On 23 Feb 2009, at 12:48, Fred Kiefer wrote:
I tried your first version, where you had to ask for the parallel
build
more verbosely, and even on my single processor machine it appeared to
be about 10% faster. This could have been an error in my measurement,
though.
The general rule of thumb for parallel make is either number of CPUs +
1, or number of CPUs * 1.5, depending on who you ask. In a sequential
make, you have a lot of time when the CPU is idle, waiting for I/O,
typically at the start and end of each compilation step.
With a parallel make, you want enough floating processes that, when
one is waiting for I/O, another can be given CPU time. For GNUstep
programs, which are often composed of short, easy-to-compile, sources,
you may find -j3 or even -j4 is faster on a single-processor system.
Doing a sequential make of EtoileFoundation on my (slow) machine gives
these numbers:
real 0m13.089s
user 0m5.777s
sys 0m5.862s
This means that the CPU spent 1.45 seconds, or around 11% of the time,
either doing unrelated things (unlikely - nothing else was running) or
waiting for I/O during this build. The maximum speed-up I would
expect from parallel compilation here is around 10%. With more CPUs,
faster CPUs, or slower disks, the speedup will be more.
David
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