On Mon, Jan 11, 2010 at 08:37:10AM -0600, Anthony Liguori wrote: > 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 >
vhost net does this already: it has a mode where it poll when skbs have left send queue: for tap this is when they have crossed the bridge, for packet socket this is when they have been transmitted. -- MST
