Date: Sun, 16 Sep 2001 00:33:59 +0200 From: Erik Trulsson <ertr1013@student.uu.se> To: stable@FreeBSD.ORG Subject: Re: make -j4 vs -j8... 4 works, but 8 does not Message-ID: <20010916003359.A48953@student.uu.se> In-Reply-To: <p05101001b7c97cb4f801@[128.113.24.47]> References: <5.1.0.14.0.20010915123606.035fcc18@192.168.0.12> <p05101001b7c97cb4f801@[128.113.24.47]>
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On Sat, Sep 15, 2001 at 06:06:10PM -0400, Garance A Drosihn wrote: > [aside on those multiple builds that I did: It was interesting that > even though it was on a dual-processor system, there was not much of > a speed improvement (on 4.3-stable) when going from -j4 to -j10. Big > improvement going from -j1 to -j4, but after that it didn't help much] That sounds exactly like what I would expect. More than 2 (or maybe 3) jobs per CPU is normally not going to make things go faster but might actually slow it down. Increasing the number of jobs running in parallell up to the number of CPUs will naturally increase performance. (Since otherwise you have some unused processors.) The reason why running more than one job/CPU might be good is that the processes uses a mixture of I/O and CPU. If you run two jobs in parallell then one can use the CPU while the other waits for I/O. This increases the efficiency in total resource usage. If you run more than that things will not improve much since you can only have one process using a CPU at a particular time. And if you have several processes trying to access a disk at the same time the disk will spend a lot of time seeking back and forth which is a sure way of making a system slow. So for a system with P processors the speed improvements for using -jN with N > 2*P is normally going to be fairly small. (While the speed improvments when increasing N from 1 up to 2*P can often be noticable, especially when going from 1 to P.) -- <Insert your favourite quote here.> Erik Trulsson ertr1013@student.uu.se To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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