Track average wait times under our maximum spin time threshold

Further observation shows that spin times slightly over the average wait time
produce the best results. This change keeps a heavily weighted average
of the wait times under 1.5ms (1.5 times the max spin threshold), and adjusts
the spin time to be 150% of the average wait time.

Introduce spin time adjustments when spin locking works, and adjust to
between 150% of the average wait time and the maximum threshold.

Reviewed-by: joao

4 files changed, 21 insertions, 8 deletions

diff --git a/src/corelib/thread/qmutex.cpp b/src/corelib/thread/qmutex.cpp
index 03e623f..1009f7b 100644
--- a/src/corelib/thread/qmutex.cpp
+++ b/src/corelib/thread/qmutex.cpp
@@ -453,14 +453,20 @@ void QMutex::lockInternal()
             Q_UNUSED(isLocked);
 
             qint64 maximumSpinTime = d->maximumSpinTime;
-            qint64 waitTime = elapsedTimer.nsecsElapsed();
+            qint64 averageWaitTime = d->averageWaitTime;
+            qint64 actualWaitTime = elapsedTimer.nsecsElapsed();
+            if (actualWaitTime < (QMutexPrivate::MaximumSpinTimeThreshold * 3 / 2)) {
+                // measure the wait times
+                averageWaitTime = d->averageWaitTime = qMin((averageWaitTime + actualWaitTime) / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
+            }
+
             // adjust the spin count when spinning does not benefit contention performance
-            if (spinTime + waitTime > QMutexPrivate::MaximumSpinTimeThreshold) {
+            if ((spinTime + actualWaitTime) - qint64(QMutexPrivate::MaximumSpinTimeThreshold) >= qint64(QMutexPrivate::MaximumSpinTimeThreshold)) {
                 // long waits, stop spinning
                 d->maximumSpinTime = 0;
-            } else if (waitTime < maximumSpinTime) {
-                // never spin more than the minimum wait time (otherwise we may perform worse)
-                d->maximumSpinTime = waitTime;
+            } else {
+                // allow spinning if wait times decrease, but never spin more than the average wait time (otherwise we may perform worse)
+                d->maximumSpinTime = qBound(qint64(averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
             }
             return;
         }
@@ -468,7 +474,13 @@ void QMutex::lockInternal()
         QThread::yieldCurrentThread();
     } while (d->contenders != 0 || !d->contenders.testAndSetAcquire(0, 1));
 
-    // spinning is working, do not change the spin time
+    // spinning is working, do not change the spin time (unless we are using much less time than allowed to spin)
+    qint64 maximumSpinTime = d->maximumSpinTime;
+    qint64 spinTime = elapsedTimer.nsecsElapsed();
+    if (spinTime < maximumSpinTime / 2) {
+        // we are using much less time than we need, adjust the limit
+        d->maximumSpinTime = qBound(qint64(d->averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
+    }
 }
 
 /*!
diff --git a/src/corelib/thread/qmutex_p.h b/src/corelib/thread/qmutex_p.h
index fa6e879..9d40bea 100644
--- a/src/corelib/thread/qmutex_p.h
+++ b/src/corelib/thread/qmutex_p.h
@@ -75,6 +75,7 @@ public:
     // 1ms = 1000000ns
     enum { MaximumSpinTimeThreshold = 1000000 };
     volatile qint64 maximumSpinTime;
+    volatile qint64 averageWaitTime;
     Qt::HANDLE owner;
     uint count;
 
diff --git a/src/corelib/thread/qmutex_unix.cpp b/src/corelib/thread/qmutex_unix.cpp
index 48014cc..0e09ea0 100644
--- a/src/corelib/thread/qmutex_unix.cpp
+++ b/src/corelib/thread/qmutex_unix.cpp
@@ -74,7 +74,7 @@ static void report_error(int code, const char *where, const char *what)
 
 
 QMutexPrivate::QMutexPrivate(QMutex::RecursionMode mode)
-    : QMutexData(mode), maximumSpinTime(MaximumSpinTimeThreshold), owner(0), count(0)
+    : QMutexData(mode), maximumSpinTime(MaximumSpinTimeThreshold), averageWaitTime(0), owner(0), count(0)
 {
 #if defined(Q_OS_MAC)
     kern_return_t r = semaphore_create(mach_task_self(), &mach_semaphore, SYNC_POLICY_FIFO, 0);
diff --git a/src/corelib/thread/qmutex_win.cpp b/src/corelib/thread/qmutex_win.cpp
index 89e8b87..a759caa 100644
--- a/src/corelib/thread/qmutex_win.cpp
+++ b/src/corelib/thread/qmutex_win.cpp
@@ -48,7 +48,7 @@
 QT_BEGIN_NAMESPACE
 
 QMutexPrivate::QMutexPrivate(QMutex::RecursionMode mode)
-    : QMutexData(mode), maximumSpinTime(MaximumSpinTimeThreshold), owner(0), count(0)
+    : QMutexData(mode), maximumSpinTime(MaximumSpinTimeThreshold), averageWaitTime(0), owner(0), count(0)
 {
     event = CreateEvent(0, FALSE, FALSE, 0);
     if (!event)