[Lustre-discuss] high CPU load limits bandwidth?

Bernd Schubert bs_lists at aakef.fastmail.fm
Wed Oct 20 10:03:15 PDT 2010


On Wednesday, October 20, 2010, Andreas Dilger wrote:
> 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.

I think checksums are only visible in ptlrpc CPU time (and most also only for 
reads), but not in the user space benchmark process.


Cheers,
Bernd

-- 
Bernd Schubert
DataDirect Networks



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