Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device

[Avi Kivity]

On 01/11/2010 03:49 PM, Anthony Liguori wrote:
> > So instead of disabling notify while requests are active we might want
> > to only disable it while we are inside virtio_blk_handle_output.
> > Something like the following minimally tested patch:
>
>
> I'd suggest that we get even more aggressive and install an idle 
> bottom half that checks the queue for newly submitted requests.  If we 
> keep getting requests submitted before a new one completes, we'll 
> never take an I/O exit.

That has the downside of bouncing a cache line on unrelated exits.  It 
probably doesn't matter with qemu as it is now, since it will bounce 
qemu_mutex, but it will hurt with large guests (especially if they have 
many rings).

IMO we should get things to work well without riding on unrelated exits, 
especially as we're trying to reduce those exits.

> The same approach is probably a good idea for virtio-net.

With vhost-net you don't see exits.

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