Bump field trial allocator size to 64kb and report usage

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index 63f549e5729d69b3587de96dbef7ad61764bac26..65611e3fc5fd220a1ba1dcda7ebaeeb04c776b8b 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -43,8 +43,12 @@ const bool kUseSharedMemoryForFieldTrials = false;
 // Constants for the field trial allocator.
 const char kAllocatorName[] = "FieldTrialAllocator";
 const uint32_t kFieldTrialType = 0xABA17E13 + 1;  // SHA1(FieldTrialEntry) v1

[bcwhite, 2016/11/08 20:30:02]

Blank line after.

[lawrencewu, 2016/11/09 18:06:46]

Done.

+// We allocate 64 KiB to hold all the field trial data. This should be enough,

[bcwhite, 2016/11/08 20:30:02]

Include the date when this was true.  Also, comment what will happen if this
isn't large enough.

[lawrencewu, 2016/11/09 18:06:46]

Done.

+// as currently we use ~8KiB for the field trials, and ~10KiB for experiment
+// parameters. This also doesn't allocate all 64 KiB at once -- the pages only
+// get mapped to physical memory when they are touched.
 #if !defined(OS_NACL)
-const size_t kFieldTrialAllocationSize = 4 << 10;  // 4 KiB = one page
+const size_t kFieldTrialAllocationSize = 4 << 14;

[bcwhite, 2016/11/08 20:14:32]

Please do this as

64 << 10 // 64 KiB

[lawrencewu, 2016/11/09 18:06:46]

Done.

 #endif
 
 // We create one FieldTrialEntry per field trial in shared memory, via
@@ -179,6 +183,15 @@ HANDLE CreateReadOnlyHandle(SharedPersistentMemoryAllocator* allocator) {
     return nullptr;
   return dst;
 }
+
+void ReportFieldTrialAllocatorUsage(

[bcwhite, 2016/11/08 20:14:32]

Don't need this.

[lawrencewu, 2016/11/09 18:06:46]

Removed.

+    SharedPersistentMemoryAllocator* allocator) {
+  size_t used = allocator->used();
+  UMA_HISTOGRAM_COUNTS_100000("UMA.FieldTrialAllocator.Used", used);
+  int used_percent = used / kFieldTrialAllocationSize;
+  UMA_HISTOGRAM_PERCENTAGE("UMA.FieldTrialAllocator.UsedPct",
+                           used_percent * 100);
+}
 #endif
 
 }  // namespace
@@ -728,8 +741,7 @@ void FieldTrialList::CopyFieldTrialStateToFlags(
                                      std::to_string(field_trial_length);
 
     cmd_line->AppendSwitchASCII(field_trial_handle_switch, field_trial_handle);
-    UMA_HISTOGRAM_COUNTS_10000("UMA.FieldTrialAllocator.Size",
-                               field_trial_length);
+    ReportFieldTrialAllocatorUsage(global_->field_trial_allocator_.get());

[bcwhite, 2016/11/08 20:14:32]

When you create the allocator, call

allocator->CreateTrackingHistograms(allocator->Name());

Here, call

allocator->UpdateTrackingHistograms();


See here for how the histograms are named:
https://cs.chromium.org/chromium/src/base/metrics/persistent_memory_allocator...

[lawrencewu, 2016/11/09 18:06:46]

Done. But this will get called on every subprocess launch. Is that okay?

[bcwhite, 2016/11/09 18:57:37]

Not ideal but acceptable.  If you can call it once after everything has been
loaded, that would be best but it's not overly important since we're really just
looking for the maximum value it reaches.

[lawrencewu, 2016/11/09 19:15:00]

Acknowledged.

     return;
   }
 #endif