Date: Mon, 12 Mar 2012 14:39:26 +0200 From: Vitaly Magerya <vmagerya@gmail.com> To: Fabien Thomas <fabien.thomas@netasq.com> Cc: freebsd-hackers@freebsd.org Subject: Re: pmc(3): when are the counters updated? Message-ID: <4F5DEE7E.5030008@gmail.com> In-Reply-To: <C8C86488-254E-45B3-9636-9272DC67CD0C@netasq.com> References: <CAL409Kxng3Yzw=grvP=fvbdLrqcOG_bG_ajaO8thZEPJtk2YaQ@mail.gmail.com> <C8C86488-254E-45B3-9636-9272DC67CD0C@netasq.com>
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Fabien Thomas wrote:
>> So, what's going on here? Is this the intended behavior, or can it
>> be changed? And how do I get accurate readings?
>
> If i remember well:
> The current code will get real HW PMC if the PMC is running and attached to owner.
> The first case is not true in your code so you get the saved value which is updated at
> process switch out time.
>
> I think you can do:
> pmc_read
> pmc_stop.
Changing the code to this:
[...]
pmc_read(pmcid, &value);
pmc_stop(pmcid);
printf("reading #1: %lu\n", (unsigned long)value);
pmc_read(pmcid, &value);
printf("reading #2: %lu\n", (unsigned long)value);
sleep(1);
pmc_read(pmcid, &value);
printf("reading #3: %lu\n", (unsigned long)value);
[...]
Gives me this output:
reading #1: 15039441
reading #2: 0
reading #3: 15084037
So, it seems that you are correct; pmc_read reports current values if
the PMC is not stopped, after which it's updated on context switches.
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