cleanup and notes

include/xrpl/basics/MallocTrim.h

@@ -8,6 +8,20 @@
 
 namespace ripple {
 
+// -----------------------------------------------------------------------------
+// Allocator interaction note:
+// - This facility invokes glibc's malloc_trim(0) on Linux/glibc to request that
+//   ptmalloc return free heap pages to the OS.
+// - If an alternative allocator (e.g. jemalloc or tcmalloc) is linked or
+//   preloaded (LD_PRELOAD), calling glibc's malloc_trim typically has no effect
+//   on the *active* heap. The call is harmless but may not reclaim memory
+//   because those allocators manage their own arenas.
+// - Only glibc sbrk/arena space is eligible for trimming; large mmap-backed
+//   allocations are usually returned to the OS on free regardless of trimming.
+// - Call at known reclamation points (e.g., after cache sweeps / online delete)
+//   and consider rate limiting to avoid churn.
+// -----------------------------------------------------------------------------
+
 struct MallocTrimReport
 {
     bool supported{false};
@@ -15,24 +29,38 @@ struct MallocTrimReport
     long rssBeforeKB{-1};
     long rssAfterKB{-1};
 
-    long
-    deltaKB() const
+    [[nodiscard]] long
+    deltaKB() const noexcept
     {
+        if (rssBeforeKB < 0 || rssAfterKB < 0)
+            return 0;
         return rssAfterKB - rssBeforeKB;
     }
 };
 
-/** Attempt to return freed memory to the operating system.
+/**
+ * @brief Attempt to return freed memory to the operating system.
  *
- * This function is only effective on Linux with glibc. On other platforms,
- * it returns a report indicating the operation is unsupported.
+ * On Linux with glibc malloc, this issues ::malloc_trim(0), which may release
+ * free space from ptmalloc arenas back to the kernel. On other platforms, or if
+ * a different allocator is in use, this function is a no-op and the report will
+ * indicate that trimming is unsupported or had no effect.
  *
- * @param tag Optional identifier for logging/debugging purposes
- * @param journal Journal for logging diagnostic information
- * @return Report containing before/after memory metrics
+ * @param tag     Optional identifier for logging/debugging purposes.
+ * @param journal Journal for diagnostic logging.
+ * @return Report containing before/after metrics and the trim result.
  *
- * @note This is intended for use after cache sweeps or other operations
- *       that free significant amounts of memory.
+ * @note If an alternative allocator (jemalloc/tcmalloc) is linked or preloaded,
+ *       calling glibc's malloc_trim may have no effect on the active heap. The
+ *       call is harmless but typically does not reclaim memory under those
+ *       allocators.
+ *
+ * @note Only memory served from glibc's sbrk/arena heaps is eligible for trim.
+ *       Large allocations satisfied via mmap are usually returned on free
+ *       independently of trimming.
+ *
+ * @note Intended for use after operations that free significant memory (e.g.,
+ *       cache sweeps, ledger cleanup, online delete). Consider rate limiting.
  */
 MallocTrimReport
 mallocTrim(std::optional<std::string> const& tag, beast::Journal journal);