Messages in this thread | | | Date | Mon, 14 Mar 2005 10:42:59 -0800 | From | "Martin J. Bligh" <> | Subject | Re: [PATCH] Prefaulting |
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--On Saturday, March 12, 2005 10:10:36 +0100 Arjan van de Ven <arjan@infradead.org> wrote:
> >> > From a quick peek it seems that the patch makes negligible difference for a >> kernel compilation when prefaulting 1-2 pages and slows the workload down >> quite a lot when prefaulting up to 16 pages. > > well the last time I saw prefaulting experiments (Ingo was involved > iirc) the problem was that the hitrate for the prefaults was such that > the costs for tearing down the extra redundant rmap chains was more > expensive than taking the "extra" faults. It seems linux has pretty > cheap faulting logic invalidating some of traditional OS assumptions...
Yes - is pretty much exactly what I saw with this too, even if we only prefaulted up to 4 pages that were already in the pagecache. The additional cost in zap_pte_rage etc killed us, and it was wholly detrimental.
I think we need better locality in glibc, etc before this is of much use. I had a debug patch somewhere to show how sparse the layout of ptes was, I'll see if I can find it.
M.
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