// Copyright 2018 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
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "absl/container/internal/hashtablez_sampler.h"
#include <atomic>
#include <cassert>
#include <cmath>
#include <functional>
#include <limits>
#include "absl/base/attributes.h"
#include "absl/base/internal/exponential_biased.h"
#include "absl/container/internal/have_sse.h"
#include "absl/debugging/stacktrace.h"
#include "absl/memory/memory.h"
#include "absl/synchronization/mutex.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace container_internal {
constexpr int HashtablezInfo::kMaxStackDepth;
namespace {
ABSL_CONST_INIT std::atomic<bool> g_hashtablez_enabled{
false
};
ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_sample_parameter{1 << 10};
ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_max_samples{1 << 20};
#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
ABSL_PER_THREAD_TLS_KEYWORD absl::base_internal::ExponentialBiased
g_exponential_biased_generator;
#endif
} // namespace
#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample = 0;
#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
HashtablezSampler& HashtablezSampler::Global() {
static auto* sampler = new HashtablezSampler();
return *sampler;
}
HashtablezSampler::DisposeCallback HashtablezSampler::SetDisposeCallback(
DisposeCallback f) {
return dispose_.exchange(f, std::memory_order_relaxed);
}
HashtablezInfo::HashtablezInfo() { PrepareForSampling(); }
HashtablezInfo::~HashtablezInfo() = default;
void HashtablezInfo::PrepareForSampling() {
capacity.store(0, std::memory_order_relaxed);
size.store(0, std::memory_order_relaxed);
num_erases.store(0, std::memory_order_relaxed);
max_probe_length.store(0, std::memory_order_relaxed);
total_probe_length.store(0, std::memory_order_relaxed);
hashes_bitwise_or.store(0, std::memory_order_relaxed);
hashes_bitwise_and.store(~size_t{}, std::memory_order_relaxed);
create_time = absl::Now();
// The inliner makes hardcoded skip_count difficult (especially when combined
// with LTO). We use the ability to exclude stacks by regex when encoding
// instead.
depth = absl::GetStackTrace(stack, HashtablezInfo::kMaxStackDepth,
/* skip_count= */ 0);
dead = nullptr;
}
HashtablezSampler::HashtablezSampler()
: dropped_samples_(0), size_estimate_(0), all_(nullptr), dispose_(nullptr) {
absl::MutexLock l(&graveyard_.init_mu);
graveyard_.dead = &graveyard_;
}
HashtablezSampler::~HashtablezSampler() {
HashtablezInfo* s = all_.load(std::memory_order_acquire);
while (s != nullptr) {
HashtablezInfo* next = s->next;
delete s;
s = next;
}
}
void HashtablezSampler::PushNew(HashtablezInfo* sample) {
sample->next = all_.load(std::memory_order_relaxed);
while (!all_.compare_exchange_weak(sample->next, sample,
std::memory_order_release,
std::memory_order_relaxed)) {
}
}
void HashtablezSampler::PushDead(HashtablezInfo* sample) {
if (auto* dispose = dispose_.load(std::memory_order_relaxed)) {
dispose(*sample);
}
absl::MutexLock graveyard_lock(&graveyard_.init_mu);
absl::MutexLock sample_lock(&sample->init_mu);
sample->dead = graveyard_.dead;
graveyard_.dead = sample;
}
HashtablezInfo* HashtablezSampler::PopDead() {
absl::MutexLock graveyard_lock(&graveyard_.init_mu);
// The list is circular, so eventually it collapses down to
// graveyard_.dead == &graveyard_
// when it is empty.
HashtablezInfo* sample = graveyard_.dead;
if (sample == &graveyard_) return nullptr;
absl::MutexLock sample_lock(&sample->init_mu);
graveyard_.dead = sample->dead;
sample->PrepareForSampling();
return sample;
}
HashtablezInfo* HashtablezSampler::Register() {
int64_t size = size_estimate_.fetch_add(1, std::memory_order_relaxed);
if (size > g_hashtablez_max_samples.load(std::memory_order_relaxed)) {
size_estimate_.fetch_sub(1, std::memory_order_relaxed);
dropped_samples_.fetch_add(1, std::memory_order_relaxed);
return nullptr;
}
HashtablezInfo* sample = PopDead();
if (sample == nullptr) {
// Resurrection failed. Hire a new warlock.
sample = new HashtablezInfo();
PushNew(sample);
}
return sample;
}
void HashtablezSampler::Unregister(HashtablezInfo* sample) {
PushDead(sample);
size_estimate_.fetch_sub(1, std::memory_order_relaxed);
}
int64_t HashtablezSampler::Iterate(
const std::function<void(const HashtablezInfo& stack)>& f) {
HashtablezInfo* s = all_.load(std::memory_order_acquire);
while (s != nullptr) {
absl::MutexLock l(&s->init_mu);
if (s->dead == nullptr) {
f(*s);
}
s = s->next;
}
return dropped_samples_.load(std::memory_order_relaxed);
}
static bool ShouldForceSampling() {
enum ForceState {
kDontForce,
kForce,
kUninitialized
};
ABSL_CONST_INIT static std::atomic<ForceState> global_state{
kUninitialized};
ForceState state = global_state.load(std::memory_order_relaxed);
if (ABSL_PREDICT_TRUE(state == kDontForce)) return false;
if (state == kUninitialized) {
state = AbslContainerInternalSampleEverything() ? kForce : kDontForce;
global_state.store(state, std::memory_order_relaxed);
}
return state == kForce;
}
HashtablezInfo* SampleSlow(int64_t* next_sample) {
if (ABSL_PREDICT_FALSE(ShouldForceSampling())) {
*next_sample = 1;
return HashtablezSampler::Global().Register();
}
#if !defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
*next_sample = std::numeric_limits<int64_t>::max();
return nullptr;
#else
bool first = *next_sample < 0;
*next_sample = g_exponential_biased_generator.GetStride(
g_hashtablez_sample_parameter.load(std::memory_order_relaxed));
// Small values of interval are equivalent to just sampling next time.
ABSL_ASSERT(*next_sample >= 1);
// g_hashtablez_enabled can be dynamically flipped, we need to set a threshold
// low enough that we will start sampling in a reasonable time, so we just use
// the default sampling rate.
if (!g_hashtablez_enabled.load(std::memory_order_relaxed)) return nullptr;
// We will only be negative on our first count, so we should just retry in
// that case.
if (first) {
if (ABSL_PREDICT_TRUE(--*next_sample > 0)) return nullptr;
return SampleSlow(next_sample);
}
return HashtablezSampler::Global().Register();
#endif
}
void UnsampleSlow(HashtablezInfo* info) {
HashtablezSampler::Global().Unregister(info);
}
void RecordInsertSlow(HashtablezInfo* info, size_t hash,
size_t distance_from_desired) {
// SwissTables probe in groups of 16, so scale this to count items probes and
// not offset from desired.
size_t probe_length = distance_from_desired;
#if ABSL_INTERNAL_RAW_HASH_SET_HAVE_SSE2
probe_length /= 16;
#else
probe_length /= 8;
#endif
info->hashes_bitwise_and.fetch_and(hash, std::memory_order_relaxed);
info->hashes_bitwise_or.fetch_or(hash, std::memory_order_relaxed);
info->max_probe_length.store(
std::max(info->max_probe_length.load(std::memory_order_relaxed),
probe_length),
std::memory_order_relaxed);
info->total_probe_length.fetch_add(probe_length, std::memory_order_relaxed);
info->size.fetch_add(1, std::memory_order_relaxed);
}
void SetHashtablezEnabled(bool enabled) {
g_hashtablez_enabled.store(enabled, std::memory_order_release);
}
void SetHashtablezSampleParameter(int32_t rate) {
if (rate > 0) {
g_hashtablez_sample_parameter.store(rate, std::memory_order_release);
} else {
ABSL_RAW_LOG(ERROR, "Invalid hashtablez sample rate: %lld",
static_cast<long long>(rate)); // NOLINT(runtime/int)
}
}
void SetHashtablezMaxSamples(int32_t max) {
if (max > 0) {
g_hashtablez_max_samples.store(max, std::memory_order_release);
} else {
ABSL_RAW_LOG(ERROR, "Invalid hashtablez max samples: %lld",
static_cast<long long>(max)); // NOLINT(runtime/int)
}
}
} // namespace container_internal
ABSL_NAMESPACE_END
} // namespace absl
