Improve precision of testlib benchmarking.

20 ms is a bit too to get a precise measurement due to limited timer
resolution. We increase the limit to 50 ms and introduce a warm-up iteration
to prevent any state or cache-related issues which can be hard to account
for in the tests.

On the N900 this seems to decrease the variance of test results by 30 %,
and fixes some cases where the results were way off due to hidden state.

Task-number: QT-3390
Reviewed-by: Morten Sørvig

2 files changed, 7 insertions, 1 deletions

diff --git a/src/testlib/qbenchmarkmeasurement.cpp b/src/testlib/qbenchmarkmeasurement.cpp
index c03cbff..0a84792 100644
--- a/src/testlib/qbenchmarkmeasurement.cpp
+++ b/src/testlib/qbenchmarkmeasurement.cpp
@@ -66,7 +66,7 @@ qint64 QBenchmarkTimeMeasurer::stop()
 
 bool QBenchmarkTimeMeasurer::isMeasurementAccepted(qint64 measurement)
 {
-    return (measurement > 20);
+    return (measurement > 50);
 }
 
 int QBenchmarkTimeMeasurer::adjustIterationCount(int suggestion)
@@ -74,6 +74,11 @@ int QBenchmarkTimeMeasurer::adjustIterationCount(int suggestion)
     return suggestion; 
 }
 
+bool QBenchmarkTimeMeasurer::needsWarmupIteration()
+{
+    return true;
+}
+
 int QBenchmarkTimeMeasurer::adjustMedianCount(int)
 { 
     return 1; 
diff --git a/src/testlib/qbenchmarkmeasurement_p.h b/src/testlib/qbenchmarkmeasurement_p.h
index 8ad3613..932852c 100644
--- a/src/testlib/qbenchmarkmeasurement_p.h
+++ b/src/testlib/qbenchmarkmeasurement_p.h
@@ -84,6 +84,7 @@ public:
     bool isMeasurementAccepted(qint64 measurement);
     int adjustIterationCount(int sugestion);
     int adjustMedianCount(int suggestion);
+    bool needsWarmupIteration();
     QTest::QBenchmarkMetric metricType();
 private:
     QTime time;