Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932429AbWEQGLK (ORCPT ); Wed, 17 May 2006 02:11:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932430AbWEQGLK (ORCPT ); Wed, 17 May 2006 02:11:10 -0400 Received: from smtp004.mail.ukl.yahoo.com ([217.12.11.35]:1453 "HELO smtp004.mail.ukl.yahoo.com") by vger.kernel.org with SMTP id S932429AbWEQGLJ (ORCPT ); Wed, 17 May 2006 02:11:09 -0400 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.it; h=Received:From:To:Subject:Date:User-Agent:Cc:References:In-Reply-To:MIME-Version:Content-Type:Content-Transfer-Encoding:Content-Disposition:Message-Id; b=tZ4fClPr/Z+nk/wWo8UfZklv8oPdvaEOb9GQc8VnejuQcHalkJw+SPGyBgWud0M7VYsb7gPEDwMQ1wyRXpP/hv6f0aC/ZRuUSbPxQ9jTviKC+uAwAE07tFOj/Pls0iPQ9KJ36QOBhWhMjBx6wAaqGxGf650gkIOjoxyHGjDT9Q4= ; From: Blaisorblade To: Andreas Mohr Subject: Re: [patch 00/14] remap_file_pages protection support Date: Wed, 17 May 2006 08:10:58 +0200 User-Agent: KMail/1.8.3 Cc: Valerie Henson , Nick Piggin , Ulrich Drepper , Andrew Morton , linux-kernel@vger.kernel.org, Linux Memory Management , Val Henson References: <20060430172953.409399000@zion.home.lan> <20060516163111.GK9612@goober> <20060516164743.GA23893@rhlx01.fht-esslingen.de> In-Reply-To: <20060516164743.GA23893@rhlx01.fht-esslingen.de> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200605170810.59589.blaisorblade@yahoo.it> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3353 Lines: 72 On Tuesday 16 May 2006 18:47, Andreas Mohr wrote: > Hi, > > On Tue, May 16, 2006 at 09:31:12AM -0700, Valerie Henson wrote: > > On Tue, May 16, 2006 at 03:51:35PM +0200, Andreas Mohr wrote: > > > I cannot offer much other than some random confirmation that from my > > > own oprofiling, whatever I did (often running a load test script > > > consisting of launching 30 big apps at the same time), find_vma > > > basically always showed up very prominently in the list of > > > vmlinux-based code (always ranking within the top 4 or 5 kernel > > > hotspots, such as timer interrupts, ACPI idle I/O etc.pp.). > > > call-tracing showed it originating from mmap syscalls etc., and AFAIR > > > quite some find_vma activity from oprofile itself. > > > > This is important: Which kernel? I'd also add (for all peoples): on which processors? L2 cache size probably plays an important role, if (as I'm convinced) the problem are cache misses during rb-tree traversal. > I had some traces still showing find_vma prominently during a profiling run > just yesterday, with a very fresh 2.6.17-rc4-ck1 (IOW, basically > 2.6.17-rc4). I added some cache prefetching in the list traversal a while > ago, You mean the rb-tree traversal, I guess! Or was the base kernel so old? > and IIRC that improved profiling times there, but cache prefetching is > very often a bandaid in search for a real solution: a better data-handling > algorithm. Ok, finally I find the time to kick in and ask a couple of question. The current algorithm is good but has poor cache locality (IMHO). First, since you can get find_vma on the profile, I've read (the article talked about userspace apps but I think it applies to kernelspace too) that oprofile can trace L2 cache misses. I think such a profiling, if possible, would be particularly interesting: there's no reason whatsoever for that lookup, even on a 32-level tree (theoretical maximum since we have max 64K vmas and height_rbtree <= 2 logN), should be so slow, unless you add cache misses into the picture. The fact that cache prefetching helps shows this even more. The lookup has very poor cache locality: the rb-node (3 pointers i.e. 12 bytes, and we need only 2 pointers on searches) is surrounded by non-relevant data we fetch (we don't need the VMA itself for nodes we traverse). For cache-locality the best data structure I know of are radix trees; but changing the implementation is absolutely non-trivial (the find_vma_prev() and friends API is tightly coupled with the rb-tree), and the size of the tree grows with the virtual address space (which is a problem on 64-bit archs); finally, you have locality when you do multiple searches, especially for the root nodes, but not across different levels inside a single search. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894) http://www.user-mode-linux.org/~blaisorblade ___________________________________ Yahoo! Mail: gratis 1GB per i messaggi e allegati da 10MB http://mail.yahoo.it - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/