[net] Measure DNS.CacheEvicted and DNS.CacheExpired.

net/base/host_cache.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 "net/base/host_cache.h"
 
 #include "base/logging.h"
+#include "base/metrics/histogram.h"
 #include "net/base/net_errors.h"
 
 namespace net {
 
 //-----------------------------------------------------------------------------
 
 HostCache::Entry::Entry(int error, const AddressList& addrlist,
                         base::TimeDelta ttl)
     : error(error),
       addrlist(addrlist),
@@ skipping 66 matching lines @@
   // Increase HostCache size for the duration of the async DNS field trial.
   // http://crbug.com/143454
   // TODO(szym): Determine the best size. http://crbug.com/114277
   static const size_t kMaxHostCacheEntries = 1000;
 #else
   static const size_t kMaxHostCacheEntries = 100;
 #endif
   return make_scoped_ptr(new HostCache(kMaxHostCacheEntries));
 }
 
+void HostCache::EvictionHandler::handle(
+    const Key& key,
+    const Entry& entry,
+    const base::TimeTicks& expiration,
+    const base::TimeTicks& now,
+    bool onGet) const {
+  if (onGet) {
+    DCHECK(now >= expiration);
+    UMA_HISTOGRAM_CUSTOM_TIMES("DNS.CacheExpiredOnGet", now - expiration,
+        base::TimeDelta::FromSeconds(1), base::TimeDelta::FromDays(1), 100);

[jar (doing other things), 2012/11/05 17:44:56]

nit: early returns tend to make this sort of code easier to read.  Otherwise
readers have to scan forward to check on the logic in the else clauses, and the
remainder of the function.

+  } else if (expiration > now) {
+    UMA_HISTOGRAM_CUSTOM_TIMES("DNS.CacheEvicted", expiration - now,
+        base::TimeDelta::FromSeconds(1), base::TimeDelta::FromDays(1), 100);
+  } else {
+    UMA_HISTOGRAM_CUSTOM_TIMES("DNS.CacheExpired", now - expiration,
+        base::TimeDelta::FromSeconds(1), base::TimeDelta::FromDays(1), 100);
+  }
+}
+
 }  // namespace net