linux-kernel - Re: [PATCH] kernel/timekeeping: move xtime_cache to be in the same cache line as the lock

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Date:	Fri, 22 Jan 2010 11:10:14 +0000
From:	Richard Kennedy <richard@....demon.co.uk>
To:	john stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/timekeeping:  move xtime_cache to be in the
 same cache line as the lock

On Thu, 2010-01-21 at 09:19 -0800, john stultz wrote:

> 
> Hrm.. I'm hoping to kill off the xtime_cache at some point soon, so I'm
> not sure if this patch will do much for long. That said, I'm not opposed
> to it in the mean time, and when xtime_cache does get yanked, I'd
> appreciate similar performance review to make sure we're not regressing.
> 
OK, removing it will be even better. I can re-run the test anytime you
like, just let me know if you've got a patch that needs testing. 

> > ---
> > patch against v2.6.33-rc4
> > compiled & tested on AMD64X2 x86_64
> > 
> > 
> > BTW on 64 bit timespec is a 16 byte structure so the aligned 16 doesn't
> > do much, and on 32bit timepec is 8bytes so this just seems to spread
> > these variables across more cache lines than necessary. Any ideas what
> > this is here for?
> 
> I think it was a copy-paste from the xtime and wall_to_monotonic
> definitions, which both have the same alignment. 
Yes, that's what I thought. In that case I think we can remove all of
those attribute aligned, which should be a small improvement for 32 bit
builds.

regards
Richard



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