[Qt-creator] parallel processing in make

[Carter, Nathan]

On Jul 15, 2010, at 9:49 PM, Coda Highland wrote:

> I can't imagine why Creator couldn't guess at a default value anyway;
> just ask QThreadPool what the ideal number of threads is. :P

Actually, if Creator can do it with QThreadPool, why can't qmake do it with QThreadPool?  Then my Makefile would always have the right -j argument for the machine it was created on.


On Jul 16, 2010, at 6:29 AM, Danny Price wrote:

> I used to do that but the setting is lost when the pro.user file is regenerated. Is there a way to set this in the .pro file?

Yes, but even better, let's have it as the default behavior for qmake to choose a -j N argument for make, but you can override it if you'd rather not use all cores or something like that.


> Doesn't make automatically use all available cores if the 'j' argument is not set? I'm sure I read that somewhere.

No, here's what I just tried on my 4-core system with one small/medium-sized Qt project:

	make clean ; time make
		Time: 1m10s
		Activity monitor showed:
			Two cores nearly unused the whole time
			Other two partially used the whole time (up+down)

	make clean ; time make -j 2
		Time: 42s
		Activity monitor showed:
			Two cores only slightly (30%?) used the whole time
			Other two maxed out the whole time

	make clean ; time make -j 4
		Time: 34s
		Activity monitor showed:
			All four cores maxed out the whole time

So make does not guess correctly from the point of view of "please finish as fast as possible."

Nathan