64_t limit = 0;
+
+    if (in >> limit && limit < (std::uint64_t{1} << 62))
+        return limit;
+
+    return 0;
+}
+
+// Whether the cgroup directory contains this process. /proc/self/cgroup
+// paths are namespace-relative, so a resolved directory can name-collide
+// with a different cgroup when the cgroup mount shows another view; only
+// trust a directory this process is actually in.
+[[nodiscard]] bool
+cgroupContainsSelf(std::string const& dir)
+{
+    std::ifstream in(dir + "/cgroup.procs");
+    pid_t const self = ::getpid();
+    pid_t pid = 0;
+
+    while (in >> pid)
+    {
+        if (pid == self)
+            return true;
+    }
+
+    return false;
+}
+
+// The smallest numeric limit in `file` from the leaf cgroup up through its
+// ancestors (the effective limit is the minimum over the hierarchy); 0 when
+// none is set or the leaf does not belong to this process.
+[[nodiscard]] std::uint64_t
+minCgroupLimit(std::string const& mount, std::string path, char const* file)
+{
+    if (!cgroupContainsSelf(mount + path))
+        return 0;
+
+    std::uint64_t best = 0;
+    auto const consider = [&best](std::uint64_t limit) {
+        if (limit != 0 && (best == 0 || limit < best))
+            best = limit;
+    };
+
+    while (!path.empty() && path != "/")
+    {
+        consider(readCgroupLimit(mount + path + "/" + file));
+
+        auto const slash = path.find_last_of('/');
+        if (slash == std::string::npos)
+            break;
+        path.resize(slash);
+    }
+    consider(readCgroupLimit(mount + "/" + file));
+
+    return best;
+}
+
+// The mount point of the cgroup v2 unified hierarchy (`controller` empty) or
+// of the cgroup v1 hierarchy whose co-mounted controllers include
+// `controller`; empty when neither is mounted. Read from /proc/self/mountinfo
+// rather than a fixed path, since a container runtime or an init system can
+// mount either hierarchy somewhere other than /sys/fs/cgroup. Field 5, the
+// mount point, is already expressed relative to this process's own root
+// (man 5 proc), so no further namespace translation is needed; its octal
+// escapes are decoded so the path opens as the kernel names it.
+[[nodiscard]] std::string
+findCgroupMount(std::string_view controller)
+{
+    std::ifstream in("/proc/self/mountinfo");
+    std::string line;
+
+    while (std::getline(in, line))
+    {
+        auto const dash = line.find(" - ");
+        if (dash == std::string::npos)
+            continue;
+
+        std::istringstream prefix(line.substr(0, dash));
+        std::string id, parentId, majorMinor, root, mountPoint;
+        prefix >> id >> parentId >> majorMinor >> root >> mountPoint;
+
+        std::istringstream suffix(line.substr(dash + 3));
+        std::string fsType, source, superOptions;
+        suffix >> fsType >> source >> superOptions;
+
+        if (controller.empty())
+        {
+            if (fsType == "cgroup2")
+                return decodeMountinfoPath(mountPoint);
+            continue;
+        }
+
+        if (fsType != "cgroup")
+            continue;
+
+        std::stringstream opts(superOptions);
+        std::string opt;
+        while (std::getline(opts, opt, ','))
+        {
+            if (opt == controller)
+                return decodeMountinfoPath(mountPoint);
+        }
+    }
+
+    return {};
+}
+
+// The cgroup (v2, then v1) memory limit in bytes; 0 when absent or
+// unlimited. Checks this process's own cgroup and its ancestors (covering
+// nested limits such as systemd MemoryMax=) before the root-level files
+// containers expose.
+[[nodiscard]] std::uint64_t
+getCgroupMemoryLimit()
+{
+    if (auto const mount = findCgroupMount(""); !mount.empty())
+    {
+        if (auto const path = getOwnCgroupPath(""); !path.empty() && path != "/")
+        {
+            if (auto const limit = minCgroupLimit(mount, path, "memory.max"))
+                return limit;
+        }
+
+        if (auto const limit = readCgroupLimit(mount + "/memory.max"))
+            return limit;
+    }
+
+    if (auto const mount = findCgroupMount("memory"); !mount.empty())
+    {
+        if (auto const path = getOwnCgroupPath("memory"); !path.empty() && path != "/")
+        {
+            if (auto const limit = minCgroupLimit(mount, path, "memory.limit_in_bytes"))
+                return limit;
+        }
+
+        if (auto const limit = readCgroupLimit(mount + "/memory.limit_in_bytes"))
+            return limit;
+    }
+
+    return 0;
+}
+
 [[nodiscard]] std::uint64_t
 getMemorySize()
 {
-    if (struct sysinfo si{}; sysinfo(&si) == 0)
-        return static_cast<std::uint64_t>(si.totalram) * si.mem_unit;
+    std::uint64_t ram = 0;
 
-    return 0;
+    if (struct sysinfo si{}; sysinfo(&si) == 0)
+        ram = static_cast<std::uint64_t>(si.totalram) * si.mem_unit;
+
+    if (auto const limit = getCgroupMemoryLimit(); limit != 0 && (ram == 0 || limit < ram))
+        return limit;
+
+    return ram;
 }
 
 }  // namespace xrpl::detail
@@ -109,50 +325,6 @@ getMemorySize()
 
 namespace xrpl {
 
-// clang-format off
-// The configurable node sizes are "tiny", "small", "medium", "large", "huge"
-inline constexpr std::array<std::pair<SizedItem, std::array<int, 5>>, 13>
-kSizedItems
-{{
-    // FIXME: We should document each of these items, explaining exactly
-    //        what they control and whether there exists an explicit
-    //        config option that can be used to override the default.
-
-    //                                   tiny    small   medium    large     huge
-    {SizedItem::SweepInterval,      {{     10,      30,      60,      90,     120 }}},
-    {SizedItem::TreeCacheSize,      {{ 262144,  524288, 2097152, 4194304, 8388608 }}},
-    {SizedItem::TreeCacheAge,       {{     30,      60,      90,     120,     900 }}},
-    {SizedItem::LedgerSize,         {{     32,      32,      64,     256,     384 }}},
-    {SizedItem::LedgerAge,          {{     30,      60,     180,     300,     600 }}},
-    {SizedItem::LedgerFetch,        {{      2,       3,       4,       5,       8 }}},
-    {SizedItem::HashNodeDbCache,    {{      4,      12,      24,      64,     128 }}},
-    {SizedItem::TxnDbCache,         {{      4,      12,      24,      64,     128 }}},
-    {SizedItem::LgrDbCache,         {{      4,       8,      16,      32,     128 }}},
-    {SizedItem::OpenFinalLimit,     {{      8,      16,      32,      64,     128 }}},
-    {SizedItem::BurstSize,          {{      4,       8,      16,      32,      48 }}},
-    {SizedItem::RamSizeGb,          {{      6,       8,      12,      24,       0 }}},
-    {SizedItem::AccountIdCacheSize, {{  20047,   50053,   77081,  150061,  300007 }}}
-}};
-// clang-format on
-
-// Ensure that the order of entries in the table corresponds to the
-// order of entries in the enum:
-static_assert(
-    []() constexpr -> bool {
-        std::underlying_type_t<SizedItem> idx = 0;
-
-        for (auto const& i : kSizedItems)
-        {
-            if (static_cast<std::underlying_type_t<SizedItem>>(i.first) != idx)
-                return false;
-
-            ++idx;
-        }
-
-        return true;
-    }(),
-    "Mismatch between sized item enum & array indices");
-
 //
 // TODO: Check permissions on config file before using it.
 //
@@ -257,44 +429,16 @@ getEnvVar(char const* name)
     return value;
 }
 
-Config::Config()
-    : j_(beast::Journal::getNullSink()), ramSize_(detail::getMemorySize() / (1024 * 1024 * 1024))
+Config::Config() : j_(beast::Journal::getNullSink()), ramSize_(detail::getMemorySize())
 {
 }
 
 void
 Config::setupControl(bool bQuiet, bool bSilent, bool bStandalone)
 {
-    XRPL_ASSERT(nodeSize == 0, "xrpl::Config::setupControl : node size not set");
-
     quiet_ = bQuiet || bSilent;
     silent_ = bSilent;
     runStandalone_ = bStandalone;
-
-    // We try to autodetect the appropriate node size by checking available
-    // RAM and CPU resources. We default to "tiny" for standalone mode.
-    if (!bStandalone)
-    {
-        // First, check against 'minimum' RAM requirements per node size:
-        auto const& threshold =
-            kSizedItems[std::underlying_type_t<SizedItem>(SizedItem::RamSizeGb)];
-
-        auto ns = std::ranges::find_if(threshold.second, [this](std::size_t limit) {
-            return (limit == 0) || (ramSize_ < limit);
-        });
-
-        XRPL_ASSERT(ns != threshold.second.end(), "xrpl::Config::setupControl : valid node size");
-
-        if (ns != threshold.second.end())
-            nodeSize = std::distance(threshold.second.begin(), ns);
-
-        // Adjust the size based on the number of hardware threads of
-        // execution available to us:
-        if (auto const hc = std::thread::hardware_concurrency(); hc != 0)
-            nodeSize = std::min<std::size_t>(hc / 2, nodeSize);
-    }
-
-    XRPL_ASSERT(nodeSize <= 4, "xrpl::Config::setupControl : node size is set");
 }
 
 void
@@ -577,31 +721,65 @@ Config::loadFromString(std::string const& fileContents)
         }
     }
 
+    if (getSingleSection(secConfig, Sections::kMemoryLimit, strTemp, j_))
+    {
+        // Gigabytes; 0 disables enforcement.
+        auto const gb = beast::lexicalCastThrow<std::uint64_t>(strTemp);
+        if (gb > 1024)
+        {
+            Throw<std::runtime_error>(
+                "Invalid value '" + strTemp + "' for key '" + Sections::kMemoryLimit +
+                "'; the limit is in gigabytes and may not exceed 1024");
+        }
+        memoryLimit = gb << 30;
+    }
+
     if (getSingleSection(secConfig, Sections::kNodeSize, strTemp, j_))
     {
-        if (boost::iequals(strTemp, "tiny"))
+        // Deprecated: each tier (by name or its legacy 0-4 index) is an
+        // alias for a memory budget. [memory_limit], when present, wins.
+        static constexpr std::array<std::pair<std::string_view, std::uint64_t>, 5> kTiers{
+            {{"tiny", 4}, {"small", 8}, {"medium", 32}, {"large", 64}, {"huge", 128}}};
+
+        auto const tier = std::ranges::find_if(
+            kTiers, [&strTemp](auto const& t) { return boost::iequals(strTemp, t.first); });
+
+        std::uint64_t const budgetGb = tier != kTiers.end()
+            ? tier->second
+            : kTiers[std::min<std::size_t>(4, beast::lexicalCastThrow<std::size_t>(strTemp))]
+                  .second;
+
+        if (!memoryLimit)
+            memoryLimit = budgetGb << 30;
+
+        if (!quiet_)
         {
-            nodeSize = 0;
+            std::cerr << "WARNING: [node_size] is deprecated and will be removed "
+                         "in a future release. Set [memory_limit] instead; thread "
+                         "counts derive from the core count and [workers] / "
+                         "[io_workers].\n";
         }
-        else if (boost::iequals(strTemp, "small"))
+    }
+
+    // A budget beyond the detected memory (physical RAM, or the cgroup limit
+    // when one is set) cannot be honored and recreates the oversized-preset
+    // OOM this setting exists to prevent.
+    if (memoryLimit && ramSize_ != 0 && *memoryLimit > ramSize_ && !quiet_)
+    {
+        std::cerr << "WARNING: the configured memory budget (" << (*memoryLimit >> 30)
+                  << " GB) exceeds detected memory (" << ramSize_
+                  << " bytes, RAM or cgroup limit); ";
+        // A whole-GB floor of 0 here is a nonzero byte budget rounded down,
+        // not the "disabled" value: recommending it would turn enforcement
+        // off, the opposite of this warning's point.
+        if (auto const detectedGb = ramSize_ >> 30)
         {
-            nodeSize = 1;
-        }
-        else if (boost::iequals(strTemp, "medium"))
-        {
-            nodeSize = 2;
-        }
-        else if (boost::iequals(strTemp, "large"))
-        {
-            nodeSize = 3;
-        }
-        else if (boost::iequals(strTemp, "huge"))
-        {
-            nodeSize = 4;
+            std::cerr << "set [memory_limit] to " << detectedGb << " or less.\n";
         }
         else
         {
-            nodeSize = std::min<std::size_t>(4, beast::lexicalCastThrow<std::size_t>(strTemp));
+            std::cerr << "[memory_limit] is in whole gigabytes and cannot represent it; "
+                         "remove the setting to use the detected budget automatically.\n";
         }
     }
 
@@ -744,6 +922,42 @@ Config::loadFromString(std::string const& fileContents)
         }
     }
 
+    if (getSingleSection(secConfig, Sections::kTreeCacheAge, strTemp, j_))
+    {
+        treeCacheAge = beast::lexicalCastThrow<int>(strTemp);
+
+        if (*treeCacheAge < 10 || *treeCacheAge > 3600)
+        {
+            Throw<std::runtime_error>(
+                std::string("Invalid ") + Sections::kTreeCacheAge +
+                ": must be between 10 and 3600 inclusive");
+        }
+    }
+
+    if (getSingleSection(secConfig, Sections::kLedgerCacheAge, strTemp, j_))
+    {
+        ledgerCacheAge = beast::lexicalCastThrow<int>(strTemp);
+
+        if (*ledgerCacheAge < 10 || *ledgerCacheAge > 3600)
+        {
+            Throw<std::runtime_error>(
+                std::string("Invalid ") + Sections::kLedgerCacheAge +
+                ": must be between 10 and 3600 inclusive");
+        }
+    }
+
+    if (getSingleSection(secConfig, Sections::kLedgerFetchSize, strTemp, j_))
+    {
+        ledgerFetchSize = beast::lexicalCastThrow<int>(strTemp);
+
+        if (*ledgerFetchSize < 1 || *ledgerFetchSize > 16)
+        {
+            Throw<std::runtime_error>(
+                std::string("Invalid ") + Sections::kLedgerFetchSize +
+                ": must be between 1 and 16 inclusive");
+        }
+    }
+
     if (getSingleSection(secConfig, Sections::kWorkers, strTemp, j_))
     {
         workers = beast::lexicalCastThrow<int>(strTemp);
@@ -1224,12 +1438,58 @@ Config::getDebugLogFile() const
 }
 
 int
-Config::getValueFor(SizedItem item, std::optional<std::size_t> node) const
+Config::getValueFor(SizedItem item) const
 {
-    auto const index = static_cast<std::underlying_type_t<SizedItem>>(item);
-    XRPL_ASSERT(index < kSizedItems.size(), "xrpl::Config::getValueFor : valid index input");
-    XRPL_ASSERT(!node || *node <= 4, "xrpl::Config::getValueFor : unset or valid node");
-    return kSizedItems.at(index).second.at(node.value_or(nodeSize));
+    // Memory-shaped items scale linearly with the budget between a floor and
+    // a ceiling; time and policy items are fixed. A budget of 0 (enforcement
+    // disabled) yields the floors. The 1024 bound keeps gb * 65536 within
+    // int range (the config parser enforces it too).
+    auto const gb = static_cast<int>(std::min<std::uint64_t>(cacheMemoryBudget() >> 30, 1024));
+
+    switch (item)
+    {
+        case SizedItem::SweepInterval:
+            return 30;
+        case SizedItem::TreeCacheSize:
+            // Half the budget at an estimated 8 KiB per entry (the node plus
+            // its weak-tracking entry, hash buckets, and control block):
+            // 1 GiB / 2 / 8 KiB = 65536 entries per budget GB.
+            return std::max(16384, gb * 65536);
+        case SizedItem::TreeCacheAge:
+            return treeCacheAge.value_or(300);
+        case SizedItem::LedgerSize:
+            return std::clamp(gb * 6, 32, 384);
+        case SizedItem::LedgerAge:
+            return ledgerCacheAge.value_or(180);
+        case SizedItem::LedgerFetch:
+            return ledgerFetchSize.value_or(4);
+        case SizedItem::HashNodeDbCache:
+        case SizedItem::TxnDbCache:
+        case SizedItem::LgrDbCache:
+            // HashNodeDbCache is consumed in MB (RocksDB cache_mb); the two
+            // SQLite page caches are consumed in KB.
+            return std::clamp(gb * 2, 4, 128);
+        case SizedItem::BurstSize:
+            return std::clamp(gb, 4, 48);
+        case SizedItem::AccountIdCacheSize:
+            // Fixed regardless of budget: ~22 MB at 72 bytes per slot, and
+            // the value stays prime for hash distribution.
+            return 300007;
+    }
+
+    UNREACHABLE("xrpl::Config::getValueFor : invalid item");
+    return 0;
+}
+
+std::uint64_t
+Config::cacheMemoryBudget() const
+{
+    if (memoryLimit)
+        return *memoryLimit;
+
+    // ramSize_ is in bytes; 0 when detection failed, which disables
+    // enforcement.
+    return ramSize_;
 }
 
 FeeSetup

src/xrpld/shamap/NodeFamily.cpp

@@ -11,6 +11,7 @@
 #include <xrpl/basics/Log.h>
 #include <xrpl/basics/base_uint.h>
 #include <xrpl/basics/chrono.h>
+#include <xrpl/beast/insight/NullCollector.h>
 #include <xrpl/shamap/FullBelowCache.h>
 #include <xrpl/shamap/TreeNodeCache.h>
 
@@ -39,8 +40,19 @@ NodeFamily::NodeFamily(Application& app, CollectorManager& cm)
               app.config().getValueFor(SizedItem::TreeCacheSize),
               std::chrono::seconds(app.config().getValueFor(SizedItem::TreeCacheAge)),
               stopwatch(),
-              j_))
+              j_,
+              beast::insight::NullCollector::make(),
+              // Hard cap: the clamped target, enforced on insert; 0 = off.
+              app.config().cacheMemoryBudget() != 0
+                  ? app.config().getValueFor(SizedItem::TreeCacheSize)
+                  : 0))
 {
+    // Bytes, not whole GB: a cgroup limit under 1 GiB is a nonzero budget
+    // with the cap on, and must not read as the disabled value.
+    auto const budget = app.config().cacheMemoryBudget();
+    JLOG(j_.info()) << "TreeNodeCache sizing: target="
+                    << app.config().getValueFor(SizedItem::TreeCacheSize) << " entries, budget "
+                    << budget << " bytes" << (budget == 0 ? " (enforcement disabled)" : "");
 }
 
 void