linux-kernel - Re: Some interesting observations when trying to optimize vmstat handling

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Message-Id: <20071108.152408.157342087.davem@davemloft.net>
Date:	Thu, 08 Nov 2007 15:24:08 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	clameter@....com
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, ak@...e.de,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: Some interesting observations when trying to optimize vmstat
 handling

From: Christoph Lameter <clameter@....com>
Date: Thu, 8 Nov 2007 11:58:58 -0800 (PST)

> The problem with cmpxchg_local here is that the differential has to
> be read before we execute the cmpxchg_local. So the cacheline is
> acquired first in read mode and then made exclusive on executing the
> cmpxchg_local.

I bet this can be defeated by prefetching for a write before
the read, but of course this won't help if the read is
being used to conditionally avoid the cmpxchg_local but I don't
think that's what you're trying to do here.

I've always wanted to add a write prefetch at the beginning of all of
the sparc64 atomic operation primitives because of this problem.
I just never got around to measuring if it's worthwhile or not.
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