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

[TÃrÃk Edwin]

On 2008-08-25 13:02, Peter Zijlstra wrote:
> On Mon, 2008-08-25 at 12:49 +0300, TÃrÃk Edwin wrote:
>   
> > 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?
> >     
>
> mmap_sem, its process wide, and your test prog bangs on it like there's
> no tomorrow.
>   

Well, the real program (clamd) that this testprogram tries to simulate 
does an mmap for almost every file, and I have lots of small files.
6.5G, 114122 files, average size 57k.

I'll run latencytop again, last time it has showed 100ms - 500ms latency 
for clamd, and it was about mmap, I'll provide you with the exact output.

> > > Guess what happens ;-)
> > >       
> > So the problem is that doing mmap() doesn't scale well with multiple 
> > threads, because there is contention on mmap_sem?
> >     
>
> Indeed.
>
>   
> > Why did 2.6.24 seem to work better?
> >     
>
> Perhaps the scheduler overhead did increase, can you try:
>
> echo NO_HRTICK > /debug/sched_features
>
> (after mounting debugfs on /debug, or adjusting the path to where you do
> have it mounted)
>
> That might cause some overhead on very high context switch rates.

No difference, and turning off the other features from sched_features 
doesn't seem to help either.

Best regards,
--Edwin
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