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Reduce number of allocations in AutoStopwatch

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This patch fixes two related issues.
1. The AutoStopwatch uses a stack-allocated `mozilla::Vector` to
communicate with its callback during each compartment switch.
This vector was designed to allow its contents to be stack-allocated
but they turned out to be accidentally heap-allocated.

2. During each tick, the stopwatch fills a vector `recentGroups_`.
This vector always started with minimal capacity and had to grow
repeatedly as groups were added, causing repeated reallocations.
This patch preallocates `recentGroups_` to have the same capacity as the
previous tick. We expect that this should eventually reach a stable size
that closely matches the actual needs of the process.
This commit is contained in:
David Teller 2019-01-28 23:41:20 +01:00 • committed by Roy Tam
commit 059aeea416
4 changed files with 25 additions and 11 deletions
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8
toolkit/components/perfmonitoring/nsPerformanceStats.cpp
View file

@ -1310,8 +1310,12 @@ nsPerformanceStatsService::GetResources(uint64_t* userTime,
void
nsPerformanceStatsService::NotifyJankObservers(const mozilla::Vector<uint64_t>& aPreviousJankLevels) {
GroupVector alerts;
mPendingAlerts.swap(alerts);
// The move operation is generally constant time, unless `mPendingAlerts.length()` is very small, in which case it's
// fast anyway.
GroupVector alerts(Move(mPendingAlerts));
mPendingAlerts = GroupVector(); // Reconstruct after `Move`.
if (!mPendingAlertsCollector) {
// We are shutting down.
return;