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
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