Re: [PATCH] aio: optimize free kioctx slot search

From: Matthew Wilcox
Date: Fri Dec 27 2013 - 07:33:27 EST

On Fri, Dec 27, 2013 at 06:32:24PM +0800, Gu Zheng wrote:
> Hi Matthew,
> On 12/26/2013 10:13 PM, Matthew Wilcox wrote:
> > On Thu, Dec 26, 2013 at 06:22:10PM +0800, Gu Zheng wrote:
> >> to the new free slot if it's smaller than 'next_free'. So that we can
> >> ensure that the slots before 'next_free' are all used, and the search can start
> >> from 'next_free' to reduce the search and improve the performance.
> >
> > Have you benchmarked it? What kinds of improvements are you seeing?
> Not yet. But the improvement is obvious, we can always reduce the search region and
> improve the hit rate when trying to find a free slot with this change.

It's not so obvious to me. You've added extra stores, which may dirty
an additional cacheline. That may end up costing more than it saves.
I suspect the difference is lost in the noise, but I've been wrong about
these things before.

Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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