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Date: Wed, 29 Apr 2015 10:06:58 -0400
From: Garrett Wollman <wollman@bimajority.org>
To: Mark Schouten <mark@tuxis.nl>
Cc: freebsd-net@FreeBSD.org
Subject: Re: Frequent hickups on the networking layer
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<<On Wed, 29 Apr 2015 09:30:34 +0200, Mark Schouten <mark@tuxis.nl> said:

> I'm not really (or really not) comfortable with hacking and recompiling 
> stuff. I'd rather not change anything in the kernel. So would it help in 
> my case to lower my MTU from 9000 to 4000? If I understand correctly, 
> this would need to allocate chunks of 4k, which is far more logical from 
> a memory point of view?

If you're using one of the drivers that has this problem, then yes,
keeping your layer-2 MTU/MRU below 4096 will probably cause it to use
4k (page-sized) clusters instead, which are perfectly safe.

As a side note, at least on the hardware I have to support, Infiniband
is limited to 4k MTU -- so I have one "jumbo" network with 4k frames
(that's bridged to IB) and one with 9k frames (that everything else
uses).

-GAWollman