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SubjectRe: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO


On Fri, Dec 19, 2014 at 11:48 AM, Andy Lutomirski <luto@amacapital.net>
wrote:
> On Fri, Dec 19, 2014 at 3:23 AM, Peter Zijlstra
> <peterz@infradead.org> wrote:
>> On Thu, Dec 18, 2014 at 04:22:59PM -0800, Andy Lutomirski wrote:
>>> Bad news: this patch is incorrect, I think. Take a look at
>>> update_rq_clock -- it does fancy things involving irq time and
>>> paravirt steal time. So this patch could result in extremely
>>> non-monotonic results.
>>
>> Yeah, I'm not sure how (and if) we could make all that work :/
>
> I obviously can't comment on what Facebook needs, but if I were
> rigging something up to profile my own code*, I'd want a count of
> elapsed time, including user, system, and probably interrupt as well.
> I would probably not want to count time during which I'm not
> scheduled, and I would also probably not want to count steal time.
> The latter makes any implementation kind of nasty.
>
> The API presumably doesn't need to be any particular clock id for
> clock_gettime, and it may not even need to be clock_gettime at all.
>
> Is perf self-monitoring good enough for this? If not, can we make it
> good enough?
>
> * I do this today using CLOCK_MONOTONIC

The clock_gettime calls are used for a wide variety of things, but
usually they are trying to instrument how much CPU the application is
using. So for example with the HHVM interpreter they have a ratio of
the number of hhvm instructions they were able to execute in N seconds
of cputime. This gets used to optimize the HHVM implementation and can
be used as a push blocking counter (code can't go in if it makes it
slower).

Wall time isn't a great representation of this because it includes
factors that might be outside a given HHVM patch, but it sounds like
we're saying almost the same thing.

I'm not familiar with the perf self monitoring?

-chris






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