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Date: Thu, 18 Feb 2010 11:09:51 +0200 (EET) From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi> To: Franco Fichtner <franco@...tsummer.de> cc: Andreas Petlund <apetlund@...ula.no>, Netdev <netdev@...r.kernel.org>, eric.dumazet@...il.com, hannemann@...s.rwth-aachen.de, LKML <linux-kernel@...r.kernel.org>, shemminger@...tta.com, william.allen.simpson@...il.com, damian@....rwth-aachen.de, ebiederm@...ssion.com, David Miller <davem@...emloft.net> Subject: Re: [net-next PATCH v4 2/3] net: TCP thin linear timeouts On Thu, 18 Feb 2010, Franco Fichtner wrote: > Andreas Petlund wrote: > > On 02/18/2010 09:41 AM, Ilpo Järvinen wrote: > > > > > On Wed, 17 Feb 2010, David Miller wrote: > > > > > > > > > > From: Andreas Petlund <apetlund@...ula.no> > > > > Date: Tue, 16 Feb 2010 15:40:41 +0100 > > > > > > > > > > > > > @@ -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; > > > > > > > > > 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. > > > > > > > There would be multiple bits free for use in both frto_counter and nonagle > > > byte. > > > > > > > > > > I was playing aroud with this setup: > > > > ========= > > 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; > > ========= > > > > Do you think that would do the trick? > > > > 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? I don't remember top of the hat how much of nonagle used, but for frto_counter max value was 3 iirc. However, I'm unsure if compiler is nowadays wise enough to handle bitfields in some not all so stupid way. Btw, also in ecn_flags we have some empty space. But all in all that didn't vacate enough to get 4 bytes so I haven't taken effort of combining. -- i.
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