about summary refs log tree commit diff
path: root/absl/container/internal/hashtablez_sampler.cc
blob: d03dd82e7fd57fc2cfe90b1cec3e3d1dc8a9b925 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
// 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 {
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
}  // namespace absl