Improve QThreadPool scalability.

Reduce lock contention:
- Skip locking in the accessor functions (where we can)
- exit early in tryStart (before locking the mutex) when the threadpool is running at max capacity.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/corelib/concurrent/qthreadpool.cpp b/src/corelib/concurrent/qthreadpool.cpp
index 83374da133..9c53b7ea40 100644
--- a/src/corelib/concurrent/qthreadpool.cpp
+++ b/src/corelib/concurrent/qthreadpool.cpp
@@ -222,6 +222,8 @@ void QThreadPoolPrivate::enqueueTask(QRunnable *runnable, int priority)
 
 int QThreadPoolPrivate::activeThreadCount() const
 {
+    // To improve scalability this function is called without holding 
+    // the mutex lock -- keep it thread-safe.
     return (allThreads.count()
             - expiredThreads.count()
             - waitingThreads
@@ -481,6 +483,12 @@ bool QThreadPool::tryStart(QRunnable *runnable)
         return false;
 
     Q_D(QThreadPool);
+
+    // To improve scalability perform a check on the thread count
+    // before locking the mutex.
+    if (d->allThreads.isEmpty() == false && d->activeThreadCount() >= d->maxThreadCount)
+        return false;
+
     QMutexLocker locker(&d->mutex);
     return d->tryStart(runnable);
 }
@@ -527,7 +535,6 @@ void QThreadPool::setExpiryTimeout(int expiryTimeout)
 int QThreadPool::maxThreadCount() const
 {
     Q_D(const QThreadPool);
-    QMutexLocker locker(&d->mutex);
     return d->maxThreadCount;
 }
 
@@ -556,7 +563,6 @@ void QThreadPool::setMaxThreadCount(int maxThreadCount)
 int QThreadPool::activeThreadCount() const
 {
     Q_D(const QThreadPool);
-    QMutexLocker locker(&d->mutex);
     return d->activeThreadCount();
 }