Switch the TCP reads on Windows to use non-blocking/non-async I/O.

chrome/renderer/page_load_histograms.cc

 // Copyright (c) 2012 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 "chrome/renderer/page_load_histograms.h"
 
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
@@ skipping 910 matching lines @@
                                 abandoned_page ? 1 : 0, 2);
     } else {
       DCHECK(scheme_type == URLPattern::SCHEME_HTTP);
       PLT_HISTOGRAM("PLT.StartToFinish.NoProxy.http",
                     start_to_finish_all_loads);
       UMA_HISTOGRAM_ENUMERATION("PLT.Abandoned.NoProxy.http",
                                 abandoned_page ? 1 : 0, 2);
     }
   }
 
+  // Histograms to determine if disabling overlapped TCP reads
+  // has an impact on PLT.
+  static const bool disable_overlapped_reads =
+      base::FieldTrialList::TrialExists("NBReadImpact");
+  if (disable_overlapped_reads) {
+    UMA_HISTOGRAM_ENUMERATION(
+        base::FieldTrial::MakeName("PLT.Abandoned", "NBReadImpact"),
+        abandoned_page ? 1 : 0, 2);
+    UMA_HISTOGRAM_ENUMERATION(
+        base::FieldTrial::MakeName("PLT.LoadType", "NBReadImpact"),
+        load_type, DocumentState::kLoadTypeMax);
+    switch (load_type) {
+      case DocumentState::NORMAL_LOAD:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_NormalLoad", "NBReadImpact"),
+            begin_to_finish_all_loads);
+        break;
+      case DocumentState::LINK_LOAD_NORMAL:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_LinkLoadNormal", "NBReadImpact"),
+            begin_to_finish_all_loads);
+        break;
+      case DocumentState::LINK_LOAD_RELOAD:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_LinkLoadReload", "NBReadImpact"),
+            begin_to_finish_all_loads);
+        break;
+      case DocumentState::LINK_LOAD_CACHE_STALE_OK:
+        PLT_HISTOGRAM(base::FieldTrial::MakeName(
+            "PLT.BeginToFinish_LinkLoadStaleOk", "NBReadImpact"),

[Ryan Sleevi, 2012/10/11 18:53:16]

Same comments elsewhere about over-abbreviation for "NBRead"

+            begin_to_finish_all_loads);
+        break;
+      default:
+        break;
+    }
+  }
+
   // Site isolation metrics.
   UMA_HISTOGRAM_COUNTS("SiteIsolation.PageLoadsWithCrossSiteFrameAccess",
                        cross_origin_access_count_);
   UMA_HISTOGRAM_COUNTS("SiteIsolation.PageLoadsWithSameSiteFrameAccess",
                        same_origin_access_count_);
 
   // Log the PLT to the info log.
   LogPageLoadTime(document_state, frame->dataSource());
 
   // Record prerendering histograms.
@@ skipping 51 matching lines @@
 
   DCHECK(document_state);
   DCHECK(ds);
   GURL url(ds->request().url());
   Time start = document_state->start_load_time();
   Time finish = document_state->finish_load_time();
   // TODO(mbelshe): should we log more stats?
   VLOG(1) << "PLT: " << (finish - start).InMilliseconds() << "ms "
           << url.spec();
 }