Explicit heap trimming

include/xrpl/basics/MallocTrim.h

@@ -0,0 +1,70 @@
+#ifndef XRPL_BASICS_MALLOCTRIM_H_INCLUDED
+#define XRPL_BASICS_MALLOCTRIM_H_INCLUDED
+
+#include <xrpl/beast/utility/Journal.h>
+
+#include <optional>
+#include <string>
+
+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};
+    int trimResult{-1};
+    long rssBeforeKB{-1};
+    long rssAfterKB{-1};
+
+    [[nodiscard]] long
+    deltaKB() const noexcept
+    {
+        if (rssBeforeKB < 0 || rssAfterKB < 0)
+            return 0;
+        return rssAfterKB - rssBeforeKB;
+    }
+};
+
+/**
+ * @brief Attempt to return freed memory to the operating system.
+ *
+ * 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 diagnostic logging.
+ * @return Report containing before/after metrics and the trim result.
+ *
+ * @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);
+
+}  // namespace ripple
+
+#endif

src/libxrpl/basics/MallocTrim.cpp

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+#include <xrpl/basics/Log.h>
+#include <xrpl/basics/MallocTrim.h>
+
+#include <boost/predef.h>
+
+#include <cstdio>
+#include <fstream>
+
+#if defined(__GLIBC__) && BOOST_OS_LINUX
+#include <malloc.h>
+#include <unistd.h>
+
+namespace {
+pid_t const cachedPid = ::getpid();
+}  // namespace
+#endif
+
+namespace ripple {
+
+namespace detail {
+
+#if defined(__GLIBC__) && BOOST_OS_LINUX
+
+long
+parseVmRSSkB(std::string const& status)
+{
+    std::istringstream iss(status);
+    std::string line;
+
+    while (std::getline(iss, line))
+    {
+        // Allow leading spaces/tabs before the key.
+        auto const firstNonWs = line.find_first_not_of(" \t");
+        if (firstNonWs == std::string::npos)
+            continue;
+
+        constexpr char key[] = "VmRSS:";
+        constexpr auto keyLen = sizeof(key) - 1;
+
+        // Require the line (after leading whitespace) to start with "VmRSS:".
+        // Check if we have enough characters and the substring matches.
+        if (firstNonWs + keyLen > line.size() ||
+            line.substr(firstNonWs, keyLen) != key)
+            continue;
+
+        // Move past "VmRSS:" and any following whitespace.
+        auto pos = firstNonWs + keyLen;
+        while (pos < line.size() &&
+               std::isspace(static_cast<unsigned char>(line[pos])))
+        {
+            ++pos;
+        }
+
+        long value = -1;
+        if (std::sscanf(line.c_str() + pos, "%ld", &value) == 1)
+            return value;
+
+        // Found the key but couldn't parse a number.
+        return -1;
+    }
+
+    // No VmRSS line found.
+    return -1;
+}
+
+#endif  // __GLIBC__ && BOOST_OS_LINUX
+
+}  // namespace detail
+
+MallocTrimReport
+mallocTrim(
+    [[maybe_unused]] std::optional<std::string> const& tag,
+    beast::Journal journal)
+{
+    MallocTrimReport report;
+
+#if !(defined(__GLIBC__) && BOOST_OS_LINUX)
+    JLOG(journal.debug()) << "malloc_trim not supported on this platform";
+#else
+
+    report.supported = true;
+
+    if (journal.debug())
+    {
+        auto readFile = [](std::string const& path) -> std::string {
+            std::ifstream ifs(path);
+            if (!ifs.is_open())
+                return {};
+            return std::string(
+                std::istreambuf_iterator<char>(ifs),
+                std::istreambuf_iterator<char>());
+        };
+
+        std::string const tagStr = tag.value_or("default");
+        std::string const statusPath =
+            "/proc/" + std::to_string(cachedPid) + "/status";
+
+        auto const statusBefore = readFile(statusPath);
+        report.rssBeforeKB = detail::parseVmRSSkB(statusBefore);
+
+        report.trimResult = ::malloc_trim(0);
+
+        auto const statusAfter = readFile(statusPath);
+        report.rssAfterKB = detail::parseVmRSSkB(statusAfter);
+
+        JLOG(journal.debug())
+            << "malloc_trim tag=" << tagStr << " result=" << report.trimResult
+            << " rss_before=" << report.rssBeforeKB << "kB"
+            << " rss_after=" << report.rssAfterKB << "kB"
+            << " delta=" << report.deltaKB() << "kB";
+    }
+    else
+    {
+        report.trimResult = ::malloc_trim(0);
+    }
+#endif
+
+    return report;
+}
+
+}  // namespace ripple