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Re: [Xen-devel] [PATCH 2 of 4 v7/leftover] libxl: enable automatic placement of guests on NUMA nodes

On Thu, 2012-07-26 at 14:03 +0200, Andre Przywara wrote:
> I have tested this version 7 on my box in various configurations:
> 1. Homogeneous memory distribution: 8 nodes, each 16 GB
> 2. "Odd" configuration: 8 nodes, 4 have 8 GB, 4 have 16 GB
> 3  Flat memory (by using cpus="all")
> I created up to 32 guests with 2 VCPUs and 2 GB each and observed the 
> distribution after every 8th guest.
> Another round with 2 VCPU guests, but this time with memory sizes 
> varying from 512MB to 5120MB.
Wow, that sounds like a lot of work... Thanks a ton for doing this
Andre! :-)

> The placement was running well in all cases.
It's hard to describe how much I am pleased to hear this. :-P

> The only thing I saw as a little sub-optimal placement in the odd 
> configuration (2) with the differently sized guests. This is probably a 
> side effect of Dom0 ballooning, which is not really NUMA aware AFAIK. 
> This leads to a situation, where some nodes just by chance have less 
> memory than a certain guest needs and so are not considered candidates.
Yes, that sounds a more than possible explanation. Honestly, I think
that no matter how good we are in finding algorithms and heuristics,
there always will be some weird combination of host and guest
configurations that will lead to equally weird results.

That being said, I also think the algorithm's structure is flexible
enough to allow us trying to do better in at least some of this weird
but possible situation. For example, we surely can play with some of the
parameters we pass to libxl__get_numa_candidate() and with the
comparison function in order to deal better with asymmetrical hosts...
In the near _future_ !! 

Also, instilling some NUMA-awareness in other components, such as the
ballooning machinery, should be on our todo-list, for 4.3 perhaps.

> So this patch gets my:
> Tested-by: Andre Przywara <andre.przywara@xxxxxxx>
That's the important thing for now. :-)

Thanks again and Regards,

<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://retis.sssup.it/people/faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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