spinlock_wait.cc
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1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 
15 // The OS-specific header included below must provide two calls:
16 // AbslInternalSpinLockDelay() and AbslInternalSpinLockWake().
17 // See spinlock_wait.h for the specs.
18 
19 #include <atomic>
20 #include <cstdint>
21 
23 
24 #if defined(_WIN32)
25 #include "absl/base/internal/spinlock_win32.inc"
26 #elif defined(__linux__)
27 #include "absl/base/internal/spinlock_linux.inc"
28 #elif defined(__akaros__)
29 #include "absl/base/internal/spinlock_akaros.inc"
30 #else
31 #include "absl/base/internal/spinlock_posix.inc"
32 #endif
33 
34 namespace absl {
35 namespace base_internal {
36 
37 // See spinlock_wait.h for spec.
38 uint32_t SpinLockWait(std::atomic<uint32_t> *w, int n,
39  const SpinLockWaitTransition trans[],
40  base_internal::SchedulingMode scheduling_mode) {
41  int loop = 0;
42  for (;;) {
43  uint32_t v = w->load(std::memory_order_acquire);
44  int i;
45  for (i = 0; i != n && v != trans[i].from; i++) {
46  }
47  if (i == n) {
48  SpinLockDelay(w, v, ++loop, scheduling_mode); // no matching transition
49  } else if (trans[i].to == v || // null transition
50  w->compare_exchange_strong(v, trans[i].to,
51  std::memory_order_acquire,
52  std::memory_order_relaxed)) {
53  if (trans[i].done) return v;
54  }
55  }
56 }
57 
58 static std::atomic<uint64_t> delay_rand;
59 
60 // Return a suggested delay in nanoseconds for iteration number "loop"
61 int SpinLockSuggestedDelayNS(int loop) {
62  // Weak pseudo-random number generator to get some spread between threads
63  // when many are spinning.
64  uint64_t r = delay_rand.load(std::memory_order_relaxed);
65  r = 0x5deece66dLL * r + 0xb; // numbers from nrand48()
66  delay_rand.store(r, std::memory_order_relaxed);
67 
68  if (loop < 0 || loop > 32) { // limit loop to 0..32
69  loop = 32;
70  }
71  const int kMinDelay = 128 << 10; // 128us
72  // Double delay every 8 iterations, up to 16x (2ms).
73  int delay = kMinDelay << (loop / 8);
74  // Randomize in delay..2*delay range, for resulting 128us..4ms range.
75  return delay | ((delay - 1) & static_cast<int>(r));
76 }
77 
78 } // namespace base_internal
79 } // namespace absl
int v
Definition: variant_test.cc:81
void SpinLockDelay(std::atomic< uint32_t > *w, uint32_t value, int loop, base_internal::SchedulingMode scheduling_mode)
Definition: spinlock_wait.h:85
uint32_t SpinLockWait(std::atomic< uint32_t > *w, int n, const SpinLockWaitTransition trans[], base_internal::SchedulingMode scheduling_mode)
Definition: algorithm.h:29
size_t to
static std::atomic< uint64_t > delay_rand
int SpinLockSuggestedDelayNS(int loop)


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autogenerated on Mon Feb 28 2022 21:31:20