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

[TÃrÃk Edwin]

On 2008-08-25 12:23, Peter Zijlstra wrote:
> On Mon, 2008-08-25 at 10:04 +0300, edwin wrote:
>   
> > Peter Zijlstra wrote:
> >     
> > > On Mon, 2008-08-25 at 00:01 +0300, TÃrÃk 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.
>   

Are you referring to the mmap_sem lock, or my mutex lock around 
all_thread_time?
> Guess what happens ;-)

So the problem is that doing mmap() doesn't scale well with multiple 
threads, because there is contention on mmap_sem?
Why did 2.6.24 seem to work better?

Best regards,
--Edwin
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