On Fri, May 28, 2010 at 11:56:54AM +0200, Jes Sorensen wrote: > On 05/26/10 21:50, Michael S. Tsirkin wrote: > > Here's a rewrite of the original patch with a new layout. > > I haven't tested it yet so no idea how this performs, but > > I think this addresses the cache bounce issue raised by Avi. > > Posting for early flames/comments. > > > > Generally, the Host end of the virtio ring doesn't need to see where > > Guest is up to in consuming the ring. However, to completely understand > > what's going on from the outside, this information must be exposed. > > For example, host can reduce the number of interrupts by detecting > > that the guest is currently handling previous buffers. > > > > We add a feature bit so the guest can tell the host that it's writing > > out the current value there, if it wants to use that. > > > > This differs from original approach in that the used index > > is put after avail index (they are typically written out together). > > To avoid cache bounces on descriptor access, > > and make future extensions easier, we put the ring itself at start of > > page, and move the control after it. > > Hi Michael, > > It looks pretty good to me, however one thing I have been thinking of > while reading through it: > > Rather than storing a pointer within the ring struct, pointing into a > position within the same struct. How about storing a byte offset instead > and using a cast to get to the pointer position? That would avoid the > pointer dereference, which is less effective cache wise and harder for > the CPU to predict. > > Not sure whether it really matters performance wise, just a thought. > > Cheers, > Jes I think this won't work: when PUBLUSH_USED_IDX is negotiated, the pointer is to within the ring. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
