Add UMA metrics for desktop memory pressure notifications.

content/child/memory/child_memory_message_filter.cc

Index: content/child/memory/child_memory_message_filter.cc
diff --git a/content/child/memory/child_memory_message_filter.cc b/content/child/memory/child_memory_message_filter.cc
index 91e6849eab2eb2aea5d157e6ef42ee85aa0ce031..726c7c2d59ac7ccbb2af6dbe17bf52ee33549d02 100644
--- a/content/child/memory/child_memory_message_filter.cc
+++ b/content/child/memory/child_memory_message_filter.cc
@@ -4,11 +4,60 @@
 
 #include "content/child/memory/child_memory_message_filter.h"
 
+#include "base/metrics/histogram_macros.h"
+#include "base/process/process_metrics.h"
 #include "content/common/memory_messages.h"
 
 namespace content {
 
-ChildMemoryMessageFilter::ChildMemoryMessageFilter() {}
+namespace {
+
+// The memory pressure monitoring is OSX only because as of this writing a good
+// estimate of wired pages is lacking on Windows. The number of desktop Linux
+// users doesn't warrant a special implementation for that, and ChromeOS and
+// Android generally handle memory pressure differently.
+#if defined(OS_MACOSX)
+
+// Returns an estimate of the physical memory used by the current process.
+// This is not hooked up on all platforms, it will return 0 if unsupported.
+uint64_t GetPrivateMemoryBytes() {
+  std::unique_ptr<base::ProcessMetrics> metrics = base::ProcessMetrics::CreateCurrentProcessMetrics();
+
+  base::ProcessMetrics::TaskVMInfo info = metrics->GetTaskVMInfo();
+  // This value is only nonzero on OSX 10.12 and above.
+  return info.phys_footprint;
+}
+
+// Logs a histogram of the memory saved from a pressure event. This is called
+// on a timer to allow some time for pressure events to get propagated to other
+// threads or asynchronous cleanup tasks to occur.
+void LogMemoryPressureDelta(uint64_t before_memory) {
+  uint64_t after_memory = GetPrivateMemoryBytes();
+
+  UMA_HISTOGRAM_MEMORY_KB("Memory.RendererPressureSavingsKB",
+                          (before_memory - after_memory) / 1024);
+
+  double savings_frac = 1.0 -
+      (static_cast<double>(after_memory) / static_cast<double>(before_memory));
+
+  // The histogram macros can't handle negative items well, as 0 will always be
+  // the underflow bucket. If memory increased, it's unlikely to be a result
+  // of compaction so we don't care too much about the specific value.
+  if (savings_frac < 0.0)
+    savings_frac = 0.0;
+  UMA_HISTOGRAM_PERCENTAGE("Memory.RendererPressurePercentSavings",
+                           static_cast<int>(100.0 * savings_frac));
+}
+
+#endif  // OS_MACOSX
+
+}  // namespace
+
+ChildMemoryMessageFilter::ChildMemoryMessageFilter()
+#if defined(OS_MACOSX)
+    : pressure_timer_(false, false)
+#endif
+{}
 
 ChildMemoryMessageFilter::~ChildMemoryMessageFilter() {}
 
@@ -40,9 +89,20 @@ void ChildMemoryMessageFilter::OnSimulatePressureNotification(
 
 void ChildMemoryMessageFilter::OnPressureNotification(
     base::MemoryPressureListener::MemoryPressureLevel level) {
+#if defined(OS_MACOSX)
+  uint64_t before_memory = GetPrivateMemoryBytes();
+#endif  // OS_MACOSX
+
   // Forward the message along the normal notification path. If notifications
   // are suppressed then the notification will be swallowed.
   base::MemoryPressureListener::NotifyMemoryPressure(level);
+
+#if defined(OS_MACOSX)
+  if (before_memory > 0) {
+    pressure_timer_.Start(FROM_HERE, base::TimeDelta::FromSeconds(5),
+        base::BindRepeating(&LogMemoryPressureDelta, before_memory));
+  }
+#endif  // OS_MACOSX
 }
 
 }  // namespace content