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

Török Edwin
2008-08-25T09:49:26Z

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, 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.
>

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