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From: Lawrence Stewart <lstewart@freebsd.org>
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To: Andre Oppermann <andre@FreeBSD.org>
Subject: Re: svn commit: r251297 - head/sys/dev/xen/netfront
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On 06/03/13 23:00, Andre Oppermann wrote:
> Author: andre
> Date: Mon Jun  3 13:00:33 2013
> New Revision: 251297
> URL: http://svnweb.freebsd.org/changeset/base/251297
> 
> Log:
>   Specify a maximum TSO length limiting the segment chain to what the
>   Xen host side can handle after defragmentation.
>   
>   This prevents the driver from throwing away too long TSO chains and
>   improves the performance on Amazon AWS instances with 10GigE virtual
>   interfaces to the normally expected throughput.
>   
>   Submitted by:	cperciva (earlier version)
>   Reviewed by:	cperciva
>   Tested by:	cperciva
>   MFC after:	1 week
> 
> Modified:
>   head/sys/dev/xen/netfront/netfront.c
> 
> Modified: head/sys/dev/xen/netfront/netfront.c
> ==============================================================================
> --- head/sys/dev/xen/netfront/netfront.c	Mon Jun  3 12:55:13 2013	(r251296)
> +++ head/sys/dev/xen/netfront/netfront.c	Mon Jun  3 13:00:33 2013	(r251297)
> @@ -134,6 +134,7 @@ static const int MODPARM_rx_flip = 0;
>   * to mirror the Linux MAX_SKB_FRAGS constant.
>   */
>  #define	MAX_TX_REQ_FRAGS (65536 / PAGE_SIZE + 2)
> +#define	NF_TSO_MAXBURST ((IP_MAXPACKET / PAGE_SIZE) * MCLBYTES)

For posterity's sake, can you and/or Colin please elaborate on how this
value was determined and what it is dependent upon? Could a newer
version of Xen remove the need for this reduced limit?

Cheers,
Lawrence