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Diffstat (limited to 'absl/time/clock.cc')
| -rw-r--r-- | absl/time/clock.cc | 547 |
1 files changed, 547 insertions, 0 deletions
diff --git a/absl/time/clock.cc b/absl/time/clock.cc new file mode 100644 index 0000000..e2bc01b --- /dev/null +++ b/absl/time/clock.cc @@ -0,0 +1,547 @@ +#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" |