From: Andi Kleen <ak@suse.de>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Subject: Re: [PATCH] use local_t for page statistics
Date: Sat, 7 Jan 2006 04:25:24 +0100
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Cc: Andrew Morton <akpm@osdl.org>, Benjamin LaHaise <bcrl@kvack.org>,
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On Saturday 07 January 2006 04:19, Nick Piggin wrote:
> Andi Kleen wrote:
> > On Saturday 07 January 2006 03:52, Nick Piggin wrote:
> > 
> > 
> >>No. On many load/store architectures there is no good way to do local_t,
> >>so something like ppc32 or ia64 just uses all atomic operations for
> > 
> > 
> > well, they're just broken and need to be fixed to not do that.
> > 
> 
> How?

If anything use the 3x duplicated data setup, not atomic operations.

> 
> > Also I bet with some tricks a seqlock like setup could be made to work.
> > 
> 
> I asked you how before. If you can come up with a way then it indeed
> might be a good solution... 

I'll try to work something up.

> The problem I see with seqlock is that it 
> is only fast in the read path. That path is not the issue here.

The common case - not getting interrupted would be fast.

> > 
> >>local_t, and ppc64 uses 3 counters per-cpu thus tripling the cache
> >>footprint.
> > 
> > 
> > and ppc64 has big caches so this also shouldn't be a problem.
> > 
> 
> Well it is even less of a problem for them now, by about 1/3.
> 
> Performance-wise there is really no benefit for even i386 or x86-64
> to move to local_t now either so I don't see what the fuss is about.

Actually P4 doesn't like CLI/STI. For AMD and P-M it's not that much an issue,
but NetBurst really doesn't like it.

-Andi

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