summaryrefslogtreecommitdiff
path: root/absl/container/internal/hashtablez_sampler.h
blob: 8b81653a57f6b160f0fe872a98e6fa053dcb0b4e (plain)
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
// 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.
//
// This is a low level library to sample hashtables and collect runtime
// statistics about them.
//
// `HashtablezSampler` controls the lifecycle of `HashtablezInfo` objects which
// store information about a single sample.
//
// `Record*` methods store information into samples.
// `Sample()` and `Unsample()` make use of a single global sampler with
// properties controlled by the flags hashtablez_enabled,
// hashtablez_sample_rate, and hashtablez_max_samples.

#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
#define ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_

#include <atomic>
#include <functional>
#include <memory>
#include <vector>

#include "absl/base/internal/per_thread_tls.h"
#include "absl/base/optimization.h"
#include "absl/synchronization/mutex.h"
#include "absl/utility/utility.h"

namespace absl {
namespace container_internal {

// Stores information about a sampled hashtable.  All mutations to this *must*
// be made through `Record*` functions below.  All reads from this *must* only
// occur in the callback to `HashtablezSampler::Iterate`.
struct HashtablezInfo {
  // Constructs the object but does not fill in any fields.
  HashtablezInfo();
  ~HashtablezInfo();
  HashtablezInfo(const HashtablezInfo&) = delete;
  HashtablezInfo& operator=(const HashtablezInfo&) = delete;

  // Puts the object into a clean state, fills in the logically `const` members,
  // blocking for any readers that are currently sampling the object.
  void PrepareForSampling() EXCLUSIVE_LOCKS_REQUIRED(init_mu);

  // These fields are mutated by the various Record* APIs and need to be
  // thread-safe.
  std::atomic<size_t> capacity;
  std::atomic<size_t> size;
  std::atomic<size_t> num_erases;
  std::atomic<size_t> max_probe_length;
  std::atomic<size_t> total_probe_length;
  std::atomic<size_t> hashes_bitwise_or;
  std::atomic<size_t> hashes_bitwise_and;

  // `HashtablezSampler` maintains intrusive linked lists for all samples.  See
  // comments on `HashtablezSampler::all_` for details on these.  `init_mu`
  // guards the ability to restore the sample to a pristine state.  This
  // prevents races with sampling and resurrecting an object.
  absl::Mutex init_mu;
  HashtablezInfo* next;
  HashtablezInfo* dead GUARDED_BY(init_mu);

  // All of the fields below are set by `PrepareForSampling`, they must not be
  // mutated in `Record*` functions.  They are logically `const` in that sense.
  // These are guarded by init_mu, but that is not externalized to clients, who
  // can only read them during `HashtablezSampler::Iterate` which will hold the
  // lock.
  static constexpr int kMaxStackDepth = 64;
  absl::Time create_time;
  int32_t depth;
  void* stack[kMaxStackDepth];
};

inline void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
                                     size_t capacity) {
  info->size.store(size, std::memory_order_relaxed);
  info->capacity.store(capacity, std::memory_order_relaxed);
}

void RecordInsertSlow(HashtablezInfo* info, size_t hash,
                      size_t distance_from_desired);

inline void RecordEraseSlow(HashtablezInfo* info) {
  info->size.fetch_sub(1, std::memory_order_relaxed);
  info->num_erases.fetch_add(1, std::memory_order_relaxed);
}

HashtablezInfo* SampleSlow(int64_t* next_sample);
void UnsampleSlow(HashtablezInfo* info);

class HashtablezInfoHandle {
 public:
  explicit HashtablezInfoHandle() : info_(nullptr) {}
  explicit HashtablezInfoHandle(HashtablezInfo* info) : info_(info) {}
  ~HashtablezInfoHandle() {
    if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
    UnsampleSlow(info_);
  }

  HashtablezInfoHandle(const HashtablezInfoHandle&) = delete;
  HashtablezInfoHandle& operator=(const HashtablezInfoHandle&) = delete;

  HashtablezInfoHandle(HashtablezInfoHandle&& o) noexcept
      : info_(absl::exchange(o.info_, nullptr)) {}
  HashtablezInfoHandle& operator=(HashtablezInfoHandle&& o) noexcept {
    if (ABSL_PREDICT_FALSE(info_ != nullptr)) {
      UnsampleSlow(info_);
    }
    info_ = absl::exchange(o.info_, nullptr);
    return *this;
  }

  inline void RecordStorageChanged(size_t size, size_t capacity) {
    if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
    RecordStorageChangedSlow(info_, size, capacity);
  }

  inline void RecordInsert(size_t hash, size_t distance_from_desired) {
    if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
    RecordInsertSlow(info_, hash, distance_from_desired);
  }

  inline void RecordErase() {
    if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
    RecordEraseSlow(info_);
  }

  friend inline void swap(HashtablezInfoHandle& lhs,
                          HashtablezInfoHandle& rhs) {
    std::swap(lhs.info_, rhs.info_);
  }

 private:
  friend class HashtablezInfoHandlePeer;
  HashtablezInfo* info_;
};

#if ABSL_PER_THREAD_TLS == 1
extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
#endif  // ABSL_PER_THREAD_TLS

// Returns an RAII sampling handle that manages registration and unregistation
// with the global sampler.
inline HashtablezInfoHandle Sample() {
#if ABSL_PER_THREAD_TLS == 0
  static auto* mu = new absl::Mutex;
  static int64_t global_next_sample = 0;
  absl::MutexLock l(mu);
#endif  // !ABSL_HAVE_THREAD_LOCAL

  if (ABSL_PREDICT_TRUE(--global_next_sample > 0)) {
    return HashtablezInfoHandle(nullptr);
  }
  return HashtablezInfoHandle(SampleSlow(&global_next_sample));
}

// Holds samples and their associated stack traces with a soft limit of
// `SetHashtablezMaxSamples()`.
//
// Thread safe.
class HashtablezSampler {
 public:
  // Returns a global Sampler.
  static HashtablezSampler& Global();

  HashtablezSampler();
  ~HashtablezSampler();

  // Registers for sampling.  Returns an opaque registration info.
  HashtablezInfo* Register();

  // Unregisters the sample.
  void Unregister(HashtablezInfo* sample);

  // The dispose callback will be called on all samples the moment they are
  // being unregistered. Only affects samples that are unregistered after the
  // callback has been set.
  // Returns the previous callback.
  using DisposeCallback = void (*)(const HashtablezInfo&);
  DisposeCallback SetDisposeCallback(DisposeCallback f);

  // Iterates over all the registered `StackInfo`s.  Returning the number of
  // samples that have been dropped.
  int64_t Iterate(const std::function<void(const HashtablezInfo& stack)>& f);

 private:
  void PushNew(HashtablezInfo* sample);
  void PushDead(HashtablezInfo* sample);
  HashtablezInfo* PopDead();

  std::atomic<size_t> dropped_samples_;
  std::atomic<size_t> size_estimate_;

  // Intrusive lock free linked lists for tracking samples.
  //
  // `all_` records all samples (they are never removed from this list) and is
  // terminated with a `nullptr`.
  //
  // `graveyard_.dead` is a circular linked list.  When it is empty,
  // `graveyard_.dead == &graveyard`.  The list is circular so that
  // every item on it (even the last) has a non-null dead pointer.  This allows
  // `Iterate` to determine if a given sample is live or dead using only
  // information on the sample itself.
  //
  // For example, nodes [A, B, C, D, E] with [A, C, E] alive and [B, D] dead
  // looks like this (G is the Graveyard):
  //
  //           +---+    +---+    +---+    +---+    +---+
  //    all -->| A |--->| B |--->| C |--->| D |--->| E |
  //           |   |    |   |    |   |    |   |    |   |
  //   +---+   |   | +->|   |-+  |   | +->|   |-+  |   |
  //   | G |   +---+ |  +---+ |  +---+ |  +---+ |  +---+
  //   |   |         |        |        |        |
  //   |   | --------+        +--------+        |
  //   +---+                                    |
  //     ^                                      |
  //     +--------------------------------------+
  //
  std::atomic<HashtablezInfo*> all_;
  HashtablezInfo graveyard_;

  std::atomic<DisposeCallback> dispose_;
};

// Enables or disables sampling for Swiss tables.
void SetHashtablezEnabled(bool enabled);

// Sets the rate at which Swiss tables will be sampled.
void SetHashtablezSampleParameter(int32_t rate);

// Sets a soft max for the number of samples that will be kept.
void SetHashtablezMaxSamples(int32_t max);

// Configuration override.
// This allows process-wide sampling without depending on order of
// initialization of static storage duration objects.
// The definition of this constant is weak, which allows us to inject a
// different value for it at link time.
extern "C" const bool kAbslContainerInternalSampleEverything;

}  // namespace container_internal
}  // namespace absl

#endif  // ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_