Messages in this thread |  | | | Date | Tue, 26 Feb 2002 16:33:23 -0700 | | From | Andreas Dilger <> | | 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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