linux-kernel - Re: [RFC PATCH] drivers/net/tg3.c: Raise Jumbo Frame MTU to 9216?

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Date:	Wed, 15 Sep 2010 12:14:55 -0700
From:	Joe Perches <joe@...ches.com>
To:	Michael Chan <mchan@...adcom.com>
Cc:	Matthew Carlson <mcarlson@...adcom.com>,
	Benjamin Li <benli@...adcom.com>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] drivers/net/tg3.c: Raise Jumbo Frame MTU to 9216?

On Wed, 2010-09-15 at 10:57 -0700, Michael Chan wrote:
> On Wed, 2010-09-15 at 10:41 -0700, Joe Perches wrote:
> > The TG3 apparently supports 9K frame sizes.
> > http://www.broadcom.com/collateral/pb/5704C-PB05-R.pdf
> > Is exactly 9000 a hardware limit?
> > Should the jumbo frame MTU be raised to 9216 or 9216
> > less the size of MAC, VLAN, IP and TCP headers?
> 9000 has been the de facto standard, has it been changed recently?

I know of a performance lab that's trying to use 9216 as a "standard"
jumbo frame length.

Unrelated to the performance lab:
http://www.uoregon.edu/~joe/jumbo-clean-gear.html
9216 seems popular, especially with Cisco gear.
Contrary to that link, the Cisco 3750 does work with 9216 length
jumbo frames.

> Anyway, we've never done any testing on 9216.  As it uses up to 2 more
> internal mbufs per packet, there may not be sufficient buffers inside
> the chip for optimal operations.  At best, some water marks will need to
> be tweaked.  The hardware statistics counters (ethtool -S) also may not
> work for packets bigger than 9022 bytes.

Thanks, do you have pointers to where the tweaking needs to be done?

Is this define a hardware upper bound?
#define TG3_RX_JMB_DMA_SZ		9046

cheers, Joe

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