From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <48D2BFB8.6010503@redhat.com> Date: Thu, 18 Sep 2008 16:53:12 -0400 From: Chris Snook MIME-Version: 1.0 Subject: Re: Populating multiple ptes at fault time References: <48D142B2.3040607@goop.org> <48D1625C.7000309@redhat.com> <48D17A93.4000803@goop.org> <48D29AFB.5070409@linux-foundation.org> <48D2A392.6010308@goop.org> <33307c790809181352h14f2cf26kc73de75b939177b5@mail.gmail.com> In-Reply-To: <33307c790809181352h14f2cf26kc73de75b939177b5@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Martin Bligh Cc: Jeremy Fitzhardinge , Christoph Lameter , Nick Piggin , Hugh Dickens , Linux Memory Management List , Linux Kernel Mailing List , Avi Kivity , Andrew Morton , Rik van Riel List-ID: Martin Bligh wrote: >> Thanks, that was exactly what I was hoping to see. I didn't see any >> definitive statements against the patch set, other than a concern that >> it could make things worse. Was the upshot that no consensus was >> reached about how to detect when its beneficial to preallocate anonymous >> pages? >> >> Martin, in that thread you mentioned that you had tried pre-populating >> file-backed mappings as well, but "Mmmm ... we tried doing this before >> for filebacked pages by sniffing the >> pagecache, but it crippled forky workloads (like kernel compile) with the >> extra cost in zap_pte_range, etc. ". >> >> Could you describe, or have a pointer to, what you tried and how it >> turned out? > > Don't have the patches still, but it was fairly simple - just faulted in > the next 3 pages whenever we took a fault, if the pages were already > in pagecache. I would have thought that was pretty lightweight and > non-invasive, but turns out it slowed things down. > >> Did you end up populating so many (unused) ptes that >> zap_pte_range needed to do lots more work? > > Yup, basically you're assuming good locality of reference, but it turns > out that (as davej would say) "userspace sucks". Well, *most* userspace sucks. It might still be worthwhile to do this when userspace is using madvise(). -- Chris -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org