Re: [Xen-users] Dedicating a physical CPU or Hyperthread to dom0? Strange test results

[Dominik Klein <dk@xxxxxxxxxxxxxxxx>]

Javier Guerra Giraldez schrieb:
> Dominik Klein wrote:
> > CPU1 = HT1 + HT2
> > CPU2 = HT3 + HT4
> Are you positively sure on this? 
Hum. Actually not.

When booting the machine with a non-xen Kernel, I see in /proc/cpuinfo:
processor       : 0
...
physical id     : 0
...

processor       : 1
...
physical id     : 0
...

processor       : 2
...
physical id     : 3
...

processor       : 3
...
physical id     : 3
...

This would strengthen my assumption.
In xen though, both dom0 and domU, I cannot see "physical id" in 
/proc/cpuinfo.
> i think Linux enumerates differently to 
> make it easier to spread load, so it might be
> CPU1 = HT1 + HT3
> CPU2 = HT2 + HT4
>
> which would mean a totally different interpretation to your experiments.
Correct.

> and, of course two processes running on different HTs on the same CPU 
> will affect each other's performance.  a multithreaded CPU doesn't have 
> any more ALUs, cache, schedulers, etc. than a singletreaded one; it just 
> have another set of state registers, making it easy to switch from one 
> instruction stream to another.  the advantage comes from giving the CPU 
> something else to do while one thread stalls because of a cache miss, or 
> an inter-instruction dependency.
Good point. This said, it is unlikely that the enumeration in Xen is the 
same as in a non-xen kernel.
Maybe a developer can say something about this.

Regards
Dominik

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