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authorGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
committerGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
commitc2e754829628d1e9b7a16b3389cfdace76950fdf (patch)
tree5a7f056f44e27c30e10025113b644f0b3b5801fc /absl/time/clock.cc
Initial Commit
Diffstat (limited to 'absl/time/clock.cc')
-rw-r--r--absl/time/clock.cc547
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diff --git a/absl/time/clock.cc b/absl/time/clock.cc
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+++ b/absl/time/clock.cc
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+#include "absl/time/clock.h"
+
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+#include <algorithm>
+#include <atomic>
+#include <cerrno>
+#include <cstdint>
+#include <ctime>
+#include <limits>
+
+#include "absl/base/internal/spinlock.h"
+#include "absl/base/internal/unscaledcycleclock.h"
+#include "absl/base/macros.h"
+#include "absl/base/port.h"
+#include "absl/base/thread_annotations.h"
+
+namespace absl {
+Time Now() {
+ // TODO(bww): Get a timespec instead so we don't have to divide.
+ int64_t n = absl::GetCurrentTimeNanos();
+ if (n >= 0) {
+ return time_internal::FromUnixDuration(
+ time_internal::MakeDuration(n / 1000000000, n % 1000000000 * 4));
+ }
+ return time_internal::FromUnixDuration(absl::Nanoseconds(n));
+}
+} // namespace absl
+
+// Decide if we should use the fast GetCurrentTimeNanos() algorithm
+// based on the cyclecounter, otherwise just get the time directly
+// from the OS on every call. This can be chosen at compile-time via
+// -DABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS=[0|1]
+#ifndef ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
+#if ABSL_USE_UNSCALED_CYCLECLOCK
+#define ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS 1
+#else
+#define ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS 0
+#endif
+#endif
+
+#if defined(__APPLE__)
+#include "absl/time/internal/get_current_time_ios.inc"
+#elif defined(_WIN32)
+#include "absl/time/internal/get_current_time_windows.inc"
+#else
+#include "absl/time/internal/get_current_time_posix.inc"
+#endif
+
+// Allows override by test.
+#ifndef GET_CURRENT_TIME_NANOS_FROM_SYSTEM
+#define GET_CURRENT_TIME_NANOS_FROM_SYSTEM() \
+ ::absl::time_internal::GetCurrentTimeNanosFromSystem()
+#endif
+
+#if !ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
+namespace absl {
+int64_t GetCurrentTimeNanos() {
+ return GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
+}
+} // namespace absl
+#else // Use the cyclecounter-based implementation below.
+
+// Allows override by test.
+#ifndef GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW
+#define GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW() \
+ ::absl::time_internal::UnscaledCycleClockWrapperForGetCurrentTime::Now()
+#endif
+
+// The following counters are used only by the test code.
+static int64_t stats_initializations;
+static int64_t stats_reinitializations;
+static int64_t stats_calibrations;
+static int64_t stats_slow_paths;
+static int64_t stats_fast_slow_paths;
+
+namespace absl {
+namespace time_internal {
+// This is a friend wrapper around UnscaledCycleClock::Now()
+// (needed to access UnscaledCycleClock).
+class UnscaledCycleClockWrapperForGetCurrentTime {
+ public:
+ static int64_t Now() { return base_internal::UnscaledCycleClock::Now(); }
+};
+} // namespace time_internal
+
+// uint64_t is used in this module to provide an extra bit in multiplications
+
+// Return the time in ns as told by the kernel interface. Place in *cycleclock
+// the value of the cycleclock at about the time of the syscall.
+// This call represents the time base that this module synchronizes to.
+// Ensures that *cycleclock does not step back by up to (1 << 16) from
+// last_cycleclock, to discard small backward counter steps. (Larger steps are
+// assumed to be complete resyncs, which shouldn't happen. If they do, a full
+// reinitialization of the outer algorithm should occur.)
+static int64_t GetCurrentTimeNanosFromKernel(uint64_t last_cycleclock,
+ uint64_t *cycleclock) {
+ // We try to read clock values at about the same time as the kernel clock.
+ // This value gets adjusted up or down as estimate of how long that should
+ // take, so we can reject attempts that take unusually long.
+ static std::atomic<uint64_t> approx_syscall_time_in_cycles{10 * 1000};
+
+ uint64_t local_approx_syscall_time_in_cycles = // local copy
+ approx_syscall_time_in_cycles.load(std::memory_order_relaxed);
+
+ int64_t current_time_nanos_from_system;
+ uint64_t before_cycles;
+ uint64_t after_cycles;
+ uint64_t elapsed_cycles;
+ int loops = 0;
+ do {
+ before_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
+ current_time_nanos_from_system = GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
+ after_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
+ // elapsed_cycles is unsigned, so is large on overflow
+ elapsed_cycles = after_cycles - before_cycles;
+ if (elapsed_cycles >= local_approx_syscall_time_in_cycles &&
+ ++loops == 20) { // clock changed frequencies? Back off.
+ loops = 0;
+ if (local_approx_syscall_time_in_cycles < 1000 * 1000) {
+ local_approx_syscall_time_in_cycles =
+ (local_approx_syscall_time_in_cycles + 1) << 1;
+ }
+ approx_syscall_time_in_cycles.store(
+ local_approx_syscall_time_in_cycles,
+ std::memory_order_relaxed);
+ }
+ } while (elapsed_cycles >= local_approx_syscall_time_in_cycles ||
+ last_cycleclock - after_cycles < (static_cast<uint64_t>(1) << 16));
+
+ // Number of times in a row we've seen a kernel time call take substantially
+ // less than approx_syscall_time_in_cycles.
+ static std::atomic<uint32_t> seen_smaller{ 0 };
+
+ // Adjust approx_syscall_time_in_cycles to be within a factor of 2
+ // of the typical time to execute one iteration of the loop above.
+ if ((local_approx_syscall_time_in_cycles >> 1) < elapsed_cycles) {
+ // measured time is no smaller than half current approximation
+ seen_smaller.store(0, std::memory_order_relaxed);
+ } else if (seen_smaller.fetch_add(1, std::memory_order_relaxed) >= 3) {
+ // smaller delays several times in a row; reduce approximation by 12.5%
+ const uint64_t new_approximation =
+ local_approx_syscall_time_in_cycles -
+ (local_approx_syscall_time_in_cycles >> 3);
+ approx_syscall_time_in_cycles.store(new_approximation,
+ std::memory_order_relaxed);
+ seen_smaller.store(0, std::memory_order_relaxed);
+ }
+
+ *cycleclock = after_cycles;
+ return current_time_nanos_from_system;
+}
+
+
+// ---------------------------------------------------------------------
+// An implementation of reader-write locks that use no atomic ops in the read
+// case. This is a generalization of Lamport's method for reading a multiword
+// clock. Increment a word on each write acquisition, using the low-order bit
+// as a spinlock; the word is the high word of the "clock". Readers read the
+// high word, then all other data, then the high word again, and repeat the
+// read if the reads of the high words yields different answers, or an odd
+// value (either case suggests possible interference from a writer).
+// Here we use a spinlock to ensure only one writer at a time, rather than
+// spinning on the bottom bit of the word to benefit from SpinLock
+// spin-delay tuning.
+
+// Acquire seqlock (*seq) and return the value to be written to unlock.
+static inline uint64_t SeqAcquire(std::atomic<uint64_t> *seq) {
+ uint64_t x = seq->fetch_add(1, std::memory_order_relaxed);
+
+ // We put a release fence between update to *seq and writes to shared data.
+ // Thus all stores to shared data are effectively release operations and
+ // update to *seq above cannot be re-ordered past any of them. Note that
+ // this barrier is not for the fetch_add above. A release barrier for the
+ // fetch_add would be before it, not after.
+ std::atomic_thread_fence(std::memory_order_release);
+
+ return x + 2; // original word plus 2
+}
+
+// Release seqlock (*seq) by writing x to it---a value previously returned by
+// SeqAcquire.
+static inline void SeqRelease(std::atomic<uint64_t> *seq, uint64_t x) {
+ // The unlock store to *seq must have release ordering so that all
+ // updates to shared data must finish before this store.
+ seq->store(x, std::memory_order_release); // release lock for readers
+}
+
+// ---------------------------------------------------------------------
+
+// "nsscaled" is unit of time equal to a (2**kScale)th of a nanosecond.
+enum { kScale = 30 };
+
+// The minimum interval between samples of the time base.
+// We pick enough time to amortize the cost of the sample,
+// to get a reasonably accurate cycle counter rate reading,
+// and not so much that calculations will overflow 64-bits.
+static const uint64_t kMinNSBetweenSamples = 2000 << 20;
+
+// We require that kMinNSBetweenSamples shifted by kScale
+// have at least a bit left over for 64-bit calculations.
+static_assert(((kMinNSBetweenSamples << (kScale + 1)) >> (kScale + 1)) ==
+ kMinNSBetweenSamples,
+ "cannot represent kMaxBetweenSamplesNSScaled");
+
+// A reader-writer lock protecting the static locations below.
+// See SeqAcquire() and SeqRelease() above.
+static absl::base_internal::SpinLock lock(
+ absl::base_internal::kLinkerInitialized);
+static std::atomic<uint64_t> seq(0);
+
+// data from a sample of the kernel's time value
+struct TimeSampleAtomic {
+ std::atomic<uint64_t> raw_ns; // raw kernel time
+ std::atomic<uint64_t> base_ns; // our estimate of time
+ std::atomic<uint64_t> base_cycles; // cycle counter reading
+ std::atomic<uint64_t> nsscaled_per_cycle; // cycle period
+ // cycles before we'll sample again (a scaled reciprocal of the period,
+ // to avoid a division on the fast path).
+ std::atomic<uint64_t> min_cycles_per_sample;
+};
+// Same again, but with non-atomic types
+struct TimeSample {
+ uint64_t raw_ns; // raw kernel time
+ uint64_t base_ns; // our estimate of time
+ uint64_t base_cycles; // cycle counter reading
+ uint64_t nsscaled_per_cycle; // cycle period
+ uint64_t min_cycles_per_sample; // approx cycles before next sample
+};
+
+static struct TimeSampleAtomic last_sample; // the last sample; under seq
+
+static int64_t GetCurrentTimeNanosSlowPath() ABSL_ATTRIBUTE_COLD;
+
+// Read the contents of *atomic into *sample.
+// Each field is read atomically, but to maintain atomicity between fields,
+// the access must be done under a lock.
+static void ReadTimeSampleAtomic(const struct TimeSampleAtomic *atomic,
+ struct TimeSample *sample) {
+ sample->base_ns = atomic->base_ns.load(std::memory_order_relaxed);
+ sample->base_cycles = atomic->base_cycles.load(std::memory_order_relaxed);
+ sample->nsscaled_per_cycle =
+ atomic->nsscaled_per_cycle.load(std::memory_order_relaxed);
+ sample->min_cycles_per_sample =
+ atomic->min_cycles_per_sample.load(std::memory_order_relaxed);
+ sample->raw_ns = atomic->raw_ns.load(std::memory_order_relaxed);
+}
+
+// Public routine.
+// Algorithm: We wish to compute real time from a cycle counter. In normal
+// operation, we construct a piecewise linear approximation to the kernel time
+// source, using the cycle counter value. The start of each line segment is at
+// the same point as the end of the last, but may have a different slope (that
+// is, a different idea of the cycle counter frequency). Every couple of
+// seconds, the kernel time source is sampled and compared with the current
+// approximation. A new slope is chosen that, if followed for another couple
+// of seconds, will correct the error at the current position. The information
+// for a sample is in the "last_sample" struct. The linear approximation is
+// estimated_time = last_sample.base_ns +
+// last_sample.ns_per_cycle * (counter_reading - last_sample.base_cycles)
+// (ns_per_cycle is actually stored in different units and scaled, to avoid
+// overflow). The base_ns of the next linear approximation is the
+// estimated_time using the last approximation; the base_cycles is the cycle
+// counter value at that time; the ns_per_cycle is the number of ns per cycle
+// measured since the last sample, but adjusted so that most of the difference
+// between the estimated_time and the kernel time will be corrected by the
+// estimated time to the next sample. In normal operation, this algorithm
+// relies on:
+// - the cycle counter and kernel time rates not changing a lot in a few
+// seconds.
+// - the client calling into the code often compared to a couple of seconds, so
+// the time to the next correction can be estimated.
+// Any time ns_per_cycle is not known, a major error is detected, or the
+// assumption about frequent calls is violated, the implementation returns the
+// kernel time. It records sufficient data that a linear approximation can
+// resume a little later.
+
+int64_t GetCurrentTimeNanos() {
+ // read the data from the "last_sample" struct (but don't need raw_ns yet)
+ // The reads of "seq" and test of the values emulate a reader lock.
+ uint64_t base_ns;
+ uint64_t base_cycles;
+ uint64_t nsscaled_per_cycle;
+ uint64_t min_cycles_per_sample;
+ uint64_t seq_read0;
+ uint64_t seq_read1;
+
+ // If we have enough information to interpolate, the value returned will be
+ // derived from this cycleclock-derived time estimate. On some platforms
+ // (POWER) the function to retrieve this value has enough complexity to
+ // contribute to register pressure - reading it early before initializing
+ // the other pieces of the calculation minimizes spill/restore instructions,
+ // minimizing icache cost.
+ uint64_t now_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
+
+ // Acquire pairs with the barrier in SeqRelease - if this load sees that
+ // store, the shared-data reads necessarily see that SeqRelease's updates
+ // to the same shared data.
+ seq_read0 = seq.load(std::memory_order_acquire);
+
+ base_ns = last_sample.base_ns.load(std::memory_order_relaxed);
+ base_cycles = last_sample.base_cycles.load(std::memory_order_relaxed);
+ nsscaled_per_cycle =
+ last_sample.nsscaled_per_cycle.load(std::memory_order_relaxed);
+ min_cycles_per_sample =
+ last_sample.min_cycles_per_sample.load(std::memory_order_relaxed);
+
+ // This acquire fence pairs with the release fence in SeqAcquire. Since it
+ // is sequenced between reads of shared data and seq_read1, the reads of
+ // shared data are effectively acquiring.
+ std::atomic_thread_fence(std::memory_order_acquire);
+
+ // The shared-data reads are effectively acquire ordered, and the
+ // shared-data writes are effectively release ordered. Therefore if our
+ // shared-data reads see any of a particular update's shared-data writes,
+ // seq_read1 is guaranteed to see that update's SeqAcquire.
+ seq_read1 = seq.load(std::memory_order_relaxed);
+
+ // Fast path. Return if min_cycles_per_sample has not yet elapsed since the
+ // last sample, and we read a consistent sample. The fast path activates
+ // only when min_cycles_per_sample is non-zero, which happens when we get an
+ // estimate for the cycle time. The predicate will fail if now_cycles <
+ // base_cycles, or if some other thread is in the slow path.
+ //
+ // Since we now read now_cycles before base_ns, it is possible for now_cycles
+ // to be less than base_cycles (if we were interrupted between those loads and
+ // last_sample was updated). This is harmless, because delta_cycles will wrap
+ // and report a time much much bigger than min_cycles_per_sample. In that case
+ // we will take the slow path.
+ uint64_t delta_cycles = now_cycles - base_cycles;
+ if (seq_read0 == seq_read1 && (seq_read0 & 1) == 0 &&
+ delta_cycles < min_cycles_per_sample) {
+ return base_ns + ((delta_cycles * nsscaled_per_cycle) >> kScale);
+ }
+ return GetCurrentTimeNanosSlowPath();
+}
+
+// Return (a << kScale)/b.
+// Zero is returned if b==0. Scaling is performed internally to
+// preserve precision without overflow.
+static uint64_t SafeDivideAndScale(uint64_t a, uint64_t b) {
+ // Find maximum safe_shift so that
+ // 0 <= safe_shift <= kScale and (a << safe_shift) does not overflow.
+ int safe_shift = kScale;
+ while (((a << safe_shift) >> safe_shift) != a) {
+ safe_shift--;
+ }
+ uint64_t scaled_b = b >> (kScale - safe_shift);
+ uint64_t quotient = 0;
+ if (scaled_b != 0) {
+ quotient = (a << safe_shift) / scaled_b;
+ }
+ return quotient;
+}
+
+static uint64_t UpdateLastSample(
+ uint64_t now_cycles, uint64_t now_ns, uint64_t delta_cycles,
+ const struct TimeSample *sample) ABSL_ATTRIBUTE_COLD;
+
+// The slow path of GetCurrentTimeNanos(). This is taken while gathering
+// initial samples, when enough time has elapsed since the last sample, and if
+// any other thread is writing to last_sample.
+//
+// Manually mark this 'noinline' to minimize stack frame size of the fast
+// path. Without this, sometimes a compiler may inline this big block of code
+// into the fast past. That causes lots of register spills and reloads that
+// are unnecessary unless the slow path is taken.
+//
+// TODO(b/36012148) Remove this attribute when our compiler is smart enough
+// to do the right thing.
+ABSL_ATTRIBUTE_NOINLINE
+static int64_t GetCurrentTimeNanosSlowPath() LOCKS_EXCLUDED(lock) {
+ // Serialize access to slow-path. Fast-path readers are not blocked yet, and
+ // code below must not modify last_sample until the seqlock is acquired.
+ lock.Lock();
+
+ // Sample the kernel time base. This is the definition of
+ // "now" if we take the slow path.
+ static uint64_t last_now_cycles; // protected by lock
+ uint64_t now_cycles;
+ uint64_t now_ns = GetCurrentTimeNanosFromKernel(last_now_cycles, &now_cycles);
+ last_now_cycles = now_cycles;
+
+ uint64_t estimated_base_ns;
+
+ // ----------
+ // Read the "last_sample" values again; this time holding the write lock.
+ struct TimeSample sample;
+ ReadTimeSampleAtomic(&last_sample, &sample);
+
+ // ----------
+ // Try running the fast path again; another thread may have updated the
+ // sample between our run of the fast path and the sample we just read.
+ uint64_t delta_cycles = now_cycles - sample.base_cycles;
+ if (delta_cycles < sample.min_cycles_per_sample) {
+ // Another thread updated the sample. This path does not take the seqlock
+ // so that blocked readers can make progress without blocking new readers.
+ estimated_base_ns = sample.base_ns +
+ ((delta_cycles * sample.nsscaled_per_cycle) >> kScale);
+ stats_fast_slow_paths++;
+ } else {
+ estimated_base_ns =
+ UpdateLastSample(now_cycles, now_ns, delta_cycles, &sample);
+ }
+
+ lock.Unlock();
+
+ return estimated_base_ns;
+}
+
+// Main part of the algorithm. Locks out readers, updates the approximation
+// using the new sample from the kernel, and stores the result in last_sample
+// for readers. Returns the new estimated time.
+static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
+ uint64_t delta_cycles,
+ const struct TimeSample *sample)
+ EXCLUSIVE_LOCKS_REQUIRED(lock) {
+ uint64_t estimated_base_ns = now_ns;
+ uint64_t lock_value = SeqAcquire(&seq); // acquire seqlock to block readers
+
+ // The 5s in the next if-statement limits the time for which we will trust
+ // the cycle counter and our last sample to give a reasonable result.
+ // Errors in the rate of the source clock can be multiplied by the ratio
+ // between this limit and kMinNSBetweenSamples.
+ if (sample->raw_ns == 0 || // no recent sample, or clock went backwards
+ sample->raw_ns + static_cast<uint64_t>(5) * 1000 * 1000 * 1000 < now_ns ||
+ now_ns < sample->raw_ns || now_cycles < sample->base_cycles) {
+ // record this sample, and forget any previously known slope.
+ last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
+ last_sample.base_ns.store(estimated_base_ns, std::memory_order_relaxed);
+ last_sample.base_cycles.store(now_cycles, std::memory_order_relaxed);
+ last_sample.nsscaled_per_cycle.store(0, std::memory_order_relaxed);
+ last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
+ stats_initializations++;
+ } else if (sample->raw_ns + 500 * 1000 * 1000 < now_ns &&
+ sample->base_cycles + 100 < now_cycles) {
+ // Enough time has passed to compute the cycle time.
+ if (sample->nsscaled_per_cycle != 0) { // Have a cycle time estimate.
+ // Compute time from counter reading, but avoiding overflow
+ // delta_cycles may be larger than on the fast path.
+ uint64_t estimated_scaled_ns;
+ int s = -1;
+ do {
+ s++;
+ estimated_scaled_ns = (delta_cycles >> s) * sample->nsscaled_per_cycle;
+ } while (estimated_scaled_ns / sample->nsscaled_per_cycle !=
+ (delta_cycles >> s));
+ estimated_base_ns = sample->base_ns +
+ (estimated_scaled_ns >> (kScale - s));
+ }
+
+ // Compute the assumed cycle time kMinNSBetweenSamples ns into the future
+ // assuming the cycle counter rate stays the same as the last interval.
+ uint64_t ns = now_ns - sample->raw_ns;
+ uint64_t measured_nsscaled_per_cycle = SafeDivideAndScale(ns, delta_cycles);
+
+ uint64_t assumed_next_sample_delta_cycles =
+ SafeDivideAndScale(kMinNSBetweenSamples, measured_nsscaled_per_cycle);
+
+ int64_t diff_ns = now_ns - estimated_base_ns; // estimate low by this much
+
+ // We want to set nsscaled_per_cycle so that our estimate of the ns time
+ // at the assumed cycle time is the assumed ns time.
+ // That is, we want to set nsscaled_per_cycle so:
+ // kMinNSBetweenSamples + diff_ns ==
+ // (assumed_next_sample_delta_cycles * nsscaled_per_cycle) >> kScale
+ // But we wish to damp oscillations, so instead correct only most
+ // of our current error, by solving:
+ // kMinNSBetweenSamples + diff_ns - (diff_ns / 16) ==
+ // (assumed_next_sample_delta_cycles * nsscaled_per_cycle) >> kScale
+ ns = kMinNSBetweenSamples + diff_ns - (diff_ns / 16);
+ uint64_t new_nsscaled_per_cycle =
+ SafeDivideAndScale(ns, assumed_next_sample_delta_cycles);
+ if (new_nsscaled_per_cycle != 0 &&
+ diff_ns < 100 * 1000 * 1000 && -diff_ns < 100 * 1000 * 1000) {
+ // record the cycle time measurement
+ last_sample.nsscaled_per_cycle.store(
+ new_nsscaled_per_cycle, std::memory_order_relaxed);
+ uint64_t new_min_cycles_per_sample =
+ SafeDivideAndScale(kMinNSBetweenSamples, new_nsscaled_per_cycle);
+ last_sample.min_cycles_per_sample.store(
+ new_min_cycles_per_sample, std::memory_order_relaxed);
+ stats_calibrations++;
+ } else { // something went wrong; forget the slope
+ last_sample.nsscaled_per_cycle.store(0, std::memory_order_relaxed);
+ last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
+ estimated_base_ns = now_ns;
+ stats_reinitializations++;
+ }
+ last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
+ last_sample.base_ns.store(estimated_base_ns, std::memory_order_relaxed);
+ last_sample.base_cycles.store(now_cycles, std::memory_order_relaxed);
+ } else {
+ // have a sample, but no slope; waiting for enough time for a calibration
+ stats_slow_paths++;
+ }
+
+ SeqRelease(&seq, lock_value); // release the readers
+
+ return estimated_base_ns;
+}
+} // namespace absl
+#endif // ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
+
+namespace absl {
+namespace {
+
+// Returns the maximum duration that SleepOnce() can sleep for.
+constexpr absl::Duration MaxSleep() {
+#ifdef _WIN32
+ // Windows _sleep() takes unsigned long argument in milliseconds.
+ return absl::Milliseconds(
+ std::numeric_limits<unsigned long>::max()); // NOLINT(runtime/int)
+#else
+ return absl::Seconds(std::numeric_limits<time_t>::max());
+#endif
+}
+
+// Sleeps for the given duration.
+// REQUIRES: to_sleep <= MaxSleep().
+void SleepOnce(absl::Duration to_sleep) {
+#ifdef _WIN32
+ _sleep(to_sleep / absl::Milliseconds(1));
+#else
+ struct timespec sleep_time = absl::ToTimespec(to_sleep);
+ while (nanosleep(&sleep_time, &sleep_time) != 0 && errno == EINTR) {
+ // Ignore signals and wait for the full interval to elapse.
+ }
+#endif
+}
+
+} // namespace
+} // namespace absl
+
+extern "C" {
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSleepFor(absl::Duration duration) {
+ while (duration > absl::ZeroDuration()) {
+ absl::Duration to_sleep = std::min(duration, absl::MaxSleep());
+ absl::SleepOnce(to_sleep);
+ duration -= to_sleep;
+ }
+}
+
+} // extern "C"