[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

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.


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.


Xen-users mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.