From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andreas Petlund Subject: Re: [net-next PATCH v4 2/3] net: TCP thin linear timeouts Date: Thu, 18 Feb 2010 10:18:37 +0100 Message-ID: <4B7D05ED.4060900@simula.no> References: <4B7AAE69.8020701@simula.no> <20100217.163255.133898079.davem@davemloft.net> <4B7CFDE4.4010003@simula.no> <4B7D01CE.1030101@lastsummer.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Franco Fichtner , Netdev , eric.dumazet@gmail.com, hannemann@nets.rwth-aachen.de, LKML , shemminger@vyatta.com, william.allen.simpson@gmail.com, damian@tvk.rwth-aachen.de, ebiederm@xmission.com, David Miller To: =?ISO-8859-1?Q?Ilpo_J=E4rvinen?= Return-path: Received: from mail-forward2.uio.no ([129.240.10.71]:59639 "EHLO mail-forward2.uio.no" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753530Ab0BRJSp (ORCPT ); Thu, 18 Feb 2010 04:18:45 -0500 Received: from exim by mail-out2.uio.no with local-bsmtp (Exim 4.69) (envelope-from ) id 1Ni2Wx-0003GJ-Pz for netdev@vger.kernel.org; Thu, 18 Feb 2010 10:18:43 +0100 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On 02/18/2010 10:09 AM, Ilpo J=E4rvinen wrote: > On Thu, 18 Feb 2010, Franco Fichtner wrote: >=20 >> Andreas Petlund wrote: >>> On 02/18/2010 09:41 AM, Ilpo J=E4rvinen wrote: >>> =20 >>>> On Wed, 17 Feb 2010, David Miller wrote: >>>> >>>> =20 >>>>> From: Andreas Petlund >>>>> Date: Tue, 16 Feb 2010 15:40:41 +0100 >>>>> >>>>> =20 >>>>>> @@ -341,6 +342,8 @@ struct tcp_sock { >>>>>> u16 advmss; /* Advertised MSS >>>>>> */ >>>>>> u8 frto_counter; /* Number of new acks after RTO */ >>>>>> u8 nonagle; /* Disable Nagle algorithm? >>>>>> */ >>>>>> + u8 thin_lto : 1,/* Use linear timeouts for thin >>>>>> streams */ >>>>>> + thin_undef : 7; >>>>>> =20 >>>>> There is now a gap of 3 unused bytes here in this critical >>>>> core TCP socket data structure. >>>>> >>>>> Please either find a way to avoid this hole, or document >>>>> it with a comment. >>>>> =20 >>>> There would be multiple bits free for use in both frto_counter and= nonagle >>>> byte. >>>> >>>> =20 >>> >>> I was playing aroud with this setup: >>> >>> =3D=3D=3D=3D=3D=3D=3D=3D=3D >>> u8 nonagle : 4,/* Disable Nagle algorithm? */ >>> thin_lto : 1,/* Use linear timeouts for thin streams */ >>> thin_dupack : 1,/* Fast retransmit on first dupack */ >>> thin_undef : 2; >>> =3D=3D=3D=3D=3D=3D=3D=3D=3D >>> >>> Do you think that would do the trick? >>> =20 >> >> According to Ilpo, it would be ok to reduce both ftro_counter and >> nonagle, so why not join all these into u16 and leave the remaining >> free bits documented for other people. Like this: >> >> u16 frto_counter:x; /* Number of new acks after RTO */ >> u16 nonagle:y; /* Disable Nagle algorithm? */ >> u16 thin_lto:1; /* Use linear timeouts for thin streams */ >> u16 unused:15-x-y; >> >> Not sure about the y and x. Ilpo, can you comment on those values? >=20 > I don't remember top of the hat how much of nonagle used, but for=20 > frto_counter max value was 3 iirc.=20 I think nonagle uses 4 bits: =3D=3D=3D=3D=3D=3D #define TCP_NAGLE_OFF 1 /* Nagle's algo is disabled */ #define TCP_NAGLE_CORK 2 /* Socket is corked */ #define TCP_NAGLE_PUSH 4 /* Cork is overridden for alrea= dy queued data */ =3D=3D=3D=3D=3D=3D > However, I'm unsure if compiler is=20 > nowadays wise enough to handle bitfields in some not all so stupid wa= y. Would you then recommend to use a byte for each value, thus avoiding th= e bitfields? Cheers, Andreas