Re: KVM performance vs. Xen

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>>
>> Previously, the block API only exposed non-vector interfaces and 
>> bounced vectored operations to a linear buffer.  That's been 
>> eliminated now though so we need to update the linux-aio patch to 
>> implement a vectored backend interface.
>>
>> However, it is an apples to apples comparison in terms of copying 
>> since the same is true with the thread pool.  My take away was that 
>> the thread pool overhead isn't the major source of issues.
>
> If the overhead is dominated by copying, then you won't see the 
> difference.  Once the copying is eliminated, the comparison may yield 
> different results.  We should certainly see a difference in context 
> switches.

Yes, I agree with this.  The absence of copying (in both the thread pool 
and linux-aio) could yield significantly different results.

Regards,

Anthony Liguori

> One cause of context switches won't be eliminated - the non-saturating 
> workload causes us to switch to the idle thread, which incurs a 
> heavyweight exit.  This doesn't matter since we're idle anyway, but 
> when we switch back, we incur a heavyweight entry.
>