Re: timer counter chip access mystery]

From: Jin Guojun [DSD] (j_guojun_at_lbl.gov)
Date: 07/17/03

  • Next message: Terry Lambert: "Re: Major problem with "No buffer space available" errors" 
    Date: Wed, 16 Jul 2003 16:22:58 -0700
To: Petri Helenius <pete@he.iki.fi>

    5.1 defaults to use ACPI-safe, which calls read_counter() at least
    three times. The total cost is 3098 ns. So, it meas that read_count()
    cost is about 1000 ns, which is close to what the Linux does.

    The rest question is how much error will be if not to use
    ACPI-safe? i.e., sysctl kern.timecounter.hardware=ACPI?

        -Jin

    /*
     * Fetch current time value from reliable hardware.
     */
    static unsigned
    acpi_timer_get_timecount(struct timecounter *tc)
    {
        return (read_counter());
    }

    /*
     * Fetch current time value from hardware that may not correctly
     * latch the counter.
     */
    static unsigned
    acpi_timer_get_timecount_safe(struct timecounter *tc)
    {
        unsigned u1, u2, u3;

        u2 = read_counter();
        u3 = read_counter();
        do {
            u1 = u2;
            u2 = u3;
            u3 = read_counter();
        } while (u1 > u2 || u2 > u3 || (u3 - u1) > 15);
        return (u2);
    }

    Petri Helenius wrote:

    > 5.x defaults to ACPI or TSC timers. So your wish has been transported
    > by a timemachine into the past.
    >
    > With 4.x you have to set it manually.
    >
    > Pete
    >
    > ----- Original Message -----
    > From: "Steven Hartland" <killing@barrysworld.com>
    > To: "Petri Helenius" <pete@he.iki.fi>; "Jin Guojun [DSD]" <j_guojun@lbl.gov>; <freebsd-performance@freebsd.org>
    > Sent: Thursday, July 17, 2003 12:57 AM
    > Subject: Re: timer counter chip access mystery]
    >
    > > If this could be changed / fixed it would have a marked performance increase
    > > for all game server code which use this function heavily from what I've seen
    > > and would explain the sometimes huge performance differential between
    > > FreeBSD and Linux.
    > >
    > > Steve / K
    > > ----- Original Message -----
    > > From: "Petri Helenius" <pete@he.iki.fi>
    > > To: "Jin Guojun [DSD]" <j_guojun@lbl.gov>; <freebsd-performance@freebsd.org>
    > > Sent: Wednesday, July 16, 2003 10:44 PM
    > > Subject: Re: timer counter chip access mystery]
    > >
    > >
    > > >
    > > > This access happens over ISA bus and thus happens at the speed the bus
    > > > operates at. Use TSC or ACPI for faster gettimeofday.
    > > >
    > > > Pete

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