suggest 'make -j2' for SMP machines?

Bryan Kadzban bryan at kadzban.is-a-geek.net
Tue Jun 5 19:34:07 PDT 2007


Miguel Bazdresch wrote:
> I agree with all you say, but I believe you're missing something more
> important than context switches: cache and bus conflicts. These are
> the main reason performance suffers when a core is running more than
> one active thread.

Right, I kind of assumed that stuff was part of the context-switch
overhead.  The cache wouldn't need to flush (and wouldn't suddenly be
filled with useless data, which is effectively the same thing) if the
same physical addresses were coming from the CPU; once the CPU starts
running another process, those physical addresses will all start changing.

OTOH, by your numbers, overall performance (at least on binutils)
doesn't suffer until you get to 4 jobs on the 2 cores.  And even then,
it's a fairly small difference.

Well, whatever.  I like Dan's idea, FWIW (mention the -j option in the
text, but put a cautionary note in there saying that compile errors may
happen, and to re-try the compile without -j if they do).

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