From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: After memory pressure: can't read from tape anymore Date: Tue, 14 Dec 2010 14:23:20 -0800 Message-ID: <20101214142320.27d911d5@nehalam> References: <1290971729.2814.13.camel@larosa> <20101203212453W.fujita.tomonori@lab.ntt.co.jp> <4CF905D1.6050903@kolumbus.fi> <1291388776.2881.4.camel@mulgrave.site> <1291395815.2814.376.camel@larosa> <1291399814.2881.66.camel@mulgrave.site> <4D07D519.70800@vlnb.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4D07D519.70800@vlnb.net> Sender: netdev-owner@vger.kernel.org To: Vladislav Bolkhovitin Cc: James Bottomley , Lukas Kolbe , Kai =?ISO-8859-1?B?TeRraXNhcmE=?= , FUJITA Tomonori , linux-scsi@vger.kernel.org, Kashyap Desai , netdev@vger.kernel.org List-Id: linux-scsi@vger.kernel.org On Tue, 14 Dec 2010 23:35:37 +0300 Vladislav Bolkhovitin 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. --