Date: Mon, 12 Mar 2012 10:03:35 +0100 From: Fabien Thomas <fabien.thomas@netasq.com> To: Vitaly Magerya <vmagerya@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: pmc(3): when are the counters updated? Message-ID: <C8C86488-254E-45B3-9636-9272DC67CD0C@netasq.com> In-Reply-To: <CAL409Kxng3Yzw=grvP=fvbdLrqcOG_bG_ajaO8thZEPJtk2YaQ@mail.gmail.com> References: <CAL409Kxng3Yzw=grvP=fvbdLrqcOG_bG_ajaO8thZEPJtk2YaQ@mail.gmail.com>
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Le 11 mars 2012 =E0 16:45, Vitaly Magerya a =E9crit :
> Hi, folks. I'm trying to use pmc(3) to analyze code fragments, and
> I've run into strange behavior: the counter values returned by
> pmc_read(3) sometimes show no increment between readings, but are
> updated a second later; even if the PMC in question was stopped
> before.
>=20
> Here's a test program:
>=20
> #include <sys/types.h>
> #include <pmc.h>
> #include <stdio.h>
> #include <unistd.h>
>=20
> int main() {
> pmc_id_t pmcid;
> pmc_value_t value;
> int i;
>=20
> pmc_init();
> pmc_allocate("instructions", PMC_MODE_TC, 0, PMC_CPU_ANY, =
&pmcid);
> pmc_start(pmcid);
> for (i =3D 0; i < 5000000; i++);
> pmc_stop(pmcid);
> pmc_read(pmcid, &value);
> printf("first reading: %lu\n", (unsigned long)value);
> sleep(1);
> pmc_read(pmcid, &value);
> printf("second reading: %lu\n", (unsigned long)value);
> }
>=20
> It's output on my system (FreeBSD 8.2 amd64, an Intel Atom processor)
> is something like this:
>=20
> first reading: 0
> second reading: 15090110
>=20
> I don't really like both numbers; I expect the first reading not
> to be zero (there obviously are instructions between pmc_start and
> pmc_stop), and I expect the second reading not to differ from the
> first, as the PMC was stopped before both of them, so it's value
> should not change.
>=20
> 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.
>=20
> (BTW, is this the right list for such questions?)
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