Matthew Emmerton wrote:
>
> 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
--
Kent Stewart
Richland, WA
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