[tracing] Display the resident size of the discardable memory segments

base/trace_event/process_memory_dump.h

 // Copyright 2015 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_TRACE_EVENT_PROCESS_MEMORY_DUMP_H_
 #define BASE_TRACE_EVENT_PROCESS_MEMORY_DUMP_H_
 
 #include <vector>
 
 #include "base/base_export.h"
@@ skipping 23 matching lines @@
     MemoryAllocatorDumpGuid target;
     int importance;
     const char* type;
   };
 
   // Maps allocator dumps absolute names (allocator_name/heap/subheap) to
   // MemoryAllocatorDump instances.
   using AllocatorDumpsMap =
       SmallMap<hash_map<std::string, MemoryAllocatorDump*>>;
 
+  // Returns the total bytes resident for a memory segment, with given
+  // |start_address| and |mapped_size|. |mapped_size| is specified in bytes. The
+  // value returned is valid only if the given range is currently mmapped by the
+  // process. Returns -1 if counting is not implemented in the current platform.

[reveman, 2015/10/07 09:40:04]

Can we have a IsCountResidentBytesSupported() function instead of using a
negative return value for when this is not supported on a platform? Or maybe
even better, #ifdef the function so it's only compiled and available on
platforms where this is supported.

+  static int64 CountResidentBytes(void* start_address, size_t mapped_size);
+
   ProcessMemoryDump(const scoped_refptr<MemoryDumpSessionState>& session_state);
   ~ProcessMemoryDump();
 
   // Creates a new MemoryAllocatorDump with the given name and returns the
   // empty object back to the caller.
   // Arguments:
   //   absolute_name: a name that uniquely identifies allocator dumps produced
   //       by this provider. It is possible to specify nesting by using a
   //       path-like string (e.g., v8/isolate1/heap1, v8/isolate1/heap2).
   //       Leading or trailing slashes are not allowed.
@@ skipping 94 matching lines @@
   // Keeps track of relationships between MemoryAllocatorDump(s).
   std::vector<MemoryAllocatorDumpEdge> allocator_dumps_edges_;
 
   DISALLOW_COPY_AND_ASSIGN(ProcessMemoryDump);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_PROCESS_MEMORY_DUMP_H_