Re: less cores more iops / speed

[Joao Eduardo Luis <joao.luis@inktank.com>]

On 11/07/2012 10:02 PM, Stefan Priebe wrote:
> Hello again,
> 
> I've noticed something really interesting.
> 
> I get 5000 iops / VM for rand. 4k writes while assigning 4 cores on a
> 2.5 Ghz Xeon.
> 
> When i move this VM to another kvm host with 3.6Ghz i get 8000 iops
> (still 8 cores) when i then LOWER the assigned cores from 8 to 4 i get
> 14.500 iops. If i assign only 2 cores i get 16.000 iops...
> 
> Why does less kvm cores mean more speed?

Totally going on a limb here, but might be related to the cache maybe?
When you have more cores your threads may bounce around the cores and
invalidate cache entries as they go by; will less cores you might end up
with some sort of twisted, forced cpu affinity that allows you to take
advantage of caching.

But I don't know, really. I would be amazed if what I just wrote had an
ounce of truth, and would be completely astonished if that was the cause
for such a sudden increase on iops.

  -Joao

> 
> Greets,
> Stefan
> -- 
> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html