tvl-depot/absl/container/internal/hashtablez_sampler.cc
Abseil Team 0dffca4e36 Export of internal Abseil changes.
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5804cc13b413412988248835459b90cd15ec43d9 by Abseil Team <absl-team@google.com>:

Mark raw_hash_set::clear() with the ABSL_ATTRIBUTE_REINITIALIZES attribute.

This prevents false positives in the clang-tidy check bugprone-use-after-move;
it allows reset() to be called on a moved-from raw_hash_set without any
warnings, and the raw_hash_set will thereafter be regarded as initialized again.

PiperOrigin-RevId: 230717196

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ff5961a5600ae19b69a9cba6912126cdf2858f38 by CJ Johnson <johnsoncj@google.com>:

Swaps DisableIfIntegral<> for EnableIfInputIterator<> for Iterator member functions of InlinedVector

PiperOrigin-RevId: 230559521

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3f9754ccbeecbd40f235c6f2465279e045ff51d9 by Derek Mauro <dmauro@google.com>:

Import GitHub PR 254
https://github.com/abseil/abseil-cpp/pull/254

Fixes warnings from -Wclass-memaccess (base_internal::ThreadIdentity?
with no trivial copy-assignment).

PiperOrigin-RevId: 230536048

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8af03a654ce9a4a7f55384bc7eb1ed64878ac2ec by Chris Kennelly <ckennelly@google.com>:

absl: cap SpinLock backoff to 4ms

The current backoff logic has 3 problems:
1. It can produce too high values (up to 256ms), which can negatively
affect tail latency. The value was chosen long time ago and now it's
a good idea to reconsider it.
2. It does not have low bound, so on any iteration it can produce
a very small value that will lead to unnecessary cpu consumption.
3. It does not increase low bound with the number of iterations.
So if the SpinLock is actually somehow locked for a very prolonged time,
a waiter can still wake periodically.

Rework the logic to solve these problems.
Add lower bound of 128us, no code should rely on absence of episodic
delays in this range as they can occur everywhere.
Lower upper bound to 4ms. A thread sleeping for 4ms does not consume
significant cpu time (see below).
Grow lower bound with the number of iterations.

This is cpu consumption of a process doing usleep(x) in a loop
(sampled with ps):

    64us -> 4.0%
   128us -> 2.7%
   256us -> 3.5%
   512us -> 2.8%
  1024us -> 1.6%
  2048us -> 0.6%
  4096us -> 0.3%
  8192us -> 0.0%

Few millisecond sleeps do not consume significant time.

PiperOrigin-RevId: 230534015

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37ebba92289ca556cb2412cd8b3cb4c1ead3def7 by Samuel Benzaquen <sbenza@google.com>:

Add override and dispose hooks to the hashtable sampler.

PiperOrigin-RevId: 230353438

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89c8f90175233ce9964eb3412df04e8a3cff0c0f by Andy Getzendanner <durandal@google.com>:

Fix a comment typo.

PiperOrigin-RevId: 229986838
GitOrigin-RevId: 5804cc13b413412988248835459b90cd15ec43d9
Change-Id: Iedb5e2cc9c0b924635c1c87b537780ab6b5b899f
2019-01-24 11:10:30 -05:00

308 lines
10 KiB
C++

// 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
//
// http://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/container/internal/have_sse.h"
#include "absl/debugging/stacktrace.h"
#include "absl/memory/memory.h"
#include "absl/synchronization/mutex.h"
namespace absl {
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};
// Returns the next pseudo-random value.
// pRNG is: aX+b mod c with a = 0x5DEECE66D, b = 0xB, c = 1<<48
// This is the lrand64 generator.
uint64_t NextRandom(uint64_t rnd) {
const uint64_t prng_mult = uint64_t{0x5DEECE66D};
const uint64_t prng_add = 0xB;
const uint64_t prng_mod_power = 48;
const uint64_t prng_mod_mask = ~(~uint64_t{0} << prng_mod_power);
return (prng_mult * rnd + prng_add) & prng_mod_mask;
}
// Generates a geometric variable with the specified mean.
// This is done by generating a random number between 0 and 1 and applying
// the inverse cumulative distribution function for an exponential.
// Specifically: Let m be the inverse of the sample period, then
// the probability distribution function is m*exp(-mx) so the CDF is
// p = 1 - exp(-mx), so
// q = 1 - p = exp(-mx)
// log_e(q) = -mx
// -log_e(q)/m = x
// log_2(q) * (-log_e(2) * 1/m) = x
// In the code, q is actually in the range 1 to 2**26, hence the -26 below
//
int64_t GetGeometricVariable(int64_t mean) {
#if ABSL_HAVE_THREAD_LOCAL
thread_local
#else // ABSL_HAVE_THREAD_LOCAL
// SampleSlow and hence GetGeometricVariable is guarded by a single mutex when
// there are not thread locals. Thus, a single global rng is acceptable for
// that case.
static
#endif // ABSL_HAVE_THREAD_LOCAL
uint64_t rng = []() {
// We don't get well distributed numbers from this so we call
// NextRandom() a bunch to mush the bits around. We use a global_rand
// to handle the case where the same thread (by memory address) gets
// created and destroyed repeatedly.
ABSL_CONST_INIT static std::atomic<uint32_t> global_rand(0);
uint64_t r = reinterpret_cast<uint64_t>(&rng) +
global_rand.fetch_add(1, std::memory_order_relaxed);
for (int i = 0; i < 20; ++i) {
r = NextRandom(r);
}
return r;
}();
rng = NextRandom(rng);
// Take the top 26 bits as the random number
// (This plus the 1<<58 sampling bound give a max possible step of
// 5194297183973780480 bytes.)
const uint64_t prng_mod_power = 48; // Number of bits in prng
// The uint32_t cast is to prevent a (hard-to-reproduce) NAN
// under piii debug for some binaries.
double q = static_cast<uint32_t>(rng >> (prng_mod_power - 26)) + 1.0;
// Put the computed p-value through the CDF of a geometric.
double interval = (std::log2(q) - 26) * (-std::log(2.0) * mean);
// Very large values of interval overflow int64_t. If we happen to
// hit such improbable condition, we simply cheat and clamp interval
// to largest supported value.
if (interval > static_cast<double>(std::numeric_limits<int64_t>::max() / 2)) {
return std::numeric_limits<int64_t>::max() / 2;
}
// Small values of interval are equivalent to just sampling next time.
if (interval < 1) {
return 1;
}
return static_cast<int64_t>(interval);
}
} // namespace
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);
}
HashtablezInfo* SampleSlow(int64_t* next_sample) {
if (kAbslContainerInternalSampleEverything) {
*next_sample = 1;
return HashtablezSampler::Global().Register();
}
bool first = *next_sample < 0;
*next_sample = GetGeometricVariable(
g_hashtablez_sample_parameter.load(std::memory_order_relaxed));
// 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();
}
#if ABSL_PER_THREAD_TLS == 1
ABSL_PER_THREAD_TLS_KEYWORD int64_t next_sample = 0;
#endif // ABSL_PER_THREAD_TLS == 1
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 SWISSTABLE_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
} // namespace absl