make -j$n buildworld : use of -j investigated

[Garance A Drosihn]

At 2:08 PM +0900 11/23/04, Rob wrote:
>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...

So, I finally got around to doing some timings on my newest PC.
It is a AMD Athlon(tm) XP 3000+ (2166.43-MHz 686-class CPU) with
1-gig of memory, and fast SATA disks.  It was certainly different
that what I saw on my previous single-CPU systems.  Roughly:

     Real      User      Sys   Max-LA
    2670.86   2071.66   543.49   1.25   buildworld
    2751.60   2085.95   603.69   1.35  -j1 buildworld
    2825.87   2137.19   637.15   5.58  -j2 buildworld
    2887.03   2158.60   648.37  11.85  -j3 buildworld
    2856.75   2156.48   647.43  19.06  -j4 buildworld
    2851.71   2154.39   647.19  25.43  -j5 buildworld
    2850.92   2155.40   646.19  31.59  -j6 buildworld
    2850.07   2153.77   648.41  36.19  -j7 buildworld
    2852.64   2155.74   647.82  47.00  -j8 buildworld
    2851.66   2153.43   650.23  53.51  -j9 buildworld

(I've actually done multiple runs of each, but they all show about
the same numbers).  I had a separate session doing 'uptime's every
30 seconds, and the "Max-LA" column is the maximum load-average
that was seen by that separate session.

Apparently the faster disks made a much bigger difference than I
had expected to see.

-- 
Garance Alistair Drosehn            =   gad at gilead.netel.rpi.edu
Senior Systems Programmer           or  gad at freebsd.org
Rensselaer Polytechnic Institute    or  drosih at rpi.edu