From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 91AA46B0044 for ; Fri, 19 Dec 2008 14:15:41 -0500 (EST) Message-ID: <494BF361.1090003@vlnb.net> Date: Fri, 19 Dec 2008 22:17:53 +0300 From: Vladislav Bolkhovitin MIME-Version: 1.0 Subject: Re: [RFC]: Support for zero-copy TCP transmit of user space data References: <4941590F.3070705@vlnb.net> <1229022734.3266.67.camel@localhost.localdomain> <4942BAB8.4050007@vlnb.net> <1229110673.3262.94.camel@localhost.localdomain> <49469ADB.6010709@vlnb.net> <20081215231801.GA27168@infradead.org> <4947FA1C.2090509@vlnb.net> <494A97DD.7080503@vlnb.net> <494A99EF.6070400@flurg.com> <494BDBC5.7050701@vlnb.net> <20081219190701.GP32491@kernel.dk> In-Reply-To: <20081219190701.GP32491@kernel.dk> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Jens Axboe Cc: "David M. Lloyd" , linux-mm@kvack.org, Christoph Hellwig , James Bottomley , linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org, scst-devel@lists.sourceforge.net, Bart Van Assche , netdev@vger.kernel.org List-ID: Jens Axboe, on 12/19/2008 10:07 PM wrote: > On Fri, Dec 19 2008, Vladislav Bolkhovitin wrote: >> David M. Lloyd, on 12/18/2008 09:43 PM wrote: >>> On 12/18/2008 12:35 PM, Vladislav Bolkhovitin wrote: >>>> An iSCSI target driver iSCSI-SCST was a part of the patchset >>>> (http://lkml.org/lkml/2008/12/10/293). For it a nice optimization to >>>> have TCP zero-copy transmit of user space data was implemented. Patch, >>>> implementing this optimization was also sent in the patchset, see >>>> http://lkml.org/lkml/2008/12/10/296. >>> I'm probably ignorant of about 90% of the context here, but isn't this the >>> sort of problem that was supposed to have been solved by vmsplice(2)? >> No, vmsplice can't help here. ISCSI-SCST is a kernel space driver. But, >> even if it was a user space driver, vmsplice wouldn't change anything >> much. It doesn't have a possibility for a user to know, when >> transmission of the data finished. So, it is intended to be used as: >> vmsplice() buffer -> munmap() the buffer -> mmap() new buffer -> >> vmsplice() it. But on the mmap() stage kernel has to zero all the newly >> mapped pages and zeroing memory isn't much faster, than copying it. >> Hence, there would be no considerable performance increase. > > vmsplice() isn't the right choice, but splice() very well could be. You > could easily use splice internally as well. The vmsplice() part sort-of > applies in the sense that you want to fill pages into a pipe, which is > essentially what vmsplice() does. You'd need some helper to do that. Sorry, Jens, but splice() works only if there is a file handle on the another side, so user space doesn't see data buffers. But SCST needs to serve a wider usage cases, like reading data with decompression from a virtual tape, where decompression is done in user space. For those only complete zero-copy network send, which I implemented, can give the best performance. > And > the ack-on-xmit-done bits is something that splice-to-socket needs > anyway, so I think it'd be quite a suitable choice for this. So, are you writing that splice() could also benefit from the zero-copy transmit feature, like I implemented? Thanks, Vlad -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org