[Lustre-discuss] high CPU load limits bandwidth?

[Michael Kluge]

Using O_DIRECT reduces the CPU load but the magical limit of 500 MB/s 
for one thread remains. Are the CRC sums calculated on a per thread 
base? Or stripe base? Is there a way to test the checksumming speed only?


Michael

Am 20.10.2010 18:53, schrieb Andreas Dilger:
> On 2010-10-20, at 10:40, Michael Kluge<Michael.Kluge at tu-dresden.de>  wrote:
>> It is the CPU load on the client. The dd/IOR process is using one core completely. The clients and the servers are connected via DDR IB. LNET bandwidth is at 1.8 GB/s. Servers have 1.8.3, the client has 1.8.3 patchless.
>
> If you only have a single threaded write, then this is somewhat unavoidable to saturate a CPU due to copy_from_user().  O_DIRECT will avoid this.
>
>   Also, disabling data checksums and debugging can help considerably. There is a patch in bugzilla to add support for h/w crc32c on Nehalem CPUs to reduce this overhead, but still not as fast as no checksum at all.
>
> Cheers, Andreas
>
>> Am 20.10.2010 18:15, schrieb Andreas Dilger:
>>> Is this client CPU or server CPU?  If you are using Ethernet it will definitely be CPU hungry and can easily saturate a single core.
>>>
>>> Cheers, Andreas
>>>
>>> On 2010-10-20, at 8:41, Michael Kluge<Michael.Kluge at tu-dresden.de>   wrote:
>>>
>>>> Hi list,
>>>>
>>>> is it normal, that a 'dd' or an 'IOR' pushing 10MB blocks to a lustre
>>>> file system shows up with a 100% CPU load within 'top'? The reason why I
>>>> am asking this is that I can write from one client to one OST with 500
>>>> MB/s. The CPU load will be at 100% in this case. If I stripe over two
>>>> OSTs (which use different OSS servers and different RAID controllers) I
>>>> will get 500 as well (seeing 2x250 MB/s on the OSTs). The CPU load will
>>>> be at 100% again.
>>>>
>>>> A 'dd' on my desktop pushing 10M blocks to the local disk shows 7-10%
>>>> CPU load.
>>>>
>>>> Are there ways to tune this behavior? Changing max_rpcs_in_flight and
>>>> max_dirty_mb did not help.
>>>>
>>>>
>>>> Regards, Michael
>>>>
>>>> --
>>>>
>>>> Michael Kluge, M.Sc.
>>>>
>>>> Technische Universität Dresden
>>>> Center for Information Services and
>>>> High Performance Computing (ZIH)
>>>> D-01062 Dresden
>>>> Germany
>>>>
>>>> Contact:
>>>> Willersbau, Room A 208
>>>> Phone:  (+49) 351 463-34217
>>>> Fax:    (+49) 351 463-37773
>>>> e-mail: michael.kluge at tu-dresden.de
>>>> WWW:    http://www.tu-dresden.de/zih
>>>> _______________________________________________
>>>> Lustre-discuss mailing list
>>>> Lustre-discuss at lists.lustre.org
>>>> http://lists.lustre.org/mailman/listinfo/lustre-discuss
>>>
>>
>>
>> --
>> Michael Kluge, M.Sc.
>>
>> Technische Universität Dresden
>> Center for Information Services and
>> High Performance Computing (ZIH)
>> D-01062 Dresden
>> Germany
>>
>> Contact:
>> Willersbau, Room WIL A 208
>> Phone:  (+49) 351 463-34217
>> Fax:    (+49) 351 463-37773
>> e-mail: michael.kluge at tu-dresden.de
>> WWW:    http://www.tu-dresden.de/zih
>


-- 
Michael Kluge, M.Sc.

Technische Universität Dresden
Center for Information Services and
High Performance Computing (ZIH)
D-01062 Dresden
Germany

Contact:
Willersbau, Room WIL A 208
Phone:  (+49) 351 463-34217
Fax:    (+49) 351 463-37773
e-mail: michael.kluge at tu-dresden.de
WWW:    http://www.tu-dresden.de/zih