Adjust thread count calculation and request bundling:

This commit addresses minor bugs introduced with commit
6faaa91850:

- The number of threads used by the database engine was
  incorrectly clamped to the lower possible value, such
  that the database was effectively operating in single
  threaded mode.

- The number of requests to extract at once was so high
  that it could result in increased latency. The bundle
  size is now limited to 4 and can be adjusted by a new
  configuration option `rq_bundle` in the `[node_db]`
  stanza. This is an advanced tunable and adjusting it
  should not be needed.

src/ripple/app/misc/SHAMapStore.h

@@ -55,7 +55,7 @@ public:
     clampFetchDepth(std::uint32_t fetch_depth) const = 0;
 
     virtual std::unique_ptr<NodeStore::Database>
-    makeNodeStore(std::int32_t readThreads) = 0;
+    makeNodeStore(int readThreads) = 0;
 
     /** Highest ledger that may be deleted. */
     virtual LedgerIndex

src/ripple/app/misc/SHAMapStoreImp.cpp

@@ -166,7 +166,7 @@ SHAMapStoreImp::SHAMapStoreImp(
 }
 
 std::unique_ptr<NodeStore::Database>
-SHAMapStoreImp::makeNodeStore(std::int32_t readThreads)
+SHAMapStoreImp::makeNodeStore(int readThreads)
 {
     auto nscfg = app_.config().section(ConfigSection::nodeDatabase());
 

src/ripple/app/misc/SHAMapStoreImp.h

@@ -136,7 +136,7 @@ public:
     }
 
     std::unique_ptr<NodeStore::Database>
-    makeNodeStore(std::int32_t readThreads) override;
+    makeNodeStore(int readThreads) override;
 
     LedgerIndex
     setCanDelete(LedgerIndex seq) override

src/ripple/nodestore/Database.h

@@ -324,6 +324,11 @@ protected:
     // The earliest shard index
     std::uint32_t const earliestShardIndex_;
 
+    // The maximum number of requests a thread extracts from the queue in an
+    // attempt to minimize the overhead of mutex acquisition. This is an
+    // advanced tunable, via the config file. The default value is 4.
+    int const requestBundle_;
+
     void
     storeStats(std::uint64_t count, std::uint64_t sz)
     {
@@ -368,6 +373,7 @@ private:
 
     std::atomic<bool> readStopping_ = false;
     std::atomic<int> readThreads_ = 0;
+    std::atomic<int> runningThreads_ = 0;
 
     virtual std::shared_ptr<NodeObject>
     fetchNodeObject(

src/ripple/nodestore/impl/Database.cpp

@@ -43,7 +43,8 @@ Database::Database(
     , earliestLedgerSeq_(
           get<std::uint32_t>(config, "earliest_seq", XRP_LEDGER_EARLIEST_SEQ))
     , earliestShardIndex_((earliestLedgerSeq_ - 1) / ledgersPerShard_)
-    , readThreads_(std::min(1, readThreads))
+    , requestBundle_(get<int>(config, "rq_bundle", 4))
+    , readThreads_(std::max(1, readThreads))
 {
     assert(readThreads != 0);
 
@@ -53,10 +54,15 @@ Database::Database(
     if (earliestLedgerSeq_ < 1)
         Throw<std::runtime_error>("Invalid earliest_seq");
 
-    for (int i = 0; i != readThreads_.load(); ++i)
+    if (requestBundle_ < 1 || requestBundle_ > 64)
+        Throw<std::runtime_error>("Invalid rq_bundle");
+
+    for (int i = readThreads_.load(); i != 0; --i)
     {
         std::thread t(
             [this](int i) {
+                runningThreads_++;
+
                 beast::setCurrentThreadName(
                     "db prefetch #" + std::to_string(i));
 
@@ -68,14 +74,20 @@ Database::Database(
                         std::unique_lock<std::mutex> lock(readLock_);
 
                         if (read_.empty())
+                        {
+                            runningThreads_--;
                             readCondVar_.wait(lock);
+                            runningThreads_++;
+                        }
 
                         if (isStopping())
                             continue;
 
-                        // We extract up to 64 objects to minimize the overhead
+                        // If configured, extract multiple object at a time to
-                        // of acquiring the mutex.
+                        // minimize the overhead of acquiring the mutex.
-                        for (int cnt = 0; !read_.empty() && cnt != 64; ++cnt)
+                        for (int cnt = 0;
+                             !read_.empty() && cnt != requestBundle_;
+                             ++cnt)
                             read.insert(read_.extract(read_.begin()));
                     }
 
@@ -84,7 +96,7 @@ Database::Database(
                         assert(!it->second.empty());
 
                         auto const& hash = it->first;
-                        auto const& data = std::move(it->second);
+                        auto const& data = it->second;
                         auto const seqn = data[0].first;
 
                         auto obj =
@@ -340,6 +352,16 @@ void
 Database::getCountsJson(Json::Value& obj)
 {
     assert(obj.isObject());
+
+    {
+        std::unique_lock<std::mutex> lock(readLock_);
+        obj["read_queue"] = static_cast<Json::UInt>(read_.size());
+    }
+
+    obj["read_threads_total"] = readThreads_.load();
+    obj["read_threads_running"] = runningThreads_.load();
+    obj["read_request_bundle"] = requestBundle_;
+
     obj[jss::node_writes] = std::to_string(storeCount_);
     obj[jss::node_reads_total] = std::to_string(fetchTotalCount_);
     obj[jss::node_reads_hit] = std::to_string(fetchHitCount_);