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. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
