Re: [PATCH] Prefaulting

From: Martin J. Bligh
Date: Mon Mar 14 2005 - 13:51:04 EST

--On Saturday, March 12, 2005 10:10:36 +0100 Arjan van de Ven <arjan@xxxxxxxxxxxxx> 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.


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