Re: Quad core CPUs loaded at only 50% when running a CPU and mmap intensive multi-threaded task

Peter Zijlstra
2008-08-25T09:42:18Z

On Mon, 2008-08-25 at 10:04 +0300, edwin wrote:
> Peter Zijlstra wrote:
> > On Mon, 2008-08-25 at 00:01 +0300, Török Edwin wrote:
> >
> >> Hi Ingo,
> >>
> >> When I run clamd (www.clamav.net), I can only get to load my CPU 50%
> >> (according to top), and disks at 30% (according to iostat -x 3),
> >> regardless how many threads I set (I tried 4, 8, 16, 32).
> >>
> >
> >
> > Can you share your .config, and prehaps tell what kernel version did
> > work for you?
>
> Sorry, I forgot to include the .config, its at the end of this mail (the
> cfs debug info output included the .config though)
>
> Well, I just bought this new box, so there isn't a kernel version that I
> know that worked on this hardware (but I am trying to boot some older
> versions now).
> However on my previous box (Athlon64, non-SMP) I have never seen such a
> problem (that the CPU is loaded only 50% with clamd) and I've been
> running 2.6.26 and 2.6.27-rc4 there too.
>
> Details below, short summary here:
> 2.6.24: WORKS, clamd 400% CPU, testprogram runs in 27.4 seconds, 67% CPU
> load; and 28.5 seconds w/o setting affinity
> 2.6.25+: DOES NOT WORK, clamd 200%-300% CPU, testprogram runs in 38-40
> seconds, 48-48% CPU load, and 47-56 seconds w/o setting affinity
>
> Debian has 2.6.18, 2.6.22, 2.6.24, 2.6.25, 2.6.26.
> 2.6.22 won't work with my lvm, so I can't boot that, so I tried 2.6.24:
>
> 2.6.24 doesn't have sched_debug enabled in the stock kernel
> unfortunately, but the output of cfs-debug-info.sh is available here,
> maybe it contains some useful info:
> http://edwintorok.googlepages.com/testrun-1219645937.tar.gz
>
> Is this enough info for you to reproduce the problem, or do you want me
> to try and bisect?

No, I think I know what's going on..

mmap() and munmap() need to take the mmap_sem for writing (since they
modify the memory map) and you let each thread (one for each cpu) take
that process wide lock, twice, for a million times.

Guess what happens ;-)