Index: compiler-rt/lib/fuzzer/CMakeLists.txt
===================================================================
--- compiler-rt/lib/fuzzer/CMakeLists.txt
+++ compiler-rt/lib/fuzzer/CMakeLists.txt
@@ -13,6 +13,7 @@
   FuzzerLoop.cpp
   FuzzerMerge.cpp
   FuzzerModuleRelative.cpp
+  FuzzerMonitor.cpp
   FuzzerMutate.cpp
   FuzzerSHA1.cpp
   FuzzerTracePC.cpp
Index: compiler-rt/lib/fuzzer/FuzzerDriver.cpp
===================================================================
--- compiler-rt/lib/fuzzer/FuzzerDriver.cpp
+++ compiler-rt/lib/fuzzer/FuzzerDriver.cpp
@@ -301,22 +301,6 @@
   return HasErrors ? 1 : 0;
 }
 
-static void RssThread(size_t RssLimitMb) {
-  while (true) {
-    SleepSeconds(1);
-    size_t Peak = GetPeakRSSMb();
-    if (Peak > RssLimitMb)
-      FuzzerRssLimitCallback(0);
-  }
-}
-
-static void StartRssThread(size_t RssLimitMb) {
-  if (!RssLimitMb)
-    return;
-  std::thread T(RssThread, RssLimitMb);
-  T.detach();
-}
-
 int RunOneTest(Fuzzer *F, const char *InputFilePath, size_t MaxLen) {
   Unit U = FileToVector(InputFilePath);
   if (MaxLen && MaxLen < U.size())
Index: compiler-rt/lib/fuzzer/FuzzerLoop.cpp
===================================================================
--- compiler-rt/lib/fuzzer/FuzzerLoop.cpp
+++ compiler-rt/lib/fuzzer/FuzzerLoop.cpp
@@ -47,96 +47,6 @@
 // Only one Fuzzer per process.
 static Fuzzer *F;
 
-// Leak detection is expensive, so we first check if there were more mallocs
-// than frees (using the sanitizer malloc hooks) and only then try to call lsan.
-struct MallocFreeTracer {
-  void Start(int TraceLevel) {
-    this->TraceLevel = TraceLevel;
-    if (TraceLevel)
-      Printf("MallocFreeTracer: START\n");
-    Mallocs = 0;
-    Frees = 0;
-  }
-  // Returns true if there were more mallocs than frees.
-  bool Stop() {
-    if (TraceLevel)
-      Printf("MallocFreeTracer: STOP %zd %zd (%s)\n", Mallocs.load(),
-             Frees.load(), Mallocs == Frees ? "same" : "DIFFERENT");
-    bool Result = Mallocs > Frees;
-    Mallocs = 0;
-    Frees = 0;
-    TraceLevel = 0;
-    return Result;
-  }
-  std::atomic<size_t> Mallocs;
-  std::atomic<size_t> Frees;
-  int TraceLevel = 0;
-
-  std::recursive_mutex TraceMutex;
-  bool TraceDisabled = false;
-};
-
-static MallocFreeTracer AllocTracer;
-
-// Locks printing and avoids nested hooks triggered from mallocs/frees in
-// sanitizer.
-class TraceLock {
-public:
-  TraceLock() : Lock(AllocTracer.TraceMutex) {
-    AllocTracer.TraceDisabled = !AllocTracer.TraceDisabled;
-  }
-  ~TraceLock() { AllocTracer.TraceDisabled = !AllocTracer.TraceDisabled; }
-
-  bool IsDisabled() const {
-    // This is already inverted value.
-    return !AllocTracer.TraceDisabled;
-  }
-
-private:
-  std::lock_guard<std::recursive_mutex> Lock;
-};
-
-ATTRIBUTE_NO_SANITIZE_MEMORY
-void MallocHook(const volatile void *ptr, size_t size) {
-  size_t N = AllocTracer.Mallocs++;
-  F->HandleMalloc(size);
-  if (int TraceLevel = AllocTracer.TraceLevel) {
-    TraceLock Lock;
-    if (Lock.IsDisabled())
-      return;
-    Printf("MALLOC[%zd] %p %zd\n", N, ptr, size);
-    if (TraceLevel >= 2 && EF)
-      PrintStackTrace();
-  }
-}
-
-ATTRIBUTE_NO_SANITIZE_MEMORY
-void FreeHook(const volatile void *ptr) {
-  size_t N = AllocTracer.Frees++;
-  if (int TraceLevel = AllocTracer.TraceLevel) {
-    TraceLock Lock;
-    if (Lock.IsDisabled())
-      return;
-    Printf("FREE[%zd]   %p\n", N, ptr);
-    if (TraceLevel >= 2 && EF)
-      PrintStackTrace();
-  }
-}
-
-// Crash on a single malloc that exceeds the rss limit.
-void Fuzzer::HandleMalloc(size_t Size) {
-  if (!Options.MallocLimitMb || (Size >> 20) < (size_t)Options.MallocLimitMb)
-    return;
-  Printf("==%d== ERROR: libFuzzer: out-of-memory (malloc(%zd))\n", GetPid(),
-         Size);
-  Printf("   To change the out-of-memory limit use -rss_limit_mb=<N>\n\n");
-  PrintStackTrace();
-  DumpCurrentUnit("oom-");
-  Printf("SUMMARY: libFuzzer: out-of-memory\n");
-  PrintFinalStats();
-  _Exit(Options.OOMExitCode); // Stop right now.
-}
-
 Fuzzer::Fuzzer(UserCallback CB, InputCorpus &Corpus, MutationDispatcher &MD,
                FuzzingOptions Options)
     : CB(CB), Corpus(Corpus), MD(MD), Options(Options) {
@@ -146,8 +56,7 @@
   F = this;
   TPC.ResetMaps();
   IsMyThread = true;
-  if (Options.DetectLeaks && EF->__sanitizer_install_malloc_and_free_hooks)
-    EF->__sanitizer_install_malloc_and_free_hooks(MallocHook, FreeHook);
+  AllocMonitorConfigure(Options);
   TPC.SetUseCounters(Options.UseCounters);
   TPC.SetUseValueProfileMask(Options.UseValueProfile);
   if (Options.Verbosity)
@@ -290,7 +199,7 @@
   if (LastMalloc) {
     Printf("(malloc(%zd))\n", LastMalloc);
     Printf("   To change the out-of-memory limit use -rss_limit_mb=<N>\n\n");
-    PrintStackTrace();
+    PrintStackTrace(PID);
   } else {
     Printf("(used: %zdMb; limit: %zdMb)\n", GetPeakRSSMb(), Options.RssLimitMb);
     Printf("   To change the out-of-memory limit use -rss_limit_mb=<N>\n\n");
@@ -601,7 +510,7 @@
   CurrentUnitSize = Size;
   {
     ScopedEnableMsanInterceptorChecks S;
-    AllocTracer.Start(Options.TraceMalloc);
+    AllocMonitorStartTracing(Options.TraceMalloc);
     UnitStartTime = system_clock::now();
     TPC.ResetMaps();
     RunningUserCallback = true;
@@ -610,7 +519,7 @@
     UnitStopTime = system_clock::now();
     (void)Res;
     assert(Res == 0);
-    HasMoreMallocsThanFrees = AllocTracer.Stop();
+    HasMoreMallocsThanFrees = AllocMonitorStopTracing();
   }
   if (!LooseMemeq(DataCopy, Data, Size))
     CrashOnOverwrittenData();
@@ -887,7 +796,7 @@
     // Perform several mutations and runs.
     MutateAndTestOne();
 
-    PurgeAllocator();
+    AllocMonitorPurge();
   }
 
   PrintStats("DONE  ", "\n");
Index: compiler-rt/lib/fuzzer/FuzzerMonitor.h
===================================================================
--- compiler-rt/lib/fuzzer/FuzzerMonitor.h
+++ compiler-rt/lib/fuzzer/FuzzerMonitor.h
@@ -11,9 +11,20 @@
 #ifndef LLVM_FUZZER_MONITOR_H
 #define LLVM_FUZZER_MONITOR_H
 
+#include "FuzzerOptions.h"
 #include "FuzzerPlatform.h"
 #include <cstddef>
 
+namespace fuzzer {
+
+void StartRssThread(size_t RssLimitMb);
+void AllocMonitorConfigure(const FuzzingOptions &Options);
+void AllocMonitorStartTracing(int TraceLevel);
+bool AllocMonitorStopTracing();
+void AllocMonitorPurge();
+
+} // namespace fuzzer
+
 extern "C" {
 
 ATTRIBUTE_INTERFACE void FuzzerAlarmCallback();
Index: compiler-rt/lib/fuzzer/FuzzerMonitor.cpp
===================================================================
--- /dev/null
+++ compiler-rt/lib/fuzzer/FuzzerMonitor.cpp
@@ -0,0 +1,164 @@
+//===- FuzzerMonitor.cpp ----------------------------------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Defines an interface for tracing mallocs and frees.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerMonitor.h"
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include "FuzzerPlatform.h"
+#include "FuzzerUtil.h"
+#include <atomic>
+#include <chrono>
+#include <mutex>
+#include <thread>
+
+namespace fuzzer {
+namespace {
+
+using namespace std::chrono;
+
+void MallocHook(const volatile void *ptr, size_t size);
+void FreeHook(const volatile void *ptr);
+
+// Leak detection is expensive, so we first check if there were more mallocs
+// than frees (using the sanitizer malloc hooks) and only then try to call lsan.
+static struct {
+  void Configure(const FuzzingOptions &Options) {
+    MallocLimit = Options.MallocLimitMb << 20;
+    TraceLevel = Options.TraceMalloc;
+    if (Options.DetectLeaks && EF->__sanitizer_install_malloc_and_free_hooks)
+      EF->__sanitizer_install_malloc_and_free_hooks(MallocHook, FreeHook);
+    RssLimitMb = Options.RssLimitMb;
+    PurgeIntervalSec = EF->__sanitizer_purge_allocator
+                           ? Options.PurgeAllocatorIntervalSec
+                           : -1;
+  }
+
+  void StartTracing(int TraceLevel) {
+    this->TraceLevel = TraceLevel;
+    if (TraceLevel)
+      Printf("MallocFreeTracer: START\n");
+    Mallocs = 0;
+    Frees = 0;
+  }
+
+  // Returns true if there were more mallocs than frees.
+  bool StopTracing() {
+    if (TraceLevel)
+      Printf("MallocFreeTracer: STOP %zd %zd (%s)\n", Mallocs.load(),
+             Frees.load(), Mallocs == Frees ? "same" : "DIFFERENT");
+    bool Result = Mallocs > Frees;
+    Mallocs = 0;
+    Frees = 0;
+    TraceLevel = 0;
+    return Result;
+  }
+
+  void Purge() {
+    if (PurgeIntervalSec < 0 || !EF->__sanitizer_purge_allocator)
+      return;
+    auto T = duration_cast<seconds>(system_clock::now() - LastPurgeAttempt);
+    if (T.count() < PurgeIntervalSec)
+      return;
+    if (RssLimitMb <= 0 || GetPeakRSSMb() > static_cast<size_t>(RssLimitMb / 2))
+      EF->__sanitizer_purge_allocator();
+    LastPurgeAttempt = system_clock::now();
+  }
+
+  size_t MallocLimit = 0;
+  std::atomic<size_t> Mallocs;
+  std::atomic<size_t> Frees;
+  int TraceLevel = 0;
+
+  std::recursive_mutex TraceMutex;
+  bool TraceDisabled = false;
+
+  int RssLimitMb = 0;
+  int PurgeIntervalSec = -1;
+  system_clock::time_point LastPurgeAttempt = system_clock::now();
+
+} AllocMonitor;
+
+// Locks printing and avoids nested hooks triggered from mallocs/frees in
+// sanitizer.
+class TraceLock {
+public:
+  TraceLock() : Lock(AllocMonitor.TraceMutex) {
+    AllocMonitor.TraceDisabled = !AllocMonitor.TraceDisabled;
+  }
+
+  ~TraceLock() { AllocMonitor.TraceDisabled = !AllocMonitor.TraceDisabled; }
+
+  bool IsDisabled() const {
+    // This is already inverted value.
+    return !AllocMonitor.TraceDisabled;
+  }
+
+private:
+  std::lock_guard<std::recursive_mutex> Lock;
+};
+
+ATTRIBUTE_NO_SANITIZE_MEMORY
+void MallocHook(const volatile void *ptr, size_t size) {
+  size_t N = AllocMonitor.Mallocs++;
+  if (AllocMonitor.MallocLimit && size >= AllocMonitor.MallocLimit)
+    FuzzerMallocLimitCallback(0, size);
+  if (int TraceLevel = AllocMonitor.TraceLevel) {
+    TraceLock Lock;
+    if (Lock.IsDisabled())
+      return;
+    if (ptr)
+      Printf("MALLOC[%zd] %p %zd\n", N, ptr, size);
+    if (TraceLevel >= 2 && EF)
+      PrintStackTrace();
+  }
+}
+
+ATTRIBUTE_NO_SANITIZE_MEMORY
+void FreeHook(const volatile void *ptr) {
+  size_t N = AllocMonitor.Frees++;
+  if (int TraceLevel = AllocMonitor.TraceLevel) {
+    TraceLock Lock;
+    if (Lock.IsDisabled())
+      return;
+    Printf("FREE[%zd]   %p\n", N, ptr);
+    if (TraceLevel >= 2 && EF)
+      PrintStackTrace();
+  }
+}
+
+} // namespace
+
+void StartRssThread(size_t RssLimitMb) {
+  if (!RssLimitMb)
+    return;
+  std::thread T([RssLimitMb]() {
+    while (true) {
+      SleepSeconds(1);
+      size_t Peak = GetPeakRSSMb();
+      if (Peak > RssLimitMb)
+        FuzzerRssLimitCallback(0);
+    }
+  });
+  T.detach();
+}
+
+void AllocMonitorConfigure(const FuzzingOptions &Options) {
+  AllocMonitor.Configure(Options);
+}
+
+void AllocMonitorStartTracing(int TraceLevel) {
+  AllocMonitor.StartTracing(TraceLevel);
+}
+
+bool AllocMonitorStopTracing() { return AllocMonitor.StopTracing(); }
+
+void AllocMonitorPurge() { AllocMonitor.Purge(); }
+
+} // namespace fuzzer