OpenCL performance degrades when run with Intel VTune

[Masato Nakasaka]

When running OpenCL applications on Elkhart Lake iGPU (x6425RE), we observe performance drop about 35% when run with Intel VTune GPU Compute/Media Hotspots mode.

For example, running a sgemm sample from CLBlast (github.com/CNugteren/CLBlast/samples/sgemm.cpp) results in following (modified to run 10 iterations):

$ ./clblast_sample_sgemm
Completed SGEMM in 1107.954 ms with status 0
Completed SGEMM in 0.727 ms with status 0
Completed SGEMM in 0.684 ms with status 0
Completed SGEMM in 0.900 ms with status 0
Completed SGEMM in 0.692 ms with status 0
Completed SGEMM in 0.668 ms with status 0
Completed SGEMM in 0.678 ms with status 0
Completed SGEMM in 0.668 ms with status 0
Completed SGEMM in 0.674 ms with status 0
Completed SGEMM in 0.678 ms with status 0

However if we do the same run with VTune the performance drops.

$ vtune -collect gpu-hotspots -- ./clblast_sample_sgemm
...
Completed SGEMM in 1206.028 ms with status 0
Completed SGEMM in 1.254 ms with status 0
Completed SGEMM in 1.130 ms with status 0
Completed SGEMM in 1.070 ms with status 0
Completed SGEMM in 1.035 ms with status 0
Completed SGEMM in 1.149 ms with status 0
Completed SGEMM in 1.122 ms with status 0
Completed SGEMM in 1.029 ms with status 0
Completed SGEMM in 0.984 ms with status 0
Completed SGEMM in 1.076 ms with status 0

We do not observe similar performance drop on Apollo Lake x7-E3950 with the same software setup. This may be an issue on VTune side but difficult to tell.

Environment:
- OS: Ubuntu for Intel IOT platform 20.04.4 (kernel: 5.13.0-1010-intel)
- CPU: Atom x6425RE
- VTune version: 2022.1.0
- OpenCL environment: Setup based on drivers from https://dgpu-docs.intel.com/releases/releases-20211130.html

[Ubuntu Foundations Team Bug Bot (crichton)]

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