Re: HZ = 1000 slows down application

From: Luigi Rizzo (rizzo_at_icir.org)
Date: 09/22/03

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    Date: Mon, 22 Sep 2003 14:22:02 -0700
To: "akanwar@digitarchy.com" <akanwar@digitarchy.com>

    On Mon, Sep 22, 2003 at 02:43:40PM -0400, akanwar@digitarchy.com wrote:
    ...
    > But now I noticed that my application is occassionally doing slower
    > iterations. Average iteration time used to be 0.2 ms without polling
    > enabled. With the device polling changes, the average time is still around
    > the same, but once every few minutes the application sees iterations that
    > are 3.3 seconds (*seconds*, not a typo) long.

    most likely your application makes some assumptions on the duration of
    a clock tick and then it gets confused when, say, a select returns
    quicker, or some time difference becomes negative, etc. etc. because
    of the finer granularity.

    very common type of bug.

            cheers
            luigi

    > This seems to happen as soon as I use the kernel with HZ=1000. Enabling or
    > disabling device polling does not seem to make any difference to this
    > behavior. I am trying to understand why there seem to be a few really long
    > iterations. Could it happen that the application does not get any CPU for
    > that long? Seems very counter intuitive that higher HZ should cause this.
    >
    > Could anyone shed any light on what is happening ?
    >
    > Thanks,
    > -ansh
    >
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