Thanks for the answer Michael.
I just created an account to be able to edit the perf wiki page. Before
doing that, I am asking here if someone knows the policy to update this
wiki or who are the maintainers to ask them ?
Thanks,
Manu
On 07/22/2013 06:19 AM, Michael Ellerman wrote:
On Tue, 2013-07-16 at 17:29 +0200, Manuel Selva wrote:
Hi,
My question regards a platform equipped with 2 Intel Xeon X5650.
According to the perf wiki page
(https://perf.wiki.kernel.org/index.php/Tutorial), "by default perf stat
counts for all threads of the process and subsequent child processes and
threads" and "By default, perf stat counts in per-thread mode".
So a first question is what is the default: per thread or per process ?
It's per process, which is as described in the first quote above. The
second quote is just wrong AFAICS.
Then, independently of the answer, I am wondering how does perf handles
per thread or per process regarding the scheduler and migrations. I
didn't find it explicitly in the Intel documentation but it seems
natural that hardware performance counters located on a given core are
only capable of counting event on this core and not on other cores. Is
it true ?
Moreover, the wiki page says that "When a thread migrated from one
processor to another, counters are saved on the current processor and
are restored on the new one" (this seems to confirm the answer to my
previous question above). It means that the scheduler is aware about
"perf" or that perf is able to register a hook into the scheduler. So I
guess this is done in the kernel part of perf (in the implementation of
the perf_event_open system call) and not in the user land part, is it true ?
Yes.
cheers
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