lkml.org 
[lkml]   [2005]   [Jul]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH] i386: Selectable Frequency of the Timer Interrupt
    From
    Date
    On Mon, 2005-07-11 at 17:38 -0700, George Anzinger wrote:
    > Martin J. Bligh wrote:
    > >>>Lots of people have switched from 2.4 to 2.6 (100 Hz to 1000 Hz) with no impact in
    > >>>stability, AFAIK. (I only remember some weird warning about HZ with debian woody's
    > >>>ps).
    > >>>
    > >>
    > >>Yes, that's called "progress" so no one complained. Going back is
    > >>called a "regression". People don't like those as much.
    > >
    > >
    > > That's a very subjective viewpoint. Realize that this is a balancing
    > > act between latency and overhead ... and you're firmly only looking
    > > at one side of the argument, instead of taking a compromise in the
    > > middle ...
    > >
    > > If I start arguing for 100HZ on the grounds that it's much more efficient,
    > > will that make 250/300 look much better to you? ;-)
    >
    > I would like to interject an addition data point, and I will NOT be subjective.
    > The nature of the PIT is that it can _hit_ some frequencies better than
    > others. We have had complaints about repeating timers not keeping good time.
    > These are not jitter issues, but drift issues. The standard says we may not
    > return early from a timer so any timer will either be on time or late. The
    > amount of lateness depends very much on the HZ value. Here is what the values
    > are for the standard CLOCK_TICK_RATE:
    >
    > HZ TICK RATE jiffie(ns) second(ns) error (ppbillion)
    > 100 1193180 10000000 1000000000 0
    > 200 1193180 5000098 1000019600 19600
    > 250 1193180 4000250 1000062500 62500
    > 500 1193180 1999703 1001851203 1851203
    > 1000 1193180 999848 1000847848 847848
    >
    > The jiffie values here are exactly what the kernel uses and are based on the
    > best one can do with the PIT hardware.
    >
    > I am not suggesting any given default HZ, but rather an argumentation of the
    > help text that goes with it. For those who want timers to repeat at one second
    > (or multiples there of) this is useful info.
    >
    > For you enjoyment I have attached the program used to print this. It allows you
    > to try additional values...

    If I recall, 1001 was a decent choice and is relatively close the the
    expected frequency. Also I think the error is positive instead of
    negative, so it avoids the "jiffies are shorter then I expected!"
    issues.

    >From your program's output:
    HZ TICK RATE jiffie(ns) second(ns) error (ppbillion)
    1001 1193180 999013 1000012013 12013

    thanks
    -john


    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2005-07-12 20:05    [W:4.043 / U:0.224 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site