Reduce histogram creation time by avoiding import of those just created.

base/metrics/histogram_persistence.cc

Index: base/metrics/histogram_persistence.cc
diff --git a/base/metrics/histogram_persistence.cc b/base/metrics/histogram_persistence.cc
index bd250ec888a62a1998a4e762576b6bac388408b2..500dc7faa40ff0868e3ce0926327407964895519 100644
--- a/base/metrics/histogram_persistence.cc
+++ b/base/metrics/histogram_persistence.cc
@@ -4,6 +4,7 @@
 
 #include "base/metrics/histogram_persistence.h"
 
+#include "base/atomicops.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
@@ -87,6 +88,11 @@ struct PersistentHistogramData {
   char name[1];
 };
 
+// This is the offset of the last histogram that was created. It is used to
+// avoid trying to import the same thing just to have it rejected because
+// it already exists.
+subtle::AtomicWord g_last_created_histogram = 0;
+
 // The object held here will obviously not be destructed at process exit
 // but that's okay since PersistentMemoryAllocator objects are explicitly
 // forbidden from doing anything essential at exit anyway due to the fact
@@ -225,6 +231,7 @@ ReleasePersistentHistogramMemoryAllocatorForTesting() {
   }
 
   g_allocator = nullptr;
+  subtle::NoBarrier_Store(&g_last_created_histogram, 0);
   return allocator;
 };
 
@@ -470,6 +477,7 @@ HistogramBase* AllocatePersistentHistogram(
     DCHECK(histogram);
     if (ref_ptr != nullptr)
       *ref_ptr = histogram_ref;
+    subtle::NoBarrier_Store(&g_last_created_histogram, histogram_ref);
     return histogram;
   }
 
@@ -503,11 +511,29 @@ void ImportPersistentHistograms() {
     if (iter.is_clear())
       g_allocator->CreateIterator(&iter);
 
-    while (true) {
-      HistogramBase* histogram = GetNextPersistentHistogram(g_allocator, &iter);
-      if (!histogram)
-        break;
-      StatisticsRecorder::RegisterOrDeleteDuplicate(histogram);
+    PersistentMemoryAllocator::Reference last_created =
+        subtle::NoBarrier_Load(&g_last_created_histogram);
+
+    PersistentMemoryAllocator::Reference ref;
+    uint32_t type_id;
+    while ((ref = g_allocator->GetNextIterable(&iter, &type_id)) != 0) {
+      // Ignore any other type of objects in the allocator.
+      if (type_id != kTypeIdHistogram)
+        continue;
+
+      // Skip the import if it's the histogram that was last created.
+      // Should a race condition cause the "last created" to be overwritten
+      // before it is recognized here then the histogram will be created
+      // and be ignored when it is detected as a duplicate by the
+      // statistics-recorder.
+      if (ref == last_created)
+        continue;
+
+      // Create the histogram from contents in persistent memory and add it
+      // to the set of known ones.
+      HistogramBase* histogram = GetPersistentHistogram(g_allocator, ref);
+      if (histogram)
+        StatisticsRecorder::RegisterOrDeleteDuplicate(histogram);
     }
   }
 }