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authorGravatar Abseil Team <absl-team@google.com>2019-08-08 10:56:58 -0700
committerGravatar CJ Johnson <johnsoncj@google.com>2019-08-08 14:19:45 -0400
commitaa844899c937bde5d2b24f276b59997e5b668bde (patch)
treecd18e64150abc74b85bbbf6abf990f66fa47cacd /absl/container/internal/hashtablez_sampler.cc
parentfcb104594b0bb4b8ac306cb2f55ecdad40974683 (diff)
Creation of LTS branch "lts_2019_08_08"20190808
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Diffstat (limited to 'absl/container/internal/hashtablez_sampler.cc')
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diff --git a/absl/container/internal/hashtablez_sampler.cc b/absl/container/internal/hashtablez_sampler.cc
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+// 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/container/internal/have_sse.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/memory/memory.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+inline namespace lts_2019_08_08 {
+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 = (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 global_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
+} // inline namespace lts_2019_08_08
+} // namespace absl