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Date: Tue, 26 Feb 2002 16:33:23 -0700
From: Andreas Dilger <adilger@turbolabs.com>
To: george anzinger <george@mvista.com>
Cc: Tim Schmielau <tim@physik3.uni-rostock.de>, linux-kernel@vger.kernel.org
Subject: Re: [patch][rfc] enable uptime display > 497 days on 32 bit
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On Feb 26, 2002  15:13 -0800, george anzinger wrote:
> Andreas Dilger wrote:
> > Do you think that doing a 64-bit add-with-carry to memory on each
> > timer interrupt and doing multiple volatile reads is faster than
> > doing a spinlock with an optional 32-bit increment?  
> 
> I think the memory cycle is "almost" free as we are also updating
> jiffies which is in the same cache line, so, yes, in the overall scheme
> of things the overhead of the additional add-with-carry is very small. 
> On the read side of things, the issue is not so much the lock, but the
> irq nature of it.  This will be VERY long, much longer than the double
> load of the high order bits, again from the same cache line.

I was wondering about that myself when looking at the code again.  I'm
not quite sure why we need to use the irq spinlock, since we already
make a local copy of jiffies so another timer IRQ changing the jiffies
value shouldn't affect the return value of get_jiffies64().  Then again,
that isn't exactly stuff I'm familiar with, so I could be totally
off-base here.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/

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