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// Copyright 2017 The Abseil Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include <cstdint>
#include <mutex> // NOLINT(build/c++11)
#include <vector>
#include "absl/base/config.h"
#include "absl/base/internal/cycleclock.h"
#include "absl/base/internal/spinlock.h"
#include "absl/synchronization/blocking_counter.h"
#include "absl/synchronization/internal/thread_pool.h"
#include "absl/synchronization/mutex.h"
#include "benchmark/benchmark.h"
namespace {
void BM_Mutex(benchmark::State& state) {
static absl::Mutex* mu = new absl::Mutex;
for (auto _ : state) {
absl::MutexLock lock(mu);
static void DelayNs(int64_t ns, int* data) {
int64_t end = absl::base_internal::CycleClock::Now() +
ns * absl::base_internal::CycleClock::Frequency() / 1e9;
while (absl::base_internal::CycleClock::Now() < end) {
template <typename MutexType>
class RaiiLocker {
explicit RaiiLocker(MutexType* mu) : mu_(mu) { mu_->Lock(); }
~RaiiLocker() { mu_->Unlock(); }
MutexType* mu_;
template <>
class RaiiLocker<std::mutex> {
explicit RaiiLocker(std::mutex* mu) : mu_(mu) { mu_->lock(); }
~RaiiLocker() { mu_->unlock(); }
std::mutex* mu_;
// RAII object to change the Mutex priority of the running thread.
class ScopedThreadMutexPriority {
explicit ScopedThreadMutexPriority(int priority) {
absl::base_internal::ThreadIdentity* identity =
identity->per_thread_synch.priority = priority;
// Bump next_priority_read_cycles to the infinite future so that the
// implementation doesn't re-read the thread's actual scheduler priority
// and replace our temporary scoped priority.
identity->per_thread_synch.next_priority_read_cycles =
~ScopedThreadMutexPriority() {
// Reset the "next priority read time" back to the infinite past so that
// the next time the Mutex implementation wants to know this thread's
// priority, it re-reads it from the OS instead of using our overridden
// priority.
->per_thread_synch.next_priority_read_cycles =
void BM_MutexEnqueue(benchmark::State& state) {
// In the "multiple priorities" variant of the benchmark, one of the
// threads runs with Mutex priority 0 while the rest run at elevated priority.
// This benchmarks the performance impact of the presence of a low priority
// waiter when a higher priority waiter adds itself of the queue
// (b/175224064).
// NOTE: The actual scheduler priority is not modified in this benchmark:
// all of the threads get CPU slices with the same priority. Only the
// Mutex queueing behavior is modified.
const bool multiple_priorities = state.range(0);
ScopedThreadMutexPriority priority_setter(
(multiple_priorities && state.thread_index() != 0) ? 1 : 0);
struct Shared {
absl::Mutex mu;
std::atomic<int> looping_threads{0};
std::atomic<int> blocked_threads{0};
std::atomic<bool> thread_has_mutex{false};
static Shared* shared = new Shared;
// Set up 'blocked_threads' to count how many threads are currently blocked
// in Abseil synchronization code.
// NOTE: Blocking done within the Google Benchmark library itself (e.g.
// the barrier which synchronizes threads entering and exiting the benchmark
// loop) does _not_ get registered in this counter. This is because Google
// Benchmark uses its own synchronization primitives based on std::mutex, not
// Abseil synchronization primitives. If at some point the benchmark library
// merges into Abseil, this code may break.
// The benchmark framework may run several iterations in the same process,
// reusing the same static-initialized 'shared' object. Given the semantics
// of the members, here, we expect everything to be reset to zero by the
// end of any iteration. Assert that's the case, just to be sure.
shared->looping_threads.load(std::memory_order_relaxed) == 0 &&
shared->blocked_threads.load(std::memory_order_relaxed) == 0 &&
"Shared state isn't zeroed at start of benchmark iteration");
static constexpr int kBatchSize = 1000;
while (state.KeepRunningBatch(kBatchSize)) {
for (int i = 0; i < kBatchSize; i++) {
absl::MutexLock l(&shared->mu);
shared->, std::memory_order_relaxed);
// Spin until all other threads are either out of the benchmark loop
// or blocked on the mutex. This ensures that the mutex queue is kept
// at its maximal length to benchmark the performance of queueing on
// a highly contended mutex.
while (shared->looping_threads.load(std::memory_order_relaxed) -
shared->blocked_threads.load(std::memory_order_relaxed) !=
1) {
// Spin until some other thread has acquired the mutex before we block
// again. This ensures that we always go through the slow (queueing)
// acquisition path rather than reacquiring the mutex we just released.
while (!shared->thread_has_mutex.load(std::memory_order_relaxed) &&
shared->looping_threads.load(std::memory_order_relaxed) > 1) {
// The benchmark framework uses a barrier to ensure that all of the threads
// complete their benchmark loop together before any of the threads exit
// the loop. So, we need to remove ourselves from the "looping threads"
// counter here before potentially blocking on that barrier. Otherwise,
// another thread spinning above might wait forever for this thread to
// block on the mutex while we in fact are waiting to exit.
template <typename MutexType>
void BM_Contended(benchmark::State& state) {
int priority = state.thread_index() % state.range(1);
ScopedThreadMutexPriority priority_setter(priority);
struct Shared {
MutexType mu;
int data = 0;
static auto* shared = new Shared;
int local = 0;
for (auto _ : state) {
// Here we model both local work outside of the critical section as well as
// some work inside of the critical section. The idea is to capture some
// more or less realisitic contention levels.
// If contention is too low, the benchmark won't measure anything useful.
// If contention is unrealistically high, the benchmark will favor
// bad mutex implementations that block and otherwise distract threads
// from the mutex and shared state for as much as possible.
// To achieve this amount of local work is multiplied by number of threads
// to keep ratio between local work and critical section approximately
// equal regardless of number of threads.
DelayNs(100 * state.threads(), &local);
RaiiLocker<MutexType> locker(&shared->mu);
DelayNs(state.range(0), &shared->data);
void SetupBenchmarkArgs(benchmark::internal::Benchmark* bm,
bool do_test_priorities) {
const int max_num_priorities = do_test_priorities ? 2 : 1;
// ThreadPerCpu poorly handles non-power-of-two CPU counts.
->ArgNames({"cs_ns", "num_prios"});
// Some empirically chosen amounts of work in critical section.
// 1 is low contention, 2000 is high contention and few values in between.
for (int critical_section_ns : {1, 20, 50, 200, 2000}) {
for (int num_priorities = 1; num_priorities <= max_num_priorities;
num_priorities++) {
bm->ArgPair(critical_section_ns, num_priorities);
BENCHMARK_TEMPLATE(BM_Contended, absl::Mutex)
->Apply([](benchmark::internal::Benchmark* bm) {
SetupBenchmarkArgs(bm, /*do_test_priorities=*/true);
BENCHMARK_TEMPLATE(BM_Contended, absl::base_internal::SpinLock)
->Apply([](benchmark::internal::Benchmark* bm) {
SetupBenchmarkArgs(bm, /*do_test_priorities=*/false);
BENCHMARK_TEMPLATE(BM_Contended, std::mutex)
->Apply([](benchmark::internal::Benchmark* bm) {
SetupBenchmarkArgs(bm, /*do_test_priorities=*/false);
// Measure the overhead of conditions on mutex release (when they must be
// evaluated). Mutex has (some) support for equivalence classes allowing
// Conditions with the same function/argument to potentially not be multiply
// evaluated.
// num_classes==0 is used for the special case of every waiter being distinct.
void BM_ConditionWaiters(benchmark::State& state) {
int num_classes = state.range(0);
int num_waiters = state.range(1);
struct Helper {
static void Waiter(absl::BlockingCounter* init, absl::Mutex* m, int* p) {
static_cast<bool (*)(int*)>([](int* v) { return *v == 0; }), p));
if (num_classes == 0) {
// No equivalence classes.
num_classes = num_waiters;
absl::BlockingCounter init(num_waiters);
absl::Mutex mu;
std::vector<int> equivalence_classes(num_classes, 1);
// Must be declared last to be destroyed first.
absl::synchronization_internal::ThreadPool pool(num_waiters);
for (int i = 0; i < num_waiters; i++) {
// Mutex considers Conditions with the same function and argument
// to be equivalent.
pool.Schedule([&, i] {
Helper::Waiter(&init, &mu, &equivalence_classes[i % num_classes]);
for (auto _ : state) {
mu.Unlock(); // Each unlock requires Condition evaluation for our waiters.
for (int i = 0; i < num_classes; i++) {
equivalence_classes[i] = 0;
// Some configurations have higher thread limits than others.
#if defined(__linux__) && !defined(ABSL_HAVE_THREAD_SANITIZER)
constexpr int kMaxConditionWaiters = 8192;
constexpr int kMaxConditionWaiters = 1024;
BENCHMARK(BM_ConditionWaiters)->RangePair(0, 2, 1, kMaxConditionWaiters);
} // namespace