SERVER-90863 Prevent calls to notifyOne if no waiters are present (#2…

…2559) GitOrigin-RevId: 5b453f2b6738227972bf3d1dec3f786fd3402b68
lakamson · May 28, 2024 · 2538eab · 2538eab
1 parent 137395b
commit 2538eab
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Showing 2 changed files with 12 additions and 15 deletions.
diff --git a/src/mongo/util/concurrency/semaphore_ticketholder.cpp b/src/mongo/util/concurrency/semaphore_ticketholder.cpp
@@ -86,35 +86,32 @@ boost::optional<Ticket> SemaphoreTicketHolder::_waitForTicketUntilImpl(Operation
     };
 
     while (true) {
-        auto oldAvailable = _tickets.load();
-
         if (boost::optional<Ticket> maybeTicket = _tryAcquireImpl(admCtx); maybeTicket) {
             return std::move(*maybeTicket);
         }
 
-        if (oldAvailable != _tickets.loadRelaxed()) {
-            continue;
-        }
-
         Date_t deadline = nextDeadline();
-        auto canAcquire = _tickets.waitUntil(oldAvailable, deadline);
+        _waiterCount.fetchAndAdd(1);
+        _tickets.waitUntil(0, deadline);
+        _waiterCount.fetchAndSubtract(1);
+
         if (interruptible) {
             opCtx->checkForInterrupt();
         }
 
-        if (canAcquire) {
-            if (boost::optional<Ticket> maybeTicket = _tryAcquireImpl(admCtx)) {
-                return std::move(*maybeTicket);
-            }
-        } else if (deadline == until) {
+        if (deadline == until) {
             // We hit the end of our deadline, so return nothing.
             return boost::none;
         }
     }
 }
 
 void SemaphoreTicketHolder::_releaseToTicketPoolImpl(AdmissionContext* admCtx) noexcept {
-    if (_tickets.fetchAndAdd(1) >= 0) {
+    // Notifying a futex costs a syscall. Since queued waiters guarantee that the `_waiterCount` is
+    // non-zero while they are waiting, we can avoid the needless cost if there are tickets and no
+    // queued waiters.
+    int32_t availableBeforeIncrementing = _tickets.fetchAndAdd(1);
+    if (availableBeforeIncrementing >= 0 && _waiterCount.load() > 0) {
         _tickets.notifyOne();
     }
 }

diff --git a/src/mongo/util/concurrency/semaphore_ticketholder.h b/src/mongo/util/concurrency/semaphore_ticketholder.h
@@ -50,8 +50,6 @@ class SemaphoreTicketHolder final : public TicketHolder {
                                    ResizePolicy resizePolicy = ResizePolicy::kGradual);
 
     int32_t available() const final;
-
-
     int64_t queued() const final {
         auto removed = _semaphoreStats.totalRemovedQueue.loadRelaxed();
         auto added = _semaphoreStats.totalAddedQueue.loadRelaxed();
@@ -67,6 +65,7 @@ class SemaphoreTicketHolder final : public TicketHolder {
                                                     bool interruptible) final;
 
     boost::optional<Ticket> _tryAcquireImpl(AdmissionContext* admCtx) final;
+
     void _releaseToTicketPoolImpl(AdmissionContext* admCtx) noexcept final;
 
     bool _resizeImpl(WithLock lock,
@@ -83,6 +82,7 @@ class SemaphoreTicketHolder final : public TicketHolder {
 
     ResizePolicy _resizePolicy;
     BasicWaitableAtomic<int32_t> _tickets;
+    Atomic<int32_t> _waiterCount{0};
     QueueStats _semaphoreStats;
 };