Subject: Re: sk(4) jumbo frames support - patch
To: Jeff Rizzo <>
From: Thor Lancelot Simon <>
List: tech-net
Date: 11/27/2005 16:28:54
On Sun, Nov 27, 2005 at 08:58:19AM -0800, Jeff Rizzo wrote:
> With this patch applied, I was able to use 9000 byte frames with a DLink
> DGE-530T board, and combined with lowering the interrupt moderation timer,
> I was able to improve tcp throughput of a simple test by about 15% over
> 1500-byte frames.

Two notes here:

1) You might want to look at increasing the mbuf cluster size and see
   what kind of performance you get (or does this driver use a private
   allocator for jumbograms?)

2) It's been widely noted that MTU 9000 is harmful: it causes an extra
   page to be allocated which is left almost empty, for an extra 50% VM
   system overhead for a tiny decrease in encapsulation overhead.  You
   might want to reduce the MTU to 8K minus the header length and see
   if performance goes _up_ -- it did for me in my test when I turned on
   jumbo support in the wm driver.