/*
* Copyright (c) Facebook, Inc. and its affiliates.
*
* 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.
*/
#pragma once
#include <algorithm>
#include <limits>
#include <glog/logging.h>
#include <folly/Portability.h>
#include <folly/chrono/Hardware.h>
#include <folly/detail/Futex.h>
#include <folly/portability/Asm.h>
#include <folly/portability/Unistd.h>
namespace folly {
namespace detail {
/// A TurnSequencer allows threads to order their execution according to
/// a monotonically increasing (with wraparound) "turn" value. The two
/// operations provided are to wait for turn T, and to move to the next
/// turn. Every thread that is waiting for T must have arrived before
/// that turn is marked completed (for MPMCQueue only one thread waits
/// for any particular turn, so this is trivially true).
///
/// TurnSequencer's state_ holds 26 bits of the current turn (shifted
/// left by 6), along with a 6 bit saturating value that records the
/// maximum waiter minus the current turn. Wraparound of the turn space
/// is expected and handled. This allows us to atomically adjust the
/// number of outstanding waiters when we perform a FUTEX_WAKE operation.
/// Compare this strategy to sem_t's separate num_waiters field, which
/// isn't decremented until after the waiting thread gets scheduled,
/// during which time more enqueues might have occurred and made pointless
/// FUTEX_WAKE calls.
///
/// TurnSequencer uses futex() directly. It is optimized for the
/// case that the highest awaited turn is 32 or less higher than the
/// current turn. We use the FUTEX_WAIT_BITSET variant, which lets
/// us embed 32 separate wakeup channels in a single futex. See
/// http://locklessinc.com/articles/futex_cheat_sheet for a description.
///
/// We only need to keep exact track of the delta between the current
/// turn and the maximum waiter for the 32 turns that follow the current
/// one, because waiters at turn t+32 will be awoken at turn t. At that
/// point they can then adjust the delta using the higher base. Since we
/// need to encode waiter deltas of 0 to 32 inclusive, we use 6 bits.
/// We actually store waiter deltas up to 63, since that might reduce
/// the number of CAS operations a tiny bit.
///
/// To avoid some futex() calls entirely, TurnSequencer uses an adaptive
/// spin cutoff before waiting. The overheads (and convergence rate)
/// of separately tracking the spin cutoff for each TurnSequencer would
/// be prohibitive, so the actual storage is passed in as a parameter and
/// updated atomically. This also lets the caller use different adaptive
/// cutoffs for different operations (read versus write, for example).
/// To avoid contention, the spin cutoff is only updated when requested
/// by the caller.
///
/// On x86 the latency of a spin loop varies dramatically across
/// architectures due to changes in the PAUSE instruction. Skylake
/// increases the latency by about a factor of 15 compared to previous
/// architectures. To work around this, on x86 we measure spins using
/// RDTSC rather than a loop counter.
template <template <typename> class Atom>
struct TurnSequencer {
explicit TurnSequencer(const uint32_t firstTurn = 0) noexcept
: state_(encode(firstTurn << kTurnShift, 0)) {}
/// Returns true iff a call to waitForTurn(turn, ...) won't block
bool isTurn(const uint32_t turn) const noexcept {
auto state = state_.load(std::memory_order_acquire);
return decodeCurrentSturn(state) == (turn << kTurnShift);
}
enum class TryWaitResult { SUCCESS, PAST, TIMEDOUT };
/// See tryWaitForTurn
/// Requires that `turn` is not a turn in the past.
void waitForTurn(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff) noexcept {
const auto ret = tryWaitForTurn(turn, spinCutoff, updateSpinCutoff);
DCHECK(ret == TryWaitResult::SUCCESS);
}
// Internally we always work with shifted turn values, which makes the
// truncation and wraparound work correctly. This leaves us bits at
// the bottom to store the number of waiters. We call shifted turns
// "sturns" inside this class.
/// Blocks the current thread until turn has arrived.
/// If updateSpinCutoff is true then this will spin for up to
/// kMaxSpinLimit before blocking and will adjust spinCutoff based
/// on the results, otherwise it will spin for at most spinCutoff.
/// Returns SUCCESS if the wait succeeded, PAST if the turn is in the
/// past or TIMEDOUT if the absTime time value is not nullptr and is
/// reached before the turn arrives
template <
class Clock = std::chrono::steady_clock,
class Duration = typename Clock::duration>
TryWaitResult tryWaitForTurn(
const uint32_t turn,
Atom<uint32_t>& spinCutoff,
const bool updateSpinCutoff,
const std::chrono::time_point<Clock, Duration>* absTime =
nullptr) noexcept {
uint32_t prevThresh = spinCutoff.load(std::memory_order_relaxed);
const uint32_t effectiveSpinCutoff =
updateSpinCutoff || prevThresh == 0 ? kMaxSpinLimit : prevThresh;
uint64_t begin = 0;
uint32_t tries;
const uint32_t sturn = turn << kTurnShift;
for (tries = 0;; ++tries) {
uint32_t state = state_.load(std::memory_order_acquire);
uint32_t current_sturn = decodeCurrentSturn(state);
if (current_sturn == sturn) {
break;
}
// wrap-safe version of (current_sturn >= sturn)
if (sturn - current_sturn >= std::numeric_limits<uint32_t>::max() / 2) {
// turn is in the past
return TryWaitResult::PAST;
}
// the first effectSpinCutoff tries are spins, after that we will
// record ourself as a waiter and block with futexWait
if (kSpinUsingHardwareClock) {
auto now = hardware_timestamp();
if (tries == 0) {
begin = now;
}
if (tries == 0 || now < begin + effectiveSpinCutoff) {
asm_volatile_pause();
continue;
}
} else {
if (tries < effectiveSpinCutoff) {
asm_volatile_pause();
continue;
}
}
uint32_t current_max_waiter_delta = decodeMaxWaitersDelta(state);
uint32_t our_waiter_delta = (sturn - current_sturn) >> kTurnShift;
uint32_t new_state;
if (our_waiter_delta <= current_max_waiter_delta) {
// state already records us as waiters, probably because this
// isn't our first time around this loop
new_state = state;
} else {
new_state = encode(current_sturn, our_waiter_delta);
if (state != new_state &&
!state_.compare_exchange_strong(state, new_state)) {
continue;
}
}
if (absTime) {
auto futexResult = detail::futexWaitUntil(
&state_, new_state, *absTime, futexChannel(turn));
if (futexResult == FutexResult::TIMEDOUT) {
return TryWaitResult::TIMEDOUT;
}
} else {
detail::futexWait(&state_, new_state, futexChannel(turn));
}
}
if (updateSpinCutoff || prevThresh == 0) {
// if we hit kMaxSpinLimit then spinning was pointless, so the right
// spinCutoff is kMinSpinLimit
uint32_t target;
uint64_t elapsed = !kSpinUsingHardwareClock || tries == 0
? tries
: hardware_timestamp() - begin;
if (tries >= kMaxSpinLimit) {
target = kMinSpinLimit;
} else {
// to account for variations, we allow ourself to spin 2*N when
// we think that N is actually required in order to succeed
target = std::min(
uint32_t{kMaxSpinLimit},
std::max(
uint32_t{kMinSpinLimit}, static_cast<uint32_t>(elapsed) * 2));
}
if (prevThresh == 0) {
// bootstrap
spinCutoff.store(target);
} else {
// try once, keep moving if CAS fails. Exponential moving average
// with alpha of 7/8
// Be careful that the quantity we add to prevThresh is signed.
spinCutoff.compare_exchange_weak(
prevThresh, prevThresh + int(target - prevThresh) / 8);
}
}
return TryWaitResult::SUCCESS;
}
/// Unblocks a thread running waitForTurn(turn + 1)
void completeTurn(const uint32_t turn) noexcept {
uint32_t state = state_.load(std::memory_order_acquire);
while (true) {
DCHECK(state == encode(turn << kTurnShift, decodeMaxWaitersDelta(state)));
uint32_t max_waiter_delta = decodeMaxWaitersDelta(state);
uint32_t new_state = encode(
(turn + 1) << kTurnShift,
max_waiter_delta == 0 ? 0 : max_waiter_delta - 1);
if (state_.compare_exchange_strong(state, new_state)) {
if (max_waiter_delta != 0) {
detail::futexWake(
&state_, std::numeric_limits<int>::max(), futexChannel(turn + 1));
}
break;
}
// failing compare_exchange_strong updates first arg to the value
// that caused the failure, so no need to reread state_
}
}
/// Returns the least-most significant byte of the current uncompleted
/// turn. The full 32 bit turn cannot be recovered.
uint8_t uncompletedTurnLSB() const noexcept {
return uint8_t(state_.load(std::memory_order_acquire) >> kTurnShift);
}
private:
static constexpr bool kSpinUsingHardwareClock = kIsArchAmd64;
static constexpr uint32_t kCyclesPerSpinLimit =
kSpinUsingHardwareClock ? 1 : 10;
/// kTurnShift counts the bits that are stolen to record the delta
/// between the current turn and the maximum waiter. It needs to be big
/// enough to record wait deltas of 0 to 32 inclusive. Waiters more
/// than 32 in the future will be woken up 32*n turns early (since
/// their BITSET will hit) and will adjust the waiter count again.
/// We go a bit beyond and let the waiter count go up to 63, which is
/// free and might save us a few CAS
static constexpr uint32_t kTurnShift = 6;
static constexpr uint32_t kWaitersMask = (1 << kTurnShift) - 1;
/// The minimum spin duration that we will adaptively select. The value
/// here is cycles, adjusted to the way in which the limit will actually
/// be applied.
static constexpr uint32_t kMinSpinLimit = 200 / kCyclesPerSpinLimit;
/// The maximum spin duration that we will adaptively select, and the
/// spin duration that will be used when probing to get a new data
/// point for the adaptation
static constexpr uint32_t kMaxSpinLimit = 20000 / kCyclesPerSpinLimit;
/// This holds both the current turn, and the highest waiting turn,
/// stored as (current_turn << 6) | min(63, max(waited_turn - current_turn))
Futex<Atom> state_;
/// Returns the bitmask to pass futexWait or futexWake when communicating
/// about the specified turn
uint32_t futexChannel(uint32_t turn) const noexcept {
return 1u << (turn & 31);
}
uint32_t decodeCurrentSturn(uint32_t state) const noexcept {
return state & ~kWaitersMask;
}
uint32_t decodeMaxWaitersDelta(uint32_t state) const noexcept {
return state & kWaitersMask;
}
uint32_t encode(uint32_t currentSturn, uint32_t maxWaiterD) const noexcept {
return currentSturn | std::min(uint32_t{kWaitersMask}, maxWaiterD);
}
};
} // namespace detail
} // namespace folly