[wasm] Improve module code size sampling approach.

This samples module code sizes at GC time instead of during destruction.
It hence makes sure that we also receive samples for long-lived modules
which would otherwise die with the Isolate and never be finalized. Note
that this approach is still biased and just a stop-gap until we have a
sampling tick based on actual wall-clock time.

R=clemensh@chromium.org

Change-Id: I9558d383a5aada8876bc9cbf63baca771dbe5c28
Reviewed-on: https://chromium-review.googlesource.com/1141866Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54554}

src/isolate.cc

@@ -2969,8 +2969,7 @@ bool Isolate::Init(StartupDeserializer* des) {
     wasm_engine_.reset(
         new wasm::WasmEngine(std::unique_ptr<wasm::WasmCodeManager>(
             new wasm::WasmCodeManager(kMaxWasmCodeMemory))));
-    wasm_engine_->code_manager()->SetModuleCodeSizeHistogram(
-        counters()->wasm_module_code_size_mb());
+    wasm::WasmCodeManager::InstallSamplingGCCallback(this);
   }
 
   deoptimizer_data_ = new DeoptimizerData(heap());

src/wasm/wasm-code-manager.cc

@@ -793,6 +793,29 @@ void WasmCodeManager::TryAllocate(size_t size, VirtualMemory* ret, void* hint) {
              reinterpret_cast<void*>(ret->end()), ret->size());
 }
 
+void WasmCodeManager::SampleModuleSizes(Isolate* isolate) const {
+  for (NativeModule* native_module : native_modules_) {
+    int code_size = static_cast<int>(native_module->committed_code_space_ / MB);
+    isolate->counters()->wasm_module_code_size_mb()->AddSample(code_size);
+  }
+}
+
+namespace {
+
+void ModuleSamplingCallback(v8::Isolate* v8_isolate, v8::GCType type,
+                            v8::GCCallbackFlags flags, void* data) {
+  Isolate* isolate = reinterpret_cast<Isolate*>(v8_isolate);
+  isolate->wasm_engine()->code_manager()->SampleModuleSizes(isolate);
+}
+
+}  // namespace
+
+// static
+void WasmCodeManager::InstallSamplingGCCallback(Isolate* isolate) {
+  isolate->heap()->AddGCEpilogueCallback(ModuleSamplingCallback,
+                                         v8::kGCTypeMarkSweepCompact, nullptr);
+}
+
 // static
 size_t WasmCodeManager::EstimateNativeModuleSize(const WasmModule* module) {
   constexpr size_t kCodeSizeMultiplier = 4;
@@ -825,7 +848,7 @@ std::unique_ptr<NativeModule> WasmCodeManager::NewNativeModule(
   // all isolates, at the point of commit.
   constexpr size_t kCriticalThreshold = 32 * 1024 * 1024;
   bool force_critical_notification =
-      (active_ > 1) &&
+      (native_modules_.size() > 1) &&
       (remaining_uncommitted_code_space_.load() < kCriticalThreshold);
 
   if (force_critical_notification) {
@@ -846,7 +869,7 @@ std::unique_ptr<NativeModule> WasmCodeManager::NewNativeModule(
     TRACE_HEAP("New NativeModule %p: Mem: %" PRIuPTR ",+%zu\n", this, start,
                size);
     AssignRanges(start, end, ret.get());
-    ++active_;
+    native_modules_.emplace(ret.get());
     return ret;
   }
 
@@ -899,8 +922,8 @@ bool NativeModule::SetExecutable(bool executable) {
 }
 
 void WasmCodeManager::FreeNativeModule(NativeModule* native_module) {
-  DCHECK_GE(active_, 1);
-  --active_;
+  DCHECK_EQ(1, native_modules_.count(native_module));
+  native_modules_.erase(native_module);
   TRACE_HEAP("Freeing NativeModule %p\n", this);
   for (auto& vmem : native_module->owned_code_space_) {
     lookup_map_.erase(vmem.address());
@@ -911,11 +934,6 @@ void WasmCodeManager::FreeNativeModule(NativeModule* native_module) {
 
   size_t code_size = native_module->committed_code_space_;
   DCHECK(IsAligned(code_size, AllocatePageSize()));
-
-  if (module_code_size_mb_) {
-    module_code_size_mb_->AddSample(static_cast<int>(code_size / MB));
-  }
-
   remaining_uncommitted_code_space_.fetch_add(code_size);
 }
 

src/wasm/wasm-code-manager.h

@@ -9,6 +9,7 @@
 #include <list>
 #include <map>
 #include <unordered_map>
+#include <unordered_set>
 
 #include "src/base/macros.h"
 #include "src/handles.h"
@@ -21,7 +22,6 @@ namespace internal {
 
 struct CodeDesc;
 class Code;
-class Histogram;
 
 namespace wasm {
 
@@ -433,9 +433,14 @@ class V8_EXPORT_PRIVATE WasmCodeManager final {
   WasmCode* GetCodeFromStartAddress(Address pc) const;
   size_t remaining_uncommitted_code_space() const;
 
-  void SetModuleCodeSizeHistogram(Histogram* histogram) {
-    module_code_size_mb_ = histogram;
-  }
+  // Add a sample of all module sizes.
+  void SampleModuleSizes(Isolate* isolate) const;
+
+  // TODO(v8:7424): For now we sample module sizes in a GC callback. This will
+  // bias samples towards apps with high memory pressure. We should switch to
+  // using sampling based on regular intervals independent of the GC.
+  static void InstallSamplingGCCallback(Isolate* isolate);
+
   static size_t EstimateNativeModuleSize(const WasmModule* module);
 
  private:
@@ -452,14 +457,9 @@ class V8_EXPORT_PRIVATE WasmCodeManager final {
   void AssignRanges(Address start, Address end, NativeModule*);
 
   std::map<Address, std::pair<Address, NativeModule*>> lookup_map_;
-  // Count of NativeModules not yet collected. Helps determine if it's
-  // worth requesting a GC on memory pressure.
-  size_t active_ = 0;
+  std::unordered_set<NativeModule*> native_modules_;
   std::atomic<size_t> remaining_uncommitted_code_space_;
 
-  // Histogram to update with the maximum used code space for each NativeModule.
-  Histogram* module_code_size_mb_ = nullptr;
-
   DISALLOW_COPY_AND_ASSIGN(WasmCodeManager);
 };