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Re: [processor frequency]

To: Wolfgang Denk <>
Subject: Re: [processor frequency]
From: "Kevin D. Kissell" <>
Date: Mon, 09 Jan 2006 22:53:37 +0100
Cc: Sathesh Babu Edara <>,
In-reply-to: <>
Original-recipient: rfc822;
References: <>
User-agent: Thunderbird 1.5 (X11/20051025)
Wolfgang Denk wrote:
In message <005a01c614fb$2fe76b00$10eca8c0@grendel> you wrote:
There is no "ideal" value for a given processor frequency.
The lower the value, the less interrupt processing overhead,
but the slower the response time to events that are detected
or serviced during clock interrupts. 1000 HZ *may* be a sensible
value (I have my doubts, personally) for 2+ GHz PC processors, but it's excessive (IMHO) for a 200MHz processor and unworkable for a 20MHz CPU. I think that 100HZ is still a reasonable value
for an embedded RISC CPU, but the "ideal" value is going to
be a function of the application.

We did some tests of the performance impact of 100 vs. 1000 Hz  clock
frequency on low end systems (50 MHz PowerPC); for details please see

My own results, on an SMP 2.6 kernel (as opposed to the uniprocessor
2.4 kernel used for the experiments reported) have been rather different.
Certainly the degradations I observed were far worse than the 5-10% reported
in the document you cite.  I'll try to repeat your experiment when I get
the time.

BTW, I'm puzzled by the "context switch" benchmark test results.  By what
mechanism - or by what definition of "context switch" - can having more
frequent interrupts make context switches happen more quickly?  It seems
to me that those results must be due to a systematic measurement error
being added/removed.


                Kevin K

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