Date: Sun, 28 Oct 2012 17:40:35 +0000 (UTC) From: Andre Oppermann <andre@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r242252 - head/sys/netinet Message-ID: <201210281740.q9SHeZe8064680@svn.freebsd.org>
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Author: andre Date: Sun Oct 28 17:40:35 2012 New Revision: 242252 URL: http://svn.freebsd.org/changeset/base/242252 Log: Prevent a flurry of forced window updates when an application is doing small reads on a (partially) filled receive socket buffer. Normally one would a send a window update every time the available space in the socket buffer increases by two times MSS. This leads to a flurry of window updates that do not provide any meaningful new information to the sender. There still is available space in the window and the sender can continue sending data. All window updates then get carried by the regular ACKs. Only when the socket buffer was (almost) full and the window closed accordingly a window updates delivery new information and allows the sender to start sending more data again. Send window updates only every two MSS when the socket buffer has less than 1/8 space available, or the available space in the socket buffer increased by 1/4 its full capacity, or the socket buffer is very small. The next regular data ACK will carry and report the exact window size again. Reported by: sbruno Tested by: darrenr Tested by: Darren Baginski PR: kern/116335 MFC after: 2 weeks Modified: head/sys/netinet/tcp_output.c Modified: head/sys/netinet/tcp_output.c ============================================================================== --- head/sys/netinet/tcp_output.c Sun Oct 28 17:30:28 2012 (r242251) +++ head/sys/netinet/tcp_output.c Sun Oct 28 17:40:35 2012 (r242252) @@ -545,23 +545,39 @@ after_sack_rexmit: } /* - * Compare available window to amount of window - * known to peer (as advertised window less - * next expected input). If the difference is at least two - * max size segments, or at least 50% of the maximum possible - * window, then want to send a window update to peer. - * Skip this if the connection is in T/TCP half-open state. + * Sending of standalone window updates. + * + * Window updates important when we close our window due to a full + * socket buffer and are opening it again after the application + * reads data from it. Once the window has opened again and the + * remote end starts to send again the ACK clock takes over and + * provides the most current window information. + * + * We must avoid to the silly window syndrome whereas every read + * from the receive buffer, no matter how small, causes a window + * update to be sent. We also should avoid sending a flurry of + * window updates when the socket buffer had queued a lot of data + * and the application is doing small reads. + * + * Prevent a flurry of pointless window updates by only sending + * an update when we can increase the advertized window by more + * than 1/4th of the socket buffer capacity. When the buffer is + * getting full or is very small be more aggressive and send an + * update whenever we can increase by two mss sized segments. + * In all other situations the ACK's to new incoming data will + * carry further window increases. * * Don't send an independent window update if a delayed * ACK is pending (it will get piggy-backed on it) or the * remote side already has done a half-close and won't send - * more data. + * more data. Skip this if the connection is in T/TCP + * half-open state. */ if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN) && !(tp->t_flags & TF_DELACK) && !TCPS_HAVERCVDFIN(tp->t_state)) { /* - * "adv" is the amount we can increase the window, + * "adv" is the amount we could increase the window, * taking into account that we are limited by * TCP_MAXWIN << tp->rcv_scale. */ @@ -581,9 +597,11 @@ after_sack_rexmit: */ if (oldwin >> tp->rcv_scale == (adv + oldwin) >> tp->rcv_scale) goto dontupdate; - if (adv >= (long) (2 * tp->t_maxseg)) - goto send; - if (2 * adv >= (long) so->so_rcv.sb_hiwat) + + if (adv >= (long)(2 * tp->t_maxseg) && + (adv >= (long)(so->so_rcv.sb_hiwat / 4) || + recwin <= (long)(so->so_rcv.sb_hiwat / 8) || + so->so_rcv.sb_hiwat <= 8 * tp->t_maxseg)) goto send; } dontupdate:
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