From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Whitehouse Subject: Re: [RFC 1/2] net: Introduce recvmmsg socket syscall Date: Thu, 21 May 2009 17:33:35 +0100 Message-ID: <1242923615.29604.409.camel@localhost.localdomain> References: <20090520230652.GB5956@ghostprotocols.net> <20090521161000.GA6638@ioremap.net> <20090521162753.GI5956@ghostprotocols.net> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Evgeniy Polyakov , David Miller , netdev@vger.kernel.org, Chris Van Hoof , Clark Williams To: Arnaldo Carvalho de Melo Return-path: Received: from [195.171.2.24] ([195.171.2.24]:40909 "EHLO fogou.chygwyn.com" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1752959AbZEURMm (ORCPT ); Thu, 21 May 2009 13:12:42 -0400 In-Reply-To: <20090521162753.GI5956@ghostprotocols.net> Sender: netdev-owner@vger.kernel.org List-ID: Hi, On Thu, 2009-05-21 at 13:27 -0300, Arnaldo Carvalho de Melo wrote: > Em Thu, May 21, 2009 at 08:10:00PM +0400, Evgeniy Polyakov escreveu: > > Hi Arnaldo. > > > > On Wed, May 20, 2009 at 08:06:52PM -0300, Arnaldo Carvalho de Melo (acme@redhat.com) wrote: > > > Meaning receive multiple messages, reducing the number of syscalls and > > > net stack entry/exit operations. > > > > > > Next patches will introduce mechanisms where protocols that want to > > > optimize this operation will provide an unlocked_recvmsg operation. > > > > What's the difference from the single msg with multiple iovecs? > > recvmsg consumes just one skb, a datagram, truncating if it has more > bytes than asked and giving less bytes than asked for if the skb being > consumer is smaller than requested. > > WRT iovec, it gets this skb/datagram and goes on filling iovec entry by > entry, till it exhausts the skb. > > The usecase here is: UDP socket has multiple skbs in its receive queue, > so application will make several syscalls to get those skbs while we > could return multiple datagrams in just one syscall + fd lookup + LSM > validation + lock_sock + release_sock. > Its not just UDP/SOCK_DGRAM either. SOCK_SEQPACKET has to return after each message because it is not allowed to (nor can it, given the api) return more than one message in a single call. So with small messages that can add up to a lot of calls. This way the MSG_EOR flags can be preserved in the correct places, Steve.