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[ace-users] Timer granularity on Linux

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ZXERASOGE002, Ext

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Dec 19, 2006, 11:05:44 AM12/19/06
to ace-...@cse.wustl.edu, gerald...@artal.fr

Hi,
I just tested timers with a ace Reactor with value of the order of 10 or 1 ms. I realized that for a value of 10ms, I am called back at a rate that is very close to 20ms. And for a value of 1ms, it's closer to 11ms.

100 ms is working fine, by the way.

Is there any reason to that, except that the OS can't handle such low values ?

I'm using ACE 5.5 on a Red Hat EL 4 on a x86_64 running Linux 2.6.9 kernel.

Best regards,
Philippe

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Matthew Gillen

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Dec 19, 2006, 11:23:15 AM12/19/06
to ZXERASOGE002, Ext, ace-...@cse.wustl.edu, gerald...@artal.fr
Philippe,

ZXERASOGE002, Ext wrote:
> Hi,
> I just tested timers with a ace Reactor with value of the order of 10 or
> 1 ms. I realized that for a value of 10ms, I am called back at a rate
> that is very close to 20ms. And for a value of 1ms, it's closer to 11ms.
>
> 100 ms is working fine, by the way.
>
> Is there any reason to that, except that the OS can't handle such low
> values ?
>
> I'm using ACE 5.5 on a Red Hat EL 4 on a x86_64 running Linux 2.6.9 kernel.

There are many possible causes for what you're seeing. The most important one
is OS scheduling: stock linux, and especially kernel versions prior to 2.6.18,
are /not/ real-time operating systems. Looking for consistent behavior from
the scheduler on the 10's of millisecond level of granularity or lower is
probably futile.

There are many "real-time" versions of the linux kernel (some proprietary,
some free) that you may want to investigate if you need to schedule something
every 1ms.

Matt

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