linux-kernel - Re: [-mm patch] x86_64 GTOD: offer scalable vgettimeofday II

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Date:	Thu, 1 Feb 2007 13:17:45 +0100
From:	Andi Kleen <ak@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	jbohac@...e.cz, linux-kernel@...r.kernel.org,
	Vojtech Pavlik <vojtech@...e.cz>, arjan@...radead.org,
	tglx@...utronix.de, johnstul@...ibm.com,
	Andrew Morton <akpm@...l.org>
Subject: Re: [-mm patch] x86_64 GTOD: offer scalable vgettimeofday II

On Thursday 01 February 2007 12:46, Ingo Molnar wrote:
> 
> * jbohac@...e.cz <jbohac@...e.cz> wrote:
> 
> > Inter-CPU monotonicity can not, however, be guaranteed in a vsyscall, 
> > so vsyscall is not used by default. [...]
> 
> note that this is not actually the case. My patch below, ontop of -mm, 
> implements a fully monotonic gettimeofday as an optional vsyscall 
> feature.
> 
> The 'price' paid for it is lower resolution - but it's still good for 
> those benchmarking TPC-C runs - and /alot/ simpler. 

BTW another comment: I was told that at least one of the big databases
wants ms resolution here. So to make your scheme work
would require a HZ=1024 regular interrupt. But that would also make
everything slower again due to CPU overhead as it was learned in the 2.4->2.6 HZ 
transition. 

So it might not actually be worth it.

-Andi

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