Date: Tue, 23 Nov 2004 12:51:18 -0600 (CST) From: "Jon Noack" <noackjr@alumni.rice.edu> To: "Rob" <spamrefuse@yahoo.com> Cc: freebsd-stable@freebsd.org Subject: Re: make -j$n buildworld : use of -j investigated Message-ID: <39182.69.53.57.66.1101235878.squirrel@69.53.57.66> In-Reply-To: <41A2C5C0.3080908@yahoo.com> References: <41A2C5C0.3080908@yahoo.com>
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Rob wrote: > I have tested following with FreeBSD 5.3-Stable. > > On several different PCs I have used > make -j$n buildworld > with $n ranging from 1 to 9. > > Although people suggest "-j4" as optimal in general > case, I have come to a very different conclusion: > > 1) single CPU with enough RAM (2 GHz, 512 MB) > there's no significant speed up in the range > "-j1" to "-j9". > So "-j1" is as good as "-j9". > > 2) single CPU with little RAM (333 MHz, 64 MB) > speed slows down rapidly from "-j1" to "-j9", > because of intensive swapping. > So "-j1" performs best in this case. > > 3) dual CPU with enough RAM (2 x 800 MHz, 1GB) > speed up by almost two from "-j1" to "-j2", > but after that no noticeable speed up anymore. > So "-j2" is as good as "-j9". > > ---------------------------------------- > > With these simple tests, I come to the conclusion that > "make -j$n buildworld" is best with n = number of CPUs. > Does that make sense? I believe the current recommendation for SMP machines is to use "-j(n+1)", so I've been using "-j3" on my dual CPU machines. This ensures there is always a task waiting to be executed when one of the CPUs completes a job. Note that a parallel build (using "-j") can cause problems when compiling some programs (in fact, it didn't work for the kernel last time I checked). For my UP machines I don't use "-j" at all. If you read the make(1) man page, you'll note that omitting "-j" implies "-B". As you already found, with a single CPU there is no speed gained by using "-j" and without it I get the assurance of running in compatibility mode. Jon
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