[Experimental, DO NOT LAND] bucket num 16

base/memory/shared_memory_tracker.h

Index: base/memory/shared_memory_tracker.h
diff --git a/base/memory/shared_memory_tracker.h b/base/memory/shared_memory_tracker.h
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_MEMORY_SHARED_MEMORY_TRACKER_H_
+#define BASE_MEMORY_SHARED_MEMORY_TRACKER_H_
+
+#include "base/atomicops.h"
+#include "base/memory/shared_memory_handle.h"
+#include "base/memory/singleton.h"
+#include "base/trace_event/memory_dump_provider.h"
+
+namespace base {
+
+class SharedMemory;
+
+namespace trace_event {
+class ProcessMemoryDump;
+}
+
+// SharedMemoryTracker tracks shared memory usage.
+class BASE_EXPORT SharedMemoryTracker
+    : public base::trace_event::MemoryDumpProvider {
+ public:
+  // Returns a singleton instance.
+  static SharedMemoryTracker* GetInstance();
+
+  // base::trace_event::MemoryDumpProvider implementation.
+  bool OnMemoryDump(const base::trace_event::MemoryDumpArgs& args,
+                    base::trace_event::ProcessMemoryDump* pmd) override;
+
+  // Records shared memory usage on mapping.
+  void IncrementMemoryUsage(const SharedMemory& shared_memory);
+
+  // Records shared memory usage on unmapping.
+  void DecrementMemoryUsage(const SharedMemory& shared_memory);
+
+ private:
+  // TrackerBucket represents a group of shared memory handle ids and their
+  // usages. Shared memorys are split into these buckets instead of using one
+  // big unordered_map so that locking for exclusive control would occur much
+  // less often.
+  struct TrackerBucket {
+    TrackerBucket();
+    ~TrackerBucket();
+
+    base::Lock lock;
+    std::unordered_map<const SharedMemory*, size_t> usages;
+  };
+
+  friend struct base::DefaultSingletonTraits<SharedMemoryTracker>;
+
+  // TODO(hajimehoshi): Update this number based on actual performance
+  // measurements.
+  static const int kTrackerBucketNum = 16;
+
+  TrackerBucket buckets_[kTrackerBucketNum];
+
+  SharedMemoryTracker();
+  ~SharedMemoryTracker() override;
+
+  DISALLOW_COPY_AND_ASSIGN(SharedMemoryTracker);
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_SHARED_MEMORY_TRACKER_H_