Date: Wed, 20 Nov 2002 16:52:41 -0500 (EST) From: Mike Silbersack <silby@silby.com> To: Tony Finch <dot@dotat.at> Cc: dschultz@uclink.Berkeley.EDU, <hackers@freebsd.org> Subject: Re: cvs commit: src/bin/sleep sleep.c Message-ID: <Pine.BSF.4.44.0211201646550.27339-100000@niwun.pair.com> In-Reply-To: <E18EbhV-0008O5-00@chiark.greenend.org.uk>
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On Wed, 20 Nov 2002, Tony Finch wrote: > David Schultz <dschultz@uclink.Berkeley.EDU> wrote: > > > >BSS and modified data are not shared, and Matt is only counting > >zero fill and COW faults. > > Most of the BSS is mmapped zero pages that are copy-on-write, so in simple > programs they should be mostly shared. See rtld-elf/map_object.c > > Tony. I'm curious how well COW sharing really works under FreeBSD. Earlier this year, I fixed a piece of code which was O((processes sharing a page)^2) in the VM system. When certain simple forkbombs were run, they would cause the machine to freeze for 30 seconds at a time or more once the VM cleanup routines kicked in and ran the O(N^2) piece of code. What bugged me at the time was that I couldn't seem to reproduce the problem with other programs... this led me to believe that we aren't really sharing too many pages in common use, but I didn't have time to investigate if that was true or not. Someone with an interest in VM implementations might want to take a wander through and see how well page sharing really works on a typical FreeBSD system. :) Mike "Silby" Silbersack To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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