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

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Date:	Wed, 15 Dec 2010 19:27:42 +0300
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Stephen Hemminger <shemminger@...tta.com>
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

Stephen Hemminger, on 12/15/2010 01:23 AM wrote:
> 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.

I see. Thanks for clarifying it.

Vlad



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