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Add flag for parking lot no signal when no waiter, the default is false #3079

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Merged
chenBright merged 1 commit into from
Sep 1, 2025
Merged

Add flag for parking lot no signal when no waiter, the default is false #3079

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21 changes: 17 additions & 4 deletions src/bthread/parking_lot.h
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Original file line number Diff line number Diff line change
Expand Up @@ -22,11 +22,14 @@
#ifndef BTHREAD_PARKING_LOT_H
#define BTHREAD_PARKING_LOT_H

#include <gflags/gflags.h>
#include "butil/atomicops.h"
#include "bthread/sys_futex.h"

namespace bthread {

DECLARE_bool(parking_lot_no_signal_when_no_waiter);

// Park idle workers.
class BAIDU_CACHELINE_ALIGNMENT ParkingLot {
public:
Expand All @@ -40,13 +43,15 @@ class BAIDU_CACHELINE_ALIGNMENT ParkingLot {
int val;
};

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

// Wake up at most `num_task' workers.
// Returns #workers woken up.
int signal(int num_task) {
_pending_signal.fetch_add((num_task << 1), butil::memory_order_release);
if (_waiter_num.load(butil::memory_order_relaxed) == 0) {
if (_no_signal_when_no_waiter && _waiter_num.load(butil::memory_order_relaxed) == 0) {
return 0;
}
return futex_wake_private(&_pending_signal, num_task);
Expand All @@ -64,9 +69,13 @@ class BAIDU_CACHELINE_ALIGNMENT ParkingLot {
// Fast path, no need to futex_wait.
return;
}
_waiter_num.fetch_add(1, butil::memory_order_relaxed);
if (_no_signal_when_no_waiter) {
_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);
if (_no_signal_when_no_waiter) {
_waiter_num.fetch_sub(1, butil::memory_order_relaxed);
}
}

// Wakeup suspended wait() and make them unwaitable ever.
Expand All @@ -79,6 +88,10 @@ class BAIDU_CACHELINE_ALIGNMENT ParkingLot {
// higher 31 bits for signalling, LSB for stopping.
butil::atomic<int> _pending_signal;
butil::atomic<int> _waiter_num;
// Whether to signal when there is no waiter.
// In busy worker scenarios, signal overhead
// can be reduced.
bool _no_signal_when_no_waiter;
};

} // namespace bthread
Expand Down
4 changes: 3 additions & 1 deletion src/bthread/task_control.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -45,6 +45,9 @@ DEFINE_bool(task_group_set_worker_name, true,

namespace bthread {

DEFINE_bool(parking_lot_no_signal_when_no_waiter, false,
"ParkingLot doesn't signal when there is no waiter. "
"In busy worker scenarios, signal overhead can be reduced.");
DEFINE_bool(enable_bthread_priority_queue, false, "Whether to enable priority queue");

DECLARE_int32(bthread_concurrency);
Expand Down Expand Up @@ -189,7 +192,6 @@ TaskControl::TaskControl()
, _signal_per_second(&_cumulated_signal_count)
, _status(print_rq_sizes_in_the_tc, this)
, _nbthreads("bthread_count")
, _enable_priority_queue(FLAGS_enable_bthread_priority_queue)
, _priority_queues(FLAGS_task_group_ntags)
, _pl_num_of_each_tag(FLAGS_bthread_parking_lot_of_each_tag)
, _tagged_pl(FLAGS_task_group_ntags)
Expand Down
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