When converting between units of time or data types of different precision,

we have to be careful to consistently round in the same direction.

Timeout checks usually check if Now() is less or equal to a deadline in order
to determine if a timeout has occurred. This correctly handles the case where
actual sleep times are equal or longer than requested sleep times.

But if we round down when setting the sleep delay, this can result in
unnecessary and expensive looping. Make sure, we always round up when converting
to a format with less precision.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/196053

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@27146 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 14 insertions, 2 deletions

diff --git a/webkit/glue/webkitclient_impl.cc b/webkit/glue/webkitclient_impl.cc
index 4315db7..79ddffd6 100644
--- a/webkit/glue/webkitclient_impl.cc
+++ b/webkit/glue/webkitclient_impl.cc
@@ -2,6 +2,7 @@
 // source code is governed by a BSD-style license that can be found in the
 // LICENSE file.
 
+#include <math.h>
 #include "config.h"
 
 #include "FrameView.h"
@@ -265,12 +266,23 @@ void WebKitClientImpl::setSharedTimerFiredFunction(void (*func)()) {
 }
 
 void WebKitClientImpl::setSharedTimerFireTime(double fire_time) {
-  int interval = static_cast<int>((fire_time - currentTime()) * 1000);
+  // By converting between double and int64 representation, we run the risk
+  // of losing precision due to rounding errors. Performing computations in
+  // microseconds reduces this risk somewhat. But there still is the potential
+  // of us computing a fire time for the timer that is shorter than what we
+  // need.
+  // As the event loop will check event deadlines prior to actually firing
+  // them, there is a risk of needlessly rescheduling events and of
+  // needlessly looping if sleep times are too short even by small amounts.
+  // This results in measurable performance degradation unless we use ceil() to
+  // always round up the sleep times.
+  int64 interval = static_cast<int64>(
+      ceil((fire_time - currentTime()) * base::Time::kMicrosecondsPerSecond));
   if (interval < 0)
     interval = 0;
 
   shared_timer_.Stop();
-  shared_timer_.Start(base::TimeDelta::FromMilliseconds(interval), this,
+  shared_timer_.Start(base::TimeDelta::FromMicroseconds(interval), this,
                       &WebKitClientImpl::DoTimeout);
 }