Re: make -j with j <= 4 seems to only load a single CPU core

[Jesper Juhl]

On 2/21/06, linux-os (Dick Johnson) <linux-os@xxxxxxxxxxxx> wrote:
>
> On Tue, 21 Feb 2006, Jesper Juhl wrote:
>
> > On 2/21/06, Paul Fulghum <paulkf@xxxxxxxxxxxxx> wrote:
> >> On Tue, 2006-02-21 at 22:10 +0100, Jesper Juhl wrote:
> >>
> >>> I should probably mention that the kernel I'm currently running and
> >>> observing this behaviour with is 2.6.16-rc4-mm1.
> >> ...
> >>>> I find this quite strange since anything from 'make -j 2' and up
> >>>> should be able to keep both cores resonably busy, but there seems to
> >>>> be a huge difference between j <= 4 and j > 4.
> >>
> >> I've seen the same thing (on Athlon 64x2 64 bit)
> >> but was not sure if it was a problem.
> >>
> >> The break point for me seems to be between -j 2 and -j 3
> >> -j 2 = serialized (or the appearance of)
> >> -j 3 = both cores mostly busy
> >>
> >> I'm pretty sure with an earlier 2.6 kernel source (but same environment)
> >> I did not see this. I'll start back tracking to earlier kernels
> >> to see if I can identify when this started.
> >>
> >
> > I know positively that I've seen this with previous 2.6.16-<something>
> > kernels, but not sure which ones exactely. I just dismissed it as
> > something that would probably be fixed soon and then today when I
> > build a few test kernels I noticed it again and thought "ohh, so it
> > didn't get fixed, better report it".
> >
> > I don't recall seeing it with 2.6.15 and earlier kernels, but I'm not
> > at all sure - especially since I only got this Athlon X2 box recently
> > and the first kernels I ever ran on it were 2.6.15-<something>.
> >
> > --
> > Jesper Juhl <jesper.juhl@xxxxxxxxx>
> > Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
> > Plain text mails only, please      http://www.expita.com/nomime.html
>
> Could it be, simply, that you are now I/O bound with nothing
> for these CPUs to do in user-space, being busy handling the
> kernel I/O?
>

I doubt it.
If that was the case then I should be I/O bound with 'make -j 4' and
even more so with 'make -j 5'. But what I'm seeing is that with 'make
-j 4' (and less) only ~50% of my CPU resources are being used but with
'make -j 5' I get close to 100% CPU utilization.

--
Jesper Juhl <jesper.juhl@xxxxxxxxx>
Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please      http://www.expita.com/nomime.html
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