Re: Information regarding the perf tool

[David Ahern <dsahern@gmail.com>]

[added Pawel, author of the patches]

On 11/4/13, 10:26 AM, Harald Servat wrote:
> On 31/10/13 19:22, David Ahern wrote:
>> On 10/31/13, 11:23 AM, Harald Servat wrote:
>>> Which is the relation between perf_clock() (or local_clock()) and
>>> clock_gettime (CLOCK_MONOTONIC, ). Are they the same? If not, is there a
>>> way to correlate them? Or alternatively, is it possible to call
>>> local_clock() from the userland?
>>
>> We still do not have a means of correlating perf_clock to time-of-day,
>> monotonic or any other clock source. Still trying.
>>
>> Pawell Moll has a couple of patches posted -- one uses an ioctl to get
>> perf_clock timestamps the other makes perf_clock accessible through
>> clock_gettime. See attached.
>>
>> David
>
> Hello,
>
>    thank you David! Those patches are great! I decided to access perf
> clock through clock_gettime because I'm more used to this call since I
> used it before.

My long term preference as well with a hook into vdso page.

>    I understand that the patches are a bit hardcoded, right? I tried to
> #include "/usr/include/linux/time.h" after #include <time.h> but they
> seem to conflict. If I just extract the interesting bits (#define
> CLOCK_PERF) it seems to work. Is there a cleaner way to use this right now?

They should just work. I seem to have misplaced my test case; I don't 
recall having problems with it.

>
>    JFTR, I have written a very small app that demonstrate the usage of
> this clock_gettime + perf (attached). It is very interesting to see that
> the difference between the monotonic and the perf clock is rather
> similar. However, as time goes by, at least in my machine, perf clock
> seems to advance at a faster pace than the monotonic clock.

that's why we would like to get a perf_clock/trace_clock time source 
exported by the kernel.

David