Store and track spin times in nanosecond resolution

Use the new QElapsedTimer::nsecsElapsed() and store all values in qint64
instead of int.

The maximum spin time threshold is now 1000000ns, or 1ms. Spinning for longer
than 1ms is just a waste of time, CPU, and battery.

Reviewed-by: joao

2 files changed, 8 insertions, 7 deletions

diff --git a/src/corelib/thread/qmutex.cpp b/src/corelib/thread/qmutex.cpp
index 72f87b7..03e623f 100644
--- a/src/corelib/thread/qmutex.cpp
+++ b/src/corelib/thread/qmutex.cpp
@@ -443,16 +443,17 @@ void QMutex::lockInternal()
     QElapsedTimer elapsedTimer;
     elapsedTimer.start();
     do {
-        if (elapsedTimer.hasExpired(d->maximumSpinTime)) {
+        qint64 spinTime = elapsedTimer.nsecsElapsed();
+        if (spinTime > d->maximumSpinTime) {
             // didn't get the lock, wait for it, since we're not going to gain anything by spinning more
-            int spinTime = elapsedTimer.restart();
+            elapsedTimer.start();
             bool isLocked = d->wait();
             Q_ASSERT_X(isLocked, "QMutex::lock",
                        "Internal error, infinite wait has timed out.");
             Q_UNUSED(isLocked);
 
-            int maximumSpinTime = d->maximumSpinTime;
-            int waitTime = elapsedTimer.elapsed();
+            qint64 maximumSpinTime = d->maximumSpinTime;
+            qint64 waitTime = elapsedTimer.nsecsElapsed();
             // adjust the spin count when spinning does not benefit contention performance
             if (spinTime + waitTime > QMutexPrivate::MaximumSpinTimeThreshold) {
                 // long waits, stop spinning
diff --git a/src/corelib/thread/qmutex_p.h b/src/corelib/thread/qmutex_p.h
index 6de42ad..fa6e879 100644
--- a/src/corelib/thread/qmutex_p.h
+++ b/src/corelib/thread/qmutex_p.h
@@ -72,9 +72,9 @@ public:
     bool wait(int timeout = -1);
     void wakeUp();
 
-    // half of a frame (in ms) at 60fps
-    enum { MaximumSpinTimeThreshold = 8 };
-    volatile int maximumSpinTime;
+    // 1ms = 1000000ns
+    enum { MaximumSpinTimeThreshold = 1000000 };
+    volatile qint64 maximumSpinTime;
     Qt::HANDLE owner;
     uint count;