Re: [Xen-users] xen on amd64 or intel

["M.A. Williamson" <maw48@xxxxxxxxx>]

> > as i mentioned earlier (sorry i don't remember the source anymore), i
> > read a test which said as long as you have only as many vm's as actual
> > processor cores the opteron is better, otherwise intel was better due to
> > its hyperthreading. the performance loss is already low now for both
> > plattforms (we measured less than ca. 5%), i only experience signifcant
> > performance problems with the gbit network of the domUs.
> I remember reading a mail here saying you're better off disabling HT on 
> Intel when running Xen, as it causes a major slowdown if the two threads 
> are assigned to different domUs.
> There was no explanation given, so I'm not sure if thats true. (Maybe 
> problems with pagetable cache?)
Depends what you're doing with the hyperthreads...

HT is happiest when the activities of the two threads are using the same 
working set in the cache, so they don't fight over cache space. If they're 
actually two different virtual machines then they're definitely not using 
the same working set (as things stand today, since there is no large scale 
data sharing), so the cache behaviour cannot be expected to be helpful and 
may be harmful to performance.
HOWEVER

Dedicating a hyperthread to dom0 may benefit performance. Particularly in 
the case where it's impossible to dedicate an entire core to dom0 (e.g. you 
only have a uniprocessor). Since IO has to happen through dom0, it's 
beneficial for it to have a logical CPU to itself to run on all the time, 
so that a context switch isn't necessary for IO to be performed. This 
effect is most noticeable when running under heavy network loads and had a 
fairly large benefit in our benchmarks vs the non-HT uniprocessor case.
Cheers,
Mark

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