Commit 8949d553 authored by Tudor Bosman's avatar Tudor Bosman Committed by Jordan DeLong

Speed up EventCount, now 64-bit only

Summary:
Don't use two atomic variables when only one will suffice (otherwise, the
increments in doNotify() and prepareWait() would require memory_model_seq_cst,
because they need to globally order a store before a load, and no weaker
model allows you to do that)

Of course, this makes this 64-bit only, but I don't care.

Test Plan: eventcount_test, tests outside of folly

Reviewed By: delong.j@fb.com

FB internal diff: D858963
parent a664fa91
......@@ -21,10 +21,14 @@
#include <syscall.h>
#include <linux/futex.h>
#include <sys/time.h>
#include <cassert>
#include <climits>
#include <atomic>
#include <thread>
#include "folly/Bits.h"
#include "folly/Likely.h"
namespace folly {
......@@ -90,12 +94,12 @@ inline int futex(int* uaddr, int op, int val, const timespec* timeout,
*/
class EventCount {
public:
EventCount() noexcept : epoch_(0), waiters_(0) { }
EventCount() noexcept : val_(0) { }
class Key {
friend class EventCount;
explicit Key(int e) noexcept : epoch_(e) { }
int epoch_;
explicit Key(uint32_t e) noexcept : epoch_(e) { }
uint32_t epoch_;
};
void notify() noexcept;
......@@ -118,8 +122,28 @@ class EventCount {
EventCount& operator=(const EventCount&) = delete;
EventCount& operator=(EventCount&&) = delete;
std::atomic<int> epoch_;
std::atomic<int> waiters_;
// This requires 64-bit
static_assert(sizeof(int) == 4, "bad platform");
static_assert(sizeof(uint32_t) == 4, "bad platform");
static_assert(sizeof(uint64_t) == 8, "bad platform");
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
static constexpr size_t kEpochOffset = 1;
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
static constexpr size_t kEpochOffset = 0; // in units of sizeof(int)
#else
# error Your machine uses a weird endianness!
#endif
// val_ stores the epoch in the most significant 32 bits and the
// waiter count in the least significant 32 bits.
std::atomic<uint64_t> val_;
static constexpr uint64_t kAddWaiter = uint64_t(1);
static constexpr uint64_t kSubWaiter = uint64_t(-1);
static constexpr size_t kEpochShift = 32;
static constexpr uint64_t kAddEpoch = uint64_t(1) << kEpochShift;
static constexpr uint64_t kWaiterMask = kAddEpoch - 1;
};
inline void EventCount::notify() noexcept {
......@@ -131,31 +155,36 @@ inline void EventCount::notifyAll() noexcept {
}
inline void EventCount::doNotify(int n) noexcept {
// The order is important: epoch_ is incremented before waiters_ is checked.
// prepareWait() increments waiters_ before checking epoch_, so it is
// impossible to miss a wakeup.
++epoch_;
if (waiters_ != 0) {
detail::futex(reinterpret_cast<int*>(&epoch_), FUTEX_WAKE, n, nullptr,
nullptr, 0);
uint64_t prev = val_.fetch_add(kAddEpoch, std::memory_order_acq_rel);
if (UNLIKELY(prev & kWaiterMask)) {
detail::futex(reinterpret_cast<int*>(&val_) + kEpochOffset,
FUTEX_WAKE, n, nullptr, nullptr, 0);
}
}
inline EventCount::Key EventCount::prepareWait() noexcept {
++waiters_;
return Key(epoch_);
uint64_t prev = val_.fetch_add(kAddWaiter, std::memory_order_acq_rel);
return Key(prev >> kEpochShift);
}
inline void EventCount::cancelWait() noexcept {
--waiters_;
// memory_order_relaxed would suffice for correctness, but the faster
// #waiters gets to 0, the less likely it is that we'll do spurious wakeups
// (and thus system calls).
uint64_t prev = val_.fetch_add(kSubWaiter, std::memory_order_seq_cst);
assert((prev & kWaiterMask) != 0);
}
inline void EventCount::wait(Key key) noexcept {
while (epoch_ == key.epoch_) {
detail::futex(reinterpret_cast<int*>(&epoch_), FUTEX_WAIT, key.epoch_,
nullptr, nullptr, 0);
while ((val_.load(std::memory_order_acquire) >> kEpochShift) == key.epoch_) {
detail::futex(reinterpret_cast<int*>(&val_) + kEpochOffset,
FUTEX_WAIT, key.epoch_, nullptr, nullptr, 0);
}
--waiters_;
// memory_order_relaxed would suffice for correctness, but the faster
// #waiters gets to 0, the less likely it is that we'll do spurious wakeups
// (and thus system calls)
uint64_t prev = val_.fetch_add(kSubWaiter, std::memory_order_seq_cst);
assert((prev & kWaiterMask) != 0);
}
template <class Condition>
......
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