From 3476c1d60ec29c5497123194acd7a9310b1023d2 Mon Sep 17 00:00:00 2001 From: David Teller Date: Mon, 28 Jan 2019 23:41:20 +0100 Subject: Reduce number of allocations in AutoStopwatch 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. --- js/src/vm/Stopwatch.cpp | 24 +++++++++++++++++------- 1 file changed, 17 insertions(+), 7 deletions(-) (limited to 'js/src/vm') diff --git a/js/src/vm/Stopwatch.cpp b/js/src/vm/Stopwatch.cpp index 7a6acb970..49b70c478 100644 --- a/js/src/vm/Stopwatch.cpp +++ b/js/src/vm/Stopwatch.cpp @@ -136,6 +136,9 @@ PerformanceMonitoring::start() bool PerformanceMonitoring::commit() { + // Maximum initialization size, in elements for the vector of groups. + static const size_t MAX_GROUPS_INIT_CAPACITY = 1024; + #if !defined(MOZ_HAVE_RDTSC) // The AutoStopwatch is only executed if `MOZ_HAVE_RDTSC`. return false; @@ -152,12 +155,19 @@ PerformanceMonitoring::commit() return true; } - PerformanceGroupVector recentGroups; - recentGroups_.swap(recentGroups); + // The move operation is generally constant time, unless `recentGroups_.length()` is very small, in which case it's + // fast anyway because it's small. + PerformanceGroupVector recentGroups(Move(recentGroups_)); + recentGroups_ = PerformanceGroupVector(); // Reconstruct after `Move`. bool success = true; - if (stopwatchCommitCallback) - success = stopwatchCommitCallback(iteration_, recentGroups, stopwatchCommitClosure); + if (stopwatchCommitCallback) { + success = stopwatchCommitCallback(iteration_, recentGroups, stopwatchCommitClosure); + } + + // Heuristic use: we expect to have roughly the same number of groups as in the previous iteration. + const size_t capacity = std::min(recentGroups.capacity(), MAX_GROUPS_INIT_CAPACITY); + success = recentGroups_.reserve(capacity) && success; // Reset immediately, to make sure that we're not hit by the end // of a nested event loop (which would cause `commit` to be called @@ -227,7 +237,7 @@ AutoStopwatch::AutoStopwatch(JSContext* cx MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IM MOZ_GUARD_OBJECT_NOTIFIER_INIT; JSCompartment* compartment = cx_->compartment(); - if (compartment->scheduledForDestruction) + if (MOZ_UNLIKELY(compartment->scheduledForDestruction)) return; JSRuntime* runtime = cx_->runtime(); @@ -266,11 +276,11 @@ AutoStopwatch::~AutoStopwatch() } JSCompartment* compartment = cx_->compartment(); - if (compartment->scheduledForDestruction) + if (MOZ_UNLIKELY(compartment->scheduledForDestruction)) return; JSRuntime* runtime = cx_->runtime(); - if (iteration_ != runtime->performanceMonitoring.iteration()) { + if (MOZ_UNLIKELY(iteration_ != runtime->performanceMonitoring.iteration())) { // We have entered a nested event loop at some point. // Any information we may have is obsolete. return; -- cgit v1.2.3