Date: Tue, 13 Feb 2001 14:23:53 -0700 From: Warner Losh <imp@harmony.village.org> To: void <float@firedrake.org> Cc: Jordan Hubbard <jkh@winston.osd.bsdi.com>, kstewart@urx.com, Danny Braniss <danny@cs.huji.ac.il>, freebsd-hackers@FreeBSD.ORG Subject: Re: soft updates performance Message-ID: <200102132123.f1DLNrW41419@harmony.village.org> In-Reply-To: Your message of "Tue, 13 Feb 2001 19:16:59 GMT." <20010213191659.A5429@firedrake.org> References: <20010213191659.A5429@firedrake.org> <kstewart@urx.com> <81045.982046200@winston.osd.bsdi.com>
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In message <20010213191659.A5429@firedrake.org> void writes: : On Mon, Feb 12, 2001 at 10:36:40PM -0800, Jordan Hubbard wrote: : > : > With how many running processors? If you're running -j4 on a : > uniprocessor system, you're only introducing competition for already : > scarce CPU resources, though -j2 can be a speedup since this allows : > one target build to run while another is in an I/O wait. I've only : > seen a speedup with -j4 when using at least 2 CPUs. : : Interesting. When I asked about optimal values on this list maybe a : year ago, I was told that -j(4 * NCPU) was a good choice. I guess that : doesn't work for NCPU == 1. Back at Solbourne (Sparc multiprossors running in the 50-75MHz range), the optimal value was determined imperically to be between 1.3 and 1.7 times the number of CPUs rounded up. A 8 CPU system made the kernel fasted at about -j13 or so. But the whole range from -j 10 to -j 15 gave within 5% of the optimal value. I've seen speedups on systems with very fast memory subsystems, but only decent disks with -j 4. My PII 500 was one such beast. However, -j 4 wasn't 2x faster than -j 2. It was more like 1.2 or 1.3 x faster (eg 20% or so). If a -j 1 kernel build took 600s, a -j 2 build would take 375-400s while a -j 4 would take more like 320-340. I also had a lot of memory on this box, so the extra jobs weren't fighting each other for that. NFS was involved, so maybe that throws the mix off :-) Your milage may vary. This sort of thing is fairly dependent on the actual system, but it varies from no gain to big gains. Warner To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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