Re: Does oprofile suffer from aliasing?

On 2015-04-29, William Cohen <[email protected]> wrote:

> So the issue is the over/under sampling because the top is sampling
> on intervals that pretty much match up with the timers used by the
> threads?

Or multiples thereof.  Or timers that share harmonics with the
sampling frequency.  Or any consistent correlation with when different
things run.

[To correct my earlier mis-statement, it's not top that's doing the
sampling, it's really the kernel itself which does the sampling every
timer tick.  Top just periodically harvests the sampled info from
/proc/<whatever> and compiles it.]

In order for sampling to be meaningful it has to be faster by an order
of magnitude or three than the things you're measuring (which are
probably running for anywhere from a hundred microseconds up to maybe
a millisecond or two) or (for things like CPU usage for long-lived
programs) the sampling has to be uncorrelated with the occurances of
what you're measuring.  For what I'm doing a random sampling trigger
with a uniform distribution of maybe 10-100ms would be fine.

> The linux 2.6.22 kernel does not provide the kernel perf support, so
> you will not be able to use operf, only the opcontrol will be usable
> with this kernel.

I can update to a newer kernel if needed, but I don't think it will
help.

> Looking at new kernel linux 3.19 perf
> (http://lxr.linux.no/#linux+v3.19.1/arch/arm/kernel/perf_event_cpu.c)
> it doesn't seem to be armv5 support in there either.

Agreed.

> Unfortunately, it looks like timer support is going to be the only
> thing available unless you modify xscale support to provide arm926
> (armv5) support.

Except that xscale and arm926 are completely different hardware
implementations.  The only thing they share is an instruciton set. 
Early versions of the xscale had a core by Intel that implemented the
ARMv5 instuction set.  The ARM926 has a core by ARM that also
implements the ARMv5 instruction set.  But they are otherwise
unrelated and not based on a single common gate (and certainly don't
have any peripherals like PMUs in common).

AFAICT the ARM926 core doesn't have a PMU at all -- I can find no
mention of one in either ARM docs for the ARM926 or Atmel docs for the
at91 parts.

So, I'm not sure how one would go about porting code from xscale or
any other architecture to the ARM926.

Is there such a thing as software-only PMU support?

-- 
Grant Edwards               grant.b.edwards        Yow! Did you move a lot of
                                  at               KOREAN STEAK KNIVES this
                              gmail.com            trip, Dingy?


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