Re: KVM performance vs. Xen

[Avi Kivity <avi@redhat.com>]

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.

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.

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