Performance variability

I am observing a lot of performance variability for matrix multiplication in sizes ranging from ~100 to ~1000 I have been investigating this without a lot of success. The timing can be up to twice as large, depending on the order in which I run the benchmarks. However, for any chosen order, the accuracy of the timing is high.

I am a bit at a loss here. Because the order only influences the position in memory of the input data to the benchmarks, I am inclined to think that this may be a memory alignment issue. The position of the matrices in the heap is rounded only to 16 bytes (double floats, alignment imposed by C++), but not to any other size and perhaps there is some kind of SIMD code making some kind of ugly magic that disturbs my benchmarks?

I have tested this with serial, openmp and pthreads versions, on Linux and on Windows, with similar outcomes. This is an AMD Ryzen processor, but I have witnessed even greater variability on Intel.

Es werden keine Dateien, Tests oder Einstiegspunkte genannt. Beginne damit, die Zeiten für Matrixmultiplikation für Größen im Bereich von etwa 100–1000 in den Varianten serial, OpenMP und pthreads auf den berichteten Plattformen und Prozessortypen zu reproduzieren. Als erledigt gilt die Identifizierung der Ursache der reihenfolgeabhängigen Variabilität und die Dokumentation einer reproduzierbaren Behebung oder einer bestätigten Ursache.