topic The required approach depends in Software Tuning, Performance Optimization & Platform Monitoring

Handling Overflows in Programmable Counters

Ravitheja_G_ — Wed, 14 Jun 2017 15:31:58 GMT

Hi Guys,

I am new to Programmable counters and i need some advice with handling overflows , i wrote a small kernel module to collect stats for each thread running on the cores and it is working well, i need some advice on how i can handle when the counters overflow.

I see that i can Performance Monitoring Interrupts when the overflow happens , it would be helpful if someone can explain how i use these interrupts to reset the counters the counters to zero once they reach a certain value so i can avoid overflows and get accurate readings

Thanks and Regards,
Ravi

The required approach depends

McCalpinJohn — Mon, 19 Jun 2017 15:30:54 GMT

The required approach depends on what you are trying to measure.

If you want measurements by core for any running process, and you can guarantee that the counters cannot increment more than pow(2,48) times between measurements, then you simply disable overflows (by clearing bit 20 in each of the IA32_PERFEVTSEL MSRs) and leave the counters running free. If a measurement looks smaller than a previous measurement, then add pow(2,48) to the difference to correct for the wrap-around.

Thank you , That helps a lot

Ravitheja_G_ — Mon, 19 Jun 2017 16:04:40 GMT

Thank you , That helps a lot