Re: [RFC PATCH TRIVIAL] Reading the virtio code...

[Rusty Russell <rusty@rustcorp.com.au>]

On Sat, 23 Apr 2011 18:13:34 -0500, Rob Landley <rlandley@parallels.com> wrote:
> From: Rob Landley <rlandley@parallels.com>
> 
> 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.