Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262584AbTEAVF3 (ORCPT ); Thu, 1 May 2003 17:05:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262624AbTEAVF2 (ORCPT ); Thu, 1 May 2003 17:05:28 -0400 Received: from pat.uio.no ([129.240.130.16]:35261 "EHLO pat.uio.no") by vger.kernel.org with ESMTP id S262584AbTEAVFZ convert rfc822-to-8bit (ORCPT ); Thu, 1 May 2003 17:05:25 -0400 Date: Thu, 1 May 2003 23:17:47 +0200 (MEST) From: =?iso-8859-1?Q?P=E5l_Halvorsen?= To: Mark Mielke cc: bert hubert , linux-kernel@vger.kernel.org, =?iso-8859-1?Q?P=E5l_Halvorsen?= Subject: Re: sendfile In-Reply-To: <20030501042831.GA26735@mark.mielke.cc> Message-ID: References: <20030430165103.GA3060@outpost.ds9a.nl> <20030430192809.GA8961@outpost.ds9a.nl> <20030430221834.GA23109@mark.mielke.cc> <20030501042831.GA26735@mark.mielke.cc> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3376 Lines: 76 On Thu, 1 May 2003, Mark Mielke wrote: > On Thu, May 01, 2003 at 12:34:32AM +0200, P?l Halvorsen wrote: > > On Wed, 30 Apr 2003, Mark Mielke wrote: > > > To some degree, couldn't sendto() fit this description? (Assuming the > > > kernel implemented 'zero-copy' on sendto()) The benefit of sendfile() > > > is that data isn't coming from a memory location. It is coming from disk, > > > meaning that your process doesn't have to become active in order for work > > > to be done. In the case of UDP packets, you almost always want a layer on > > > top that either times the UDP packet output, or sends output in response > > > to input, mostly defeating the purpose of sendfile()... > > Maybe, but then I'll have two system calls... > > As I mentioned before, the real benefit to sendfile(), as I understand it, is > that sendfile() makes it unnecessary for the OS to fully activate the calling > process in order to do work for the calling process. Unless you can point out > some other benefit provided by sendfile(), I fail to see how you will do: > > while (1) { > send_frame_over_udp(); > sleep(); > } > > Without two system calls. Whether send_frame_over_udp() uses sendfile() as > you seem to want it to, or whether it just calls sendto(), doesn't make a > difference. Because one of your requirements is that you need to provide a > smooth feed, the primary benefit of sendfile(), that of not having to activate > your process, becomes invalid. > > I haven't done timings, or looked deeply at this part of linux-2.5.x, > however, I fail to see why the following code should not meet your > requirements: > > void *p = mmap(0, length_of_file, PROT_READ, MAP_SHARED, fd, 0); > off_t offset = 0; > > while (offset < length_of_file) > { > int packet_size = max(512, length_of_file - offset); > send(socket, &p[offset], packet_size, 0); > offset += packet_size; > usleep(packets_size * 1000000 / packets_per_second); > } > > In theory, send() should be able to provide the zero copy benefits you > are requesting. In practice, it might be a little harder, but in this > case, from my perspective, send() and sendfile() should both provide > equivalent performance. Why would sendfile() perform better than send()? As far as i understand mmap/send, you'll have a copy operation in the kernel here. mmap shares the kernel and user buffer, but when sending the packet data is copied to the socket buffer!!?? OK, but I understand that my streaming scenario is not the target application for sendfile. Then, I have another question - so that I maybe can implement this myself. Can the network interface support gather operations - ie. collecting data several places for a packet ("DMA gather copy" from memory to NIC)? (Like described in http://delivery.acm.org/10.1145/610000/603774/6345.html?key1=603774&key2=4582281501&coll=portal&dl=ACM&CFID=10149715&CFTOKEN=89922395 - Linux Journal Volume 2003 , Issue 105 (January 2003) ) If so, does the sk_buff use the struct skb_shared_info to point to the different memory regions, or ...? -ph > mark - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/