Re: [Xen-users] gaming on multiple OS of the same machine?

[John Weekes <lists.xen@xxxxxxxxxxxxxxxxxx>]

On 5/15/2012 5:42 PM, James Harper wrote:
> Be careful with this... a hyperthread is not equivalent to a real thread. Last time I 
> checked, the hyperthread only ran while the main thread was stalled (waiting for memory 
> fetch or something). Before the various schedulers (Linux/Windows) learnt about how to 
> manage hyperthreads there were cases where performance was worse with hyperthreading 
> turned on. Even the last server build I did for running a specific application says 
> "make sure hyperthreading is disabled" in the setup notes
While hyperthreading is not anywhere close to adding a core, I've found 
that the current versions of it is are light years beyond the old P4 
implementation. That one definitely needed to be disabled, as it hurt 
performance pretty badly for latency-sensitive applications, possibly 
because of the problem you describe.
The current implementation seems to interleave two threads running on a 
single physical core well and give them roughly the same resources, with 
minimal delays involved.
Just always be aware that hyperthreads are not real cores, make sure not 
to manually set affinities (so that the OS/hypervisor can schedule 
running processes/vcpus onto separate physical cores when possible), and 
know that when you see "50%" overall CPU usage, you're actually a lot 
closer to 85%!
-John

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