Re: TSC instead of ACPI: powerd doesn't work anymore (to be expected?)

From: Robert Watson (rwatson_at_FreeBSD.org)
Date: 10/31/05

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    Date: Mon, 31 Oct 2005 10:53:45 +0000 (GMT)
To: David Xu <davidxu@freebsd.org>

    On Mon, 31 Oct 2005, David Xu wrote:

    > In thread program, we have to use clock_gettime, for example a thread
    > wants to wait for condition variable for two seconds, it has to:
    >
    > struct timespec ts;
    >
    > clock_gettime(CLOCK_REALTIME, &ts);
    > ts.tv_sec += 2;
    > pthread_cond_wait(&cond, &mtx);
    >
    > problem is who really cares time precise? how many people are really
    > handling realtime critical tasks? this is really an unpleasant side
    > effect that a simple syscall can stall cpu.

    The questions are: how good is good enough? And for what?

    The problem here is that while pthread_cond_wait() can afford to accept
    low resolution, the same clock retrieval interface is also used for things
    like scientific measurement. So it sounds like the real mistake was the
    specification for the thread API: while timespec is the New Way for
    time-related interfaces, this approach has coalesced precision time
    measurement and "how long to sleep for" time measurement. Moving to HZ
    incrementing of the clock probably exceeds the quality reduction that
    makes sense for clock_gettime(), but moving to a roughly sync'd TSC clock
    (such as the Linux approach) probably doesn't.

    Robert N M Watson
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