PAPI Forum

Hello,

I would like to understand if/how PAPI+perfctr is different from vtune and what are the advantages of using it over vtune? (I have a linux x86_64 system)

I downloaded and installed PAPI (but have not yet gotten to patching perfctr). The included documentation and other information that I found on the website do not help me in understanding the above.

Many Thanks!

Vtune measures performance events across the entire cpu. PAPI measures performance events from the perspective of your application. For example, PAPI can answer the question: "How many FLOPS occured in this subroutine.
If your linux kernel is more recent than 2.6.32, you don't need to patch it. Just configure and make. There's built-in performance counter support.

I read some about PAPI virtualizing the h/w/counters on a per process basis. I do not yet know what this really means. Is there some overhead to doing so?

vtune also has a api that allows you to start and stop counter measurements in your code (so for eg you could start it everytime you start a subroutine and stop at the end).

Thanks.

Basically, every time there's a context switch, the counters are saved along with all the other state. This does imply a small additional overhead during context switch, but it's negligible and not attributed to your application. Vtune may now be able to start and stop counters, but I believe it still counts system-wide. Although that, too, may have changed with the introduction of counter support within the kernel. If you can guarantee pretty much exclusive access to a core, the counts shouldn't be terribly different.