From: Ben Greear Subject: Re: Reading NFS file without copying to user-space? Date: Fri, 04 Sep 2009 14:12:02 -0700 Message-ID: <4AA182A2.5060507@candelatech.com> References: <4AA16F25.6050700@candelatech.com> <1252096543.2402.4.camel@heimdal.trondhjem.org> <4AA17D62.9020404@candelatech.com> <74C14419-4D21-4EC2-B01A-EAC04B354F06@fys.uio.no> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Cc: "linux-nfs@vger.kernel.org" To: Trond Myklebust Return-path: Received: from mail.candelatech.com ([208.74.158.172]:49474 "EHLO ns3.lanforge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933855AbZIDVMG (ORCPT ); Fri, 4 Sep 2009 17:12:06 -0400 In-Reply-To: <74C14419-4D21-4EC2-B01A-EAC04B354F06@fys.uio.no> Sender: linux-nfs-owner@vger.kernel.org List-ID: On 09/04/2009 01:58 PM, Trond Myklebust wrote: > On Sep 4, 2009, at 16:49, Ben Greear wrote: > >> I'm using O_DIRECT (so that the server is continually stressed even if >> the file would have otherwise been cached locally on the client). >> >> This still causes a copy of the contents to user-space when I do a >> read() call though, as far as I can tell. Since I'm normally not looking >> at this data at all, the memory copy from kernel to user is wasted >> effort in my case. > > You're missing the point. O_DIRECT does not copy data from the kernel > into userspace. The data is placed directly into the user buffer from > the socket. I may be going about things all wrong... > > The only faster alternative would be to directly discard the data in the > socket, and we offer no option to do that. I'm opening an fd like this: uint32 flgs = O_RDONLY | O_DIRECT | O_LARGEFILE; fd = open(fname, flgs); Then read from the fd it: int retval = read(fd, rcv_buffer_ptr, my_read_len); rcv_buffer_ptr is just a 1MB (or so) array of bytes. Maybe I need to use aio_read with O_DIRECT to get the benefits you speak of? Thanks, Ben -- Ben Greear Candela Technologies Inc http://www.candelatech.com