netdev - Re: After memory pressure: can't read from tape anymore

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Message-ID: <20101214142320.27d911d5@nehalam>
Date:	Tue, 14 Dec 2010 14:23:20 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Vladislav Bolkhovitin <vst@...b.net>
Cc:	James Bottomley <James.Bottomley@...e.de>,
	Lukas Kolbe <lkolbe@...hfak.uni-bielefeld.de>,
	Kai Mäkisara <kai.makisara@...umbus.fi>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-scsi@...r.kernel.org, Kashyap Desai <Kashyap.Desai@....com>,
	netdev@...r.kernel.org
Subject: Re: After memory pressure: can't read from tape anymore

On Tue, 14 Dec 2010 23:35:37 +0300
Vladislav Bolkhovitin <vst@...b.net> wrote:

> What is interesting to me in this regard is how networking with 9K jumbo
> frames manages to work acceptably reliable? Jumbo frames used
> sufficiently often, including under high memory pressure.
> 
> I'm not a deep networking guru, but network drivers need to allocate
> physically continual memory for skbs, which means 16K per 9K packet,
> which means order 2 allocations per skb.

Good network drivers support fragmentation and allocate a small portion
for the header and allocate pages for the rest. This requires no higher
order allocation.  The networking stack takes fragmented data coming
in and does the necessary copy/merging to access contiguous headers.

There are still some crap network drivers that require large contiguous
allocation. These should not be used with jumbo frames in real
environments.

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