Date: Mon, 15 Sep 2008 22:48:52 -0500 From: Stephen Montgomery-Smith <stephen@math.missouri.edu> To: Steve Kargl <sgk@troutmask.apl.washington.edu> Cc: Martin Cracauer <cracauer@cons.org>, freebsd-current@freebsd.org Subject: Re: Improved multiprocessor usage on amd64 Message-ID: <48CF2CA4.1000802@math.missouri.edu> In-Reply-To: <48CF2AEF.9070208@math.missouri.edu> References: <48CDBC78.4010409@math.missouri.edu> <20080915195021.GA69528@cons.org> <48CEFF74.8020602@math.missouri.edu> <20080916033459.GA31220@troutmask.apl.washington.edu> <48CF2AEF.9070208@math.missouri.edu>
next in thread | previous in thread | raw e-mail | index | archive | help
Stephen Montgomery-Smith wrote: > Steve Kargl wrote: >> On Mon, Sep 15, 2008 at 07:36:04PM -0500, Stephen Montgomery-Smith wrote: >>> ... and each thread is a loop of the form >>> >>> while (1) { >>> wait until told to start; >>> do massive amounts of floating point arithmetic (only additions and >>> multiplications) on large arrays; >>> tell the master process that you are done; >>> } >>> >>>> Do you have about as many threads as processor or more? >>> Both ways. The time difference between the two approaches is >>> negligible. >>> >> >> Are you using ULE? With my MPI applications, if the number of >> launched processes exceeds the number of cpus by 1, ULE falls >> through the floor. I have a nagging feeling that there is a problem >> with cpu affinity. >> >> http://lists.freebsd.org/pipermail/freebsd-current/2008-July/086917.html >> Let me say a little bit more. I have this gut feeling that the problem has a lot to do with cache management. My program has each thread doing, in effect, huge matrix multiplications, each one working on their own little bit. If a CPU core changes from one thread to another, it then has to flush out the cache to RAM, and read in a whole bunch of other RAM into cache. I have this sense that Linux and FreeBSD have something in its internals where it figures this out, and after a while starts changing the time between when it changes from one process to another. But Linux has a faster learning curve than FreeBSD. But this is all pure speculation on my part, because I have very little ideas as to how these internals work. Stephen
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?48CF2CA4.1000802>