In message <[EMAIL PROTECTED]> void writes:
: On Mon, Feb 12, 2001 at 10:36:40PM -0800, Jordan Hubbard wrote:
: >
: > 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.
:
: Interesting. When I asked about optimal values on this list maybe a
: year ago, I was told that -j(4 * NCPU) was a good choice. I guess that
: doesn't work for NCPU == 1.
Back at Solbourne (Sparc multiprossors running in the 50-75MHz range),
the optimal value was determined imperically to be between 1.3 and 1.7
times the number of CPUs rounded up. A 8 CPU system made the kernel
fasted at about -j13 or so. But the whole range from -j 10 to -j 15
gave within 5% of the optimal value.
I've seen speedups on systems with very fast memory subsystems, but
only decent disks with -j 4. My PII 500 was one such beast. However,
-j 4 wasn't 2x faster than -j 2. It was more like 1.2 or 1.3 x faster
(eg 20% or so). If a -j 1 kernel build took 600s, a -j 2 build
would take 375-400s while a -j 4 would take more like 320-340. I also
had a lot of memory on this box, so the extra jobs weren't fighting
each other for that. NFS was involved, so maybe that throws the mix
off :-)
Your milage may vary. This sort of thing is fairly dependent on the
actual system, but it varies from no gain to big gains.
Warner
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