linux-kernel - Re: [PATCH 03/19] fs: Convert nr_inodes and nr

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Message-ID: <20101017010944.GD29677@dastard>
Date:	Sun, 17 Oct 2010 12:09:44 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/19] fs: Convert nr_inodes and nr_unused to per-cpu
 counters

On Sat, Oct 16, 2010 at 07:26:48PM +0200, Peter Zijlstra wrote:
> On Sat, 2010-10-16 at 12:27 +0200, Eric Dumazet wrote:
> > 
> > In cases you mention, you want a precise count (aka
> > percpu_counter_sum_positive()), not the approximate one (aka
> > percpu_counter_read_positive()).
> > 
> > The only difference is then the possible/online cpu loop difference.
> > 
> In either case, SGI is screwed, doing millions of for_each_*_cpu() loops
> per second isn't going to work for them.
> 
> fwiw, for_each_*_cpu() takes longer than a single jiffy tick on those
> machines.

Yes, agreed. I'm not sure we need exact summation for these counters,
but I haven't wanted to bring inaccuracies into the code at this
point in time. I need to investigate the effect of using the
approximate summation values in all the cases they are used.

I don't see that there will be a problem doing this, but it's not
something that needs to be cared about right now. After we get to
the store-free path walks in place, we can revisit all these side
issues more fully.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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