Add UMA for disk latency for cache accesses.

net/disk_cache/simple/simple_synchronous_entry.cc

 // Copyright (c) 2013 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 "net/disk_cache/simple/simple_synchronous_entry.h"
 
 #include <algorithm>
 #include <cstring>
 #include <functional>
 #include <limits>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/files/file_util.h"
 #include "base/hash.h"
 #include "base/location.h"
+#include "base/metrics/histogram.h"
 #include "base/numerics/safe_conversions.h"
 #include "base/sha1.h"
 #include "base/strings/stringprintf.h"
+#include "base/time/time.h"
 #include "net/base/io_buffer.h"
 #include "net/base/net_errors.h"
 #include "net/disk_cache/simple/simple_backend_version.h"
 #include "net/disk_cache/simple/simple_histogram_macros.h"
 #include "net/disk_cache/simple/simple_util.h"
 #include "third_party/zlib/zlib.h"
 
 using base::File;
 using base::FilePath;
 using base::Time;
@@ skipping 179 matching lines @@
     : sparse_offset(sparse_offset_p),
       buf_len(buf_len_p) {}
 
 // static
 void SimpleSynchronousEntry::OpenEntry(
     net::CacheType cache_type,
     const FilePath& path,
     const uint64 entry_hash,
     bool had_index,
     SimpleEntryCreationResults *out_results) {
+  base::TimeTicks open_start_time(base::TimeTicks::Now());

[Alexei Svitkine (slow), 2015/05/14 14:53:05]

Nit: Use base/timer/elapsed_timer.h

[Randy Smith (Not in Mondays), 2015/05/14 15:25:25]

Done.

   SimpleSynchronousEntry* sync_entry =
       new SimpleSynchronousEntry(cache_type, path, "", entry_hash);
   out_results->result =
       sync_entry->InitializeForOpen(had_index,
                                     &out_results->entry_stat,
                                     &out_results->stream_0_data,
                                     &out_results->stream_0_crc32);
   if (out_results->result != net::OK) {
     sync_entry->Doom();
     delete sync_entry;
     out_results->sync_entry = NULL;
     out_results->stream_0_data = NULL;
     return;
   }
+  UMA_HISTOGRAM_TIMES("SimpleCache.DiskOpenLatency",
+                      base::TimeTicks::Now() - open_start_time);
   out_results->sync_entry = sync_entry;
 }
 
 // static
 void SimpleSynchronousEntry::CreateEntry(
     net::CacheType cache_type,
     const FilePath& path,
     const std::string& key,
     const uint64 entry_hash,
     bool had_index,
@@ skipping 1184 matching lines @@
   range.offset = offset;
   range.length = len;
   range.data_crc32 = data_crc32;
   range.file_offset = data_file_offset;
   sparse_ranges_.insert(std::make_pair(offset, range));
 
   return true;
 }
 
 }  // namespace disk_cache