Date: Tue, 13 Feb 2001 02:32:32 -0500 (EST) From: Matthew Emmerton <matt@gsicomp.on.ca> To: Jordan Hubbard <jkh@winston.osd.bsdi.com> Cc: kstewart@urx.com, Danny Braniss <danny@cs.huji.ac.il>, freebsd-hackers@FreeBSD.ORG Subject: Re: soft updates performance Message-ID: <Pine.BSF.4.21.0102130225370.64351-100000@xena.gsicomp.on.ca> In-Reply-To: <81045.982046200@winston.osd.bsdi.com>
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On Mon, 12 Feb 2001, Jordan Hubbard wrote: > > One other point that I would like to understand is why -j4 takes > > longer on all of my systems. That goes against what everyone claims > > should happen. > > 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. FWIW, I've got an ancient dual-CPU machine (Pentium 133s) with an onboard Adaptec 7870 hooked to a pair of SCSI-2 drives. With any intensive build activity (make buildworld, or a kernel recompile), -j8 gives me the best results. (I came to this conclusion after profiling a kernel build using -j2/4/6/8/10/12.) The only explanation I can give in my case is that the onboard 7870 is a PCI device and is the main bottleneck in the system (my motherboard is a very interesting EISA/PCI combo, mfgd in 1991). Although Jordan's quite right in saying that using anything larger than -j2 on a uniprocessor machine will usually be futile, in the world of SMP things are much stranger, so it's good to experiment. (-j8 is about a 50% speedup over -j2). YMMV. -- Matt Emmerton To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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