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Date: Tue, 10 Apr 2001 14:02:02 +0200
From: Andi Kleen <ak@suse.de>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Andi Kleen <ak@suse.de>, Mark Salisbury <mbs@mc.com>,
        Jeff Dike <jdike@karaya.com>, schwidefsky@de.ibm.com,
        linux-kernel@vger.kernel.org
Subject: Re: No 100 HZ timer !
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In-Reply-To: <20010410075109.A9549@gruyere.muc.suse.de> <E14mw9t-00041m-00@the-village.bc.nu>
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In-Reply-To: <E14mw9t-00041m-00@the-village.bc.nu>; from alan@lxorguk.ukuu.org.uk on Tue, Apr 10, 2001 at 12:18:03PM +0100
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On Tue, Apr 10, 2001 at 12:18:03PM +0100, Alan Cox wrote:
> > > interrupting decrementer.  (i.e just about any modern chip)
> > 
> > Just how would you do kernel/user CPU time accounting then ?  It's currently done 
> > on every timer tick, and doing it less often would make it useless.
> 
> On the contrary doing it less often but at the right time massively improves
> its accuracy. You do it on reschedule. An rdtsc instruction is cheap and all
> of a sudden you have nearly cycle accurate accounting

Does not sound very attractive all at all on non virtual machines (I see the point on
UML/VM):
making system entry/context switch/interrupts slower, making add_timer slower, just to 
process a few less timer interrupts. That's like robbing the fast paths for a slow path.

-Andi
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