tree 6f79f2819816bc679e6155e32141f1a407a5f070
parent 9286ae01ac7669e1b0a56151a3132804ed82c3d4
author David Vrabel <david.vrabel@citrix.com> 1413973026 +0100
committer David S. Miller <davem@davemloft.net> 1414376492 -0400

xen-netfront: always keep the Rx ring full of requests

A full Rx ring only requires 1 MiB of memory.  This is not enough
memory that it is useful to dynamically scale the number of Rx
requests in the ring based on traffic rates, because:

a) Even the full 1 MiB is a tiny fraction of a typically modern Linux
   VM (for example, the AWS micro instance still has 1 GiB of memory).

b) Netfront would have used up to 1 MiB already even with moderate
   data rates (there was no adjustment of target based on memory
   pressure).

c) Small VMs are going to typically have one VCPU and hence only one
   queue.

Keeping the ring full of Rx requests handles bursty traffic better
than trying to converge on an optimal number of requests to keep
filled.

On a 4 core host, an iperf -P 64 -t 60 run from dom0 to a 4 VCPU guest
improved from 5.1 Gbit/s to 5.6 Gbit/s.  Gains with more bursty
traffic are expected to be higher.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>