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

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/11/2010 08:29 AM, Avi Kivity wrote:
> 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.

The read and write sides of the ring are widely separated in physical 
memory specifically to avoid cache line bouncing.

>   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.

A block I/O request can potentially be very, very long lived.  By 
serializing requests like this, there's a high likelihood that it's 
going to kill performance with anything capable of processing multiple 
requests.

OTOH, if we aggressively poll the ring when we have an opportunity to, 
there's very little down side to that and it addresses the serialization 
problem.

>> The same approach is probably a good idea for virtio-net.
>
> With vhost-net you don't see exits.

The point is, when we've disabled notification, we should poll on the 
ring for additional requests instead of waiting for one to complete 
before looking at another one.

Even with vhost-net, this logic is still applicable although potentially 
harder to achieve.

Regards,

Anthony Liguori