Messages in this thread | | | Date | Tue, 21 Feb 2006 22:10:46 +0100 | From | "Jesper Juhl" <> | Subject | Re: make -j with j <= 4 seems to only load a single CPU core |
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On 2/21/06, Jesper Juhl <jesper.juhl@gmail.com> wrote: > This is quite odd and I've no idea where to start looking for the > cause, but let me describe what I'm seeing and maybe someone can point > me in the right direction. > > I'm running SMP 2.6.x kernels on a Athlon 64 X2 4400+ >
I should probably mention that the kernel I'm currently running and observing this behaviour with is 2.6.16-rc4-mm1.
> When I build new kernels I use 'make -j' to get both CPU cores busy > and minimize build time. > > I've observed that when I use 'make -j 2', 'make -j 3' or 'make -j 4' > only ~half of my CPU resources get used during the build and when I > look at the output it looks exactely like output from plain 'make' for > something like 95% of the build - that is, files get build > sequentially, not in parallel. > However, if I run 'make -j 5' or higher, then both cores get lots of > work to do and utilization of both cores stay close to 100% for almost > the entire build, and the output during the build shows lots of files > from different directories intermixed, so it's clearly building stuf > in parallel. > > 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. > > Another datapoint: This is most pronounced when I am also running the > make process nice'd. Which I usually do in order to be able to use the > machine for other tasks while the build is in progress. > If I don't run the build nice'd then it starts to be resonably > parallel at j >= 3, but still doesn't *really* load both cores before > j >= 5. > > > Just to be completely clear - a few examples: > > This is what I usually run to load both cores well : > > nice make -j 5 2>&1 | tee build.log > > this however gives me a more or less serial build with only half the > system resources used : > > nice make -j 4 2>&1 | tee build.log > > Without nice this loads the box somewhat OK (but not completely): > > make -j 3 2>&1 | tee build.log > > and this pretty much gives me a serial build: > > make -j 2 2>&1 | tee build.log > > > Any good explanations for this behaviour ? > >
-- Jesper Juhl <jesper.juhl@gmail.com> 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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