On Wed, Apr 27, 2011 at 02:59:27PM +0930, Rusty Russell wrote: > On Sat, 23 Apr 2011 18:13:34 -0500, Rob Landley <rlandley@xxxxxxxxxxxxx> wrote: > > From: Rob Landley <rlandley@xxxxxxxxxxxxx> > > > > Going indirect for only two buffers isn't likely to be a performance win > > because the kmalloc/kfree overhead for the indirect block can't be cheaper > > than one extra linked list traversal. > > Unfortunately it's not completely clear. QEMU sets fairly small rings, > and the virtio-net driver uses 2 descriptors minimum. The effect can be > a real bottleneck for small packets. > > Now, virtio-net could often stuff the virtio_net_hdr in the space before > the packet data (saving a descriptor) but I think that will need a > feature bit since qemu (incorrectly) used to insist on a separate > descriptor for that header. > > > Properly "tuning" the threshold would probably be workload-specific. > > (One big downside of not going indirect is extra pressure on the table > > entries, and table size varies.) But I think that in the general case, > > 2 is a defensible minimum? > > I'd be tempted to say that once we fill the ring, we should drop the > threshold. > > Michael? > > Thanks, > Rusty. Yes, one idea is to use part of a ring (e.g. 1/4 of a ring) for direct entries, and the rest for indirect. So we end up with a threshold like max(1, vq->num_free - in - out - vq->num * N) (above is pseudo-code, must take care of unsigned vs signed etc) and I think I'd try with N = 3/4 or maybe N = 1/2 -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
