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Reduce futex usage in ParkingLot #2907

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Merged
wwbmmm merged 2 commits into from
Mar 6, 2025
Merged

Reduce futex usage in ParkingLot #2907

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0cf7901
Reduce futex usage in ParkingLot
JimChengLin Mar 4, 2025
b30024c
Merge branch 'apache:master' into master
JimChengLin Mar 4, 2025
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8 changes: 7 additions & 1 deletion src/bthread/parking_lot.h
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Original file line number Diff line number Diff line change
Expand Up @@ -40,11 +40,14 @@ class BAIDU_CACHELINE_ALIGNMENT ParkingLot {
int val;
};

ParkingLot() : _pending_signal(0) {}
ParkingLot() : _pending_signal(0), _waiter_num(0) {}

// Wake up at most `num_task' workers.
// Returns #workers woken up.
int signal(int num_task) {
if (_waiter_num.load(butil::memory_order_relaxed) == 0) {
return 0;
}
_pending_signal.fetch_add((num_task << 1), butil::memory_order_release);
return futex_wake_private(&_pending_signal, num_task);
}
Expand All @@ -57,7 +60,9 @@ class BAIDU_CACHELINE_ALIGNMENT ParkingLot {
// Wait for tasks.
// If the `expected_state' does not match, wait() may finish directly.
void wait(const State& expected_state) {
_waiter_num.fetch_add(1, butil::memory_order_relaxed);
futex_wait_private(&_pending_signal, expected_state.val, NULL);
_waiter_num.fetch_sub(1, butil::memory_order_relaxed);
}

// Wakeup suspended wait() and make them unwaitable ever.
Expand All @@ -68,6 +73,7 @@ class BAIDU_CACHELINE_ALIGNMENT ParkingLot {
private:
// higher 31 bits for signalling, LSB for stopping.
butil::atomic<int> _pending_signal;
butil::atomic<int> _waiter_num;
};

} // namespace bthread
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