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..4eae0421936b1ed795290dcd967be8faeba3992d 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -43,9 +43,7 @@ const bool kUseSharedMemoryForFieldTrials = false;
 // Constants for the field trial allocator.
 const char kAllocatorName[] = "FieldTrialAllocator";
 const uint32_t kFieldTrialType = 0xABA17E13 + 1;  // SHA1(FieldTrialEntry) v1
-#if !defined(OS_NACL)
-const size_t kFieldTrialAllocationSize = 4 << 10;  // 4 KiB = one page
-#endif
+const size_t kFieldTrialAllocationSize = 4 << 14;  // 64 KiB should be enough

[Alexei Svitkine (slow), 2016/11/08 18:15:23]

The "should be enough" comment is not useful. 

I suggest moving the comment to above the constant and expanding on it. In
particular, mentioning that this doesn't want memory since parts of the block
that are not touched don't get mapped.

If you want to keep the "should be enough" comment, you should justify it - e.g.
by referencing the observed amounts currently.

 
 // We create one FieldTrialEntry per field trial in shared memory, via
 // AddToAllocatorWhileLocked. The FieldTrialEntry is followed by a base::Pickle
@@ -728,8 +726,8 @@ 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",

[Alexei Svitkine (slow), 2016/11/08 18:15:23]

Changing the macro without changing histogram name is not OK. It will renumber
all the bucket ranges and now you'll have different buckets reported by
different versions. So please rename the histogram, marking the old name as
<obsolete> in the config.

Can you also add a different histogram logging the percent used?

[Alexei Svitkine (slow), 2016/11/08 18:18:38]

For the rename, you can use "2" suffix to the name.

-                               field_trial_length);
+    UMA_HISTOGRAM_COUNTS_100000("UMA.FieldTrialAllocator.Size",
+                                global_->field_trial_allocator_->used());
     return;
   }
 #endif