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Date: Mon, 12 Mar 2012 14:39:26 +0200
From: Vitaly Magerya <vmagerya@gmail.com>
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To: Fabien Thomas <fabien.thomas@netasq.com>
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Cc: freebsd-hackers@freebsd.org
Subject: Re: pmc(3): when are the counters updated?
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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.