Use Histogram algorithm to calculate DNS timeout. Honor DnsConfig timeout if it exceeds 5s.

net/dns/dns_session.cc

Index: net/dns/dns_session.cc
diff --git a/net/dns/dns_session.cc b/net/dns/dns_session.cc
index 67d5779e4d237355d4cc0d6e48d7628479b9bef2..01da93e2b1b47e4c876e8107b26906defa81b633 100644
--- a/net/dns/dns_session.cc
+++ b/net/dns/dns_session.cc
@@ -38,6 +38,8 @@ struct DnsSession::ServerStats {
   ServerStats(base::TimeDelta rtt_estimate_param, RttBuckets* buckets)
     : last_failure_count(0), rtt_estimate(rtt_estimate_param) {
     rtt_histogram.reset(new base::SampleVector(buckets));
+    // Seed histogram with 2 samples at |rtt_estimate| timeout.
+    rtt_histogram->Accumulate(rtt_estimate.InMilliseconds(), 2);
   }
 
   // Count of consecutive failures after last success.
@@ -89,8 +91,9 @@ DnsSession::DnsSession(const DnsConfig& config,
   UMA_HISTOGRAM_CUSTOM_COUNTS(
       "AsyncDNS.ServerCount", config_.nameservers.size(), 0, 10, 10);
   for (size_t i = 0; i < config_.nameservers.size(); ++i) {
-    server_stats_.push_back(new ServerStats(config_.timeout,
-                                            rtt_buckets_.Pointer()));
+    // Histogram-based method.

[szym, 2013/07/25 22:47:24]

This comment can be misleading. Suggest undoing this change.

[mef, 2013/07/26 11:27:07]

Done.

+    server_stats_.push_back(
+        new ServerStats(config_.timeout, rtt_buckets_.Pointer()));
   }
 }
 
@@ -209,18 +212,11 @@ void DnsSession::RecordServerStats() {
 
 
 base::TimeDelta DnsSession::NextTimeout(unsigned server_index, int attempt) {
-  DCHECK_LT(server_index, server_stats_.size());
-
-  base::TimeDelta timeout = config_.timeout;
-  // If this server has not responded successfully, then don't wait too long.
-  if (server_stats_[server_index]->last_success.is_null())
-    return timeout;
-
-  // The timeout doubles every full round (each nameserver once).
-  unsigned num_backoffs = attempt / config_.nameservers.size();
-
-  return std::min(timeout * (1 << num_backoffs),
-                  base::TimeDelta::FromMilliseconds(kMaxTimeoutMs));
+  // Respect config timeout if it exceeds |kMaxTimeoutMs|.
+  if (config_.timeout.InMilliseconds() > kMaxTimeoutMs)

[szym, 2013/07/25 22:47:24]

should it be >=?

[mef, 2013/07/26 11:27:07]

Done.

+    return config_.timeout;
+  // Use Histogram to calculate next timeout.

[szym, 2013/07/25 22:47:24]

nit: superfluous comment

[mef, 2013/07/26 11:27:07]

Done.

+  return NextTimeoutFromHistogram(server_index, attempt);
 }
 
 // Allocate a socket, already connected to the server address.