svn commit: r301197 - head/sys/dev/xen/netfront

[Roger Pau Monné]

On Thu, Jun 02, 2016 at 11:14:26AM +0000, Roger Pau Monné wrote:
> Author: royger
> Date: Thu Jun  2 11:14:26 2016
> New Revision: 301197
> URL: https://svnweb.freebsd.org/changeset/base/301197
> 
> Log:
>   xen-netfront: always keep the Rx ring full of requests
>   
>   This is based on Linux commit 1f3c2eba1e2d866ef99bb9b10ade4096e3d7607c from
>   David Vrabel:
>   
>   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.
>   
>   Reviewed by:	Wei Liu <wei.liu2 at citrix.com>
>   Sponsored by:	Citrix Systems R&D

Differential revision: https://reviews.freebsd.org/D6610