memory coordinator: Use free memory to guess available memory on Linux

content/browser/memory/memory_monitor_linux.cc

 // Copyright (c) 2016 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.
 
 #include "content/browser/memory/memory_monitor_linux.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/process/process_metrics.h"
 
 namespace content {
@@ skipping 15 matching lines @@
   delegate_->GetSystemMemoryInfo(&mem_info);
 
   // According to kernel commit 34e431b0ae398fc54ea69ff85ec700722c9da773,
   // "available" is the "amount of memory that is available for a new workload
   // without pushing the system into swap"; return that value if it is valid.
   // Old linux kernels (before 3.14) don't support "available" and show zero
   // instead.
   if (mem_info.available > 0)
     return mem_info.available >> kShiftKiBtoMiB;
 
-  // If there is no "available" value, guess at it based on free memory and
-  // what the OS can easily discard.
-  return (mem_info.free + mem_info.buffers + mem_info.cached) >> kShiftKiBtoMiB;
+  // If there is no "available" value, guess at it based on free memory.
+  // Though there will be easily discardable memory (buffers and caches), we
+  // don't count them because discarding them will affect the overall
+  // performance of the OS.
+  return mem_info.free >> kShiftKiBtoMiB;
 }
 
 // static
 std::unique_ptr<MemoryMonitorLinux> MemoryMonitorLinux::Create(
     MemoryMonitorDelegate* delegate) {
   return base::MakeUnique<MemoryMonitorLinux>(delegate);
 }
 
 // Implementation of factory function defined in memory_monitor.h.
 std::unique_ptr<MemoryMonitor> CreateMemoryMonitor() {
   return MemoryMonitorLinux::Create(MemoryMonitorDelegate::GetInstance());
 }
 
 }  // namespace content