Re: [PATCH 2.6.13] lockless pagecache 2/7

Alan Cox wrote:

On Sul, 2005-09-04 at 11:01 +1000, Nick Piggin wrote:

quoted

I would be surprised if it was a big loss... but I'm assuming
a locked cmpxchg isn't outlandishly expensive. Basically:

  read_lock_irqsave(cacheline1);
  atomic_inc_return(cacheline2);
  read_unlock_irqrestore(cacheline1);

Turns into

  atomic_cmpxchg();

I'll do some microbenchmarks and get back to you. I'm quite
interested now ;) What sort of AMDs did you have in mind,

Athlon or higher give very different atomic numbers to P4. If you are
losing the read_lock/unlock then the atomic_cmpxchg should be faster on
all I agree.

Phew! I'll test them anyway, however.

One question however - atomic_foo operations are not store barriers so
you might need mb() and friends for PPC ?

Dave's documented that nicely in Documentation/atomic_ops.txt

In general, atomic ops that do not return a value are not barriers,
while operations that do return a value are.

So I think we can define the atomic_cmpxchg as providing a barrier.

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.

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