make -j optimization

[Derek Martin]

On Thu, Sep 30, 2004 at 01:47:20PM -0400, Brendan wrote:
> On Thursday 30 September 2004 13:42, David Kramer wrote:
> > In GNU make, you can specify a -j <n> option, to run <n> commands
> > simultaneously.  This is good.
> >
> > I'm having an argument with a coworker over whether specifying a value for
> > <n> greater than the number of the processors in the system actually does
> > any good or not.  I don't see how it can.

I/O wait.

> > In fact, specifying one fewer than the number of processors shoudl
> > have almost the same performance as specifying the number of
> > processors.

I don't see how this makes sense, unless you're talking about a large
number of processors (i.e. n>3).  On a dual-CPU system, the difference
is quite substantial, in my experience.

> I'm sure other know much more than I do, but with 2 processors, we
> always used -j3, because if one thread is waiting for something
> (input, i/o, whatever) then the 3rd process comes in and balances it
> back out.

I have read elsewhere that this is a common case resulting in
performance gain.  Some of the engineers at MCL also were in the habit
of compiling this way because they said it was faster.  I myself have
never tried it but...

> There are loads of benchmarks on this, so googling might be your
> best bet.

...indeed.  Or you can just use the time command to do it yourself.  It
shouldn't take very long, if you have SMP machines...

-- 
Derek D. Martin    http://www.pizzashack.org/   GPG Key ID: 0xDFBEAD02
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