Re: Timers and timing, was: MySQL Performance 6.0rc1

From: Peter Jeremy (PeterJeremy_at_optushome.com.au)
Date: 10/28/05

  • Next message: Colin Percival: "Re: TSC instead of ACPI: powerd doesn't work anymore (to be expected?)" 
    Date: Fri, 28 Oct 2005 19:34:02 +1000
To: Poul-Henning Kamp <phk@phk.freebsd.dk>

    On Fri, 2005-Oct-28 00:35:24 +0200, Poul-Henning Kamp wrote:
    > * Does threads have to return ordered timestamps ?
    >
    > Consider:
    >
    > CPU1 CPU2
    >
    > gettimeofday(t1)
    > gettimeofday(t2)
    > gettimeofday(t3)
    ...
    >
    > [t1 <= t2 <= t3]

    If this reflects kernel threads, unless there's some sort of explicit
    synchronization between the two threads, there's no need to guarantee
    anything better than t1 <[=] t3. If the threads are explicitly
    synchronised, then we should guarantee t1 <= t2 <= t3.

    On Thu, 2005-Oct-27 19:50:45 -0400, Chuck Swiger wrote:
    >I believe Darwin keeps the timecounters of the system exposed on a common
    >page mapped via the System framework (their libc+libm), which gets mapped
    >in once by init, and then shared with all of it's children copy-on-write.
    >They are using the PowerPC timebase registers according to a thread on the
    >darwin-kernel list.

    SunOS 4.x (I'm not sure about SunOS 5.x) did this as well. As Poul
    pointed out, this is much easier with sane hardware.

    FreeBSD already supports a variety of different timecounters with
    different levels of accuracy/performance/guarantees. One problem is
    that this is a system-wide knob whereas different applications may
    have different requirements. Whilst it's reasonable for MySQL to
    gather performance statistics, it only needs to measure short time
    periods and doesn't require extreme accuracy - the TSC would
    probably be good enough on a UP system (if there was a quick way
    to measure the current TSC tick rate). I don't think there's
    any way on i386 to quickly access a timecounter that returns a
    synchronised time across multiple CPUs. 

    -- 
Peter Jeremy
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