From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: Chaining sg lists for big I/O commands: Question Date: Thu, 10 May 2007 23:34:28 +0200 Message-ID: <20070510213428.GX4163@kernel.dk> References: <20070509133830.GV4163@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Jan Engelhardt Cc: Justin Piszcz , linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, Alan Piszcz List-Id: linux-raid.ids On Thu, May 10 2007, Jan Engelhardt wrote: > On May 9 2007 15:38, Jens Axboe wrote: > >> I am a mdadm/disk/hard drive fanatic, I was curious: > >> > >> >On i386, we can at most fit 256 scatterlist elements into a page, > >> >and on x86-64 we are stuck with 128. So that puts us somewhere > >> >between 512kb and 1024kb for a single IO. > >> > >> How come 32bit is 256 and 64 is only 128? > >> > >> I am sure it is something very fundamental/simple but I was curious, I > >> would think x86_64 would fit/support more scatterlists in a page. > > > >Because of the size of the scatterlist structure. As pointers are bigger > >on 64-bit archs, the scatterlist structure ends up being bigger. The > >page size on x86-64 is 4kb, hence the number of structures you can fit > >in a page is smaller. > > I take it this problem "goes away" on arches with 8KB page_size? Not really, the 8kb page size just doubles the sg size. On a 64-bit arch, that would still only get you 1mb IO size. -- Jens Axboe