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Date: Tue, 23 Nov 2004 14:08:16 +0900
From: Rob <spamrefuse@yahoo.com>
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Subject: make -j$n buildworld : use of -j investigated
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Hi,

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?

Rob.