Measure DNS resolution waste from not always performing AF_UNSPEC lookups.

net/base/host_resolver_impl.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_resolver_impl.h"
 
 #if defined(OS_WIN)
 #include <Winsock2.h>
 #elif defined(OS_POSIX)
 #include <netdb.h>
@@ skipping 1804 matching lines @@
 
 bool HostResolverImpl::ServeFromCache(const Key& key,
                                       const RequestInfo& info,
                                       int* net_error,
                                       AddressList* addresses) {
   DCHECK(addresses);
   DCHECK(net_error);
   if (!info.allow_cached_response() || !cache_.get())
     return false;
 
-  const HostCache::Entry* cache_entry = cache_->Lookup(
-      key, base::TimeTicks::Now());
+  base::TimeTicks current_time = base::TimeTicks::Now();
+  const HostCache::Entry* cache_entry = cache_->Lookup(key, current_time);
+
+  bool found = cache_entry != NULL;

[szym, 2012/08/07 23:17:32]

I suggest adding braces around this whole block so that it's apparent that all
of this serves just one histogram, and is easier to remove in the future.

+  bool ipv4 = key.address_family == ADDRESS_FAMILY_IPV4;
+  // If we couldn't find the desired address family, check to see if the other
+  // family is in the cache, which indicates waste, and we should fix
+  // crbug.com/139811.
+  bool found_other_family = false;
+  if (!found && default_address_family_ == ADDRESS_FAMILY_UNSPECIFIED) {
+    Key other_family_key = key;
+    other_family_key.address_family = ipv4 ?
+        ADDRESS_FAMILY_UNSPECIFIED : ADDRESS_FAMILY_IPV4;
+    found_other_family =
+        cache_->Lookup(other_family_key, current_time) != NULL;
+  }
+  enum {  // Used in HISTOGRAM_ENUMERATION.
+    CACHE_IPV4_ONLY_FOUND,
+    CACHE_IPV4_ONLY_MISS,
+    CACHE_FOUND_IPV4,
+    CACHE_FOUND_UNSPEC,
+    CACHE_WASTE_IPV4,
+    CACHE_WASTE_UNSPEC,
+    CACHE_MISS_IPV4,
+    CACHE_MISS_UNSPEC,
+    CACHE_MAX,  // Bounding value.
+  } category = CACHE_MAX;
+  if (default_address_family_ != ADDRESS_FAMILY_UNSPECIFIED)
+    category = found ? CACHE_IPV4_ONLY_FOUND : CACHE_IPV4_ONLY_MISS;
+  else if (found)

[szym, 2012/08/07 23:17:32]

nit: Use braces whenever if has an else clause.

+    category = ipv4 ? CACHE_FOUND_IPV4 : CACHE_FOUND_UNSPEC;
+  else if (found_other_family)
+    category = ipv4 ? CACHE_WASTE_IPV4 : CACHE_WASTE_UNSPEC;
+  else
+    category = ipv4 ? CACHE_MISS_IPV4 : CACHE_MISS_UNSPEC;
+  UMA_HISTOGRAM_ENUMERATION("DNS.ResolveCacheCategory", category, CACHE_MAX);
+
   if (!cache_entry)
     return false;
 
   *net_error = cache_entry->error;
   if (*net_error == OK) {
     *addresses = cache_entry->addrlist;
     EnsurePortOnAddressList(info.port(), addresses);
   }
   return true;
 }
@@ skipping 187 matching lines @@
     if (self)
       TryServingAllJobsFromHosts();
   }
 }
 
 bool HostResolverImpl::HaveDnsConfig() const {
   return (dns_client_.get() != NULL) && (dns_client_->GetConfig() != NULL);
 }
 
 }  // namespace net