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The TCP delayed ACK logic isn't aware of LRO passing up large aggregated

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segments thinking it received only one segment. This causes it to enable
the delay the ACK for 100ms to wait for another segment which may never
come because all the data was received already.

Doing delayed ACK for LRO segments is bogus for two reasons: a) it pushes
us further away from acking every other packet; b) it introduces additional
delay in responding to the sender.  The latter is especially bad because it
is in the nature of LRO to aggregated all segments of a burst with no more
coming until an ACK is sent back.

Change the delayed ACK logic to detect LRO segments by being larger than
the MSS for this connection and issuing an immediate ACK for them to keep
the ACK clock ticking without interruption.

Reported by:	julian, cperciva
Tested by:	cperciva
Reviewed by:	lstewart
MFC after:	3 days
This commit is contained in:
Andre Oppermann 2013-10-22 18:24:34 +00:00
parent d6454cbdb5
commit c1e5a6e5e8
1 changed files with 7 additions and 4 deletions
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11
sys/netinet/tcp_input.c
View File

@ -508,10 +508,13 @@ do { \
* the ack that opens up a 0-sized window and
* - delayed acks are enabled or
* - this is a half-synchronized T/TCP connection.
* - the segment size is not larger than the MSS and LRO wasn't used
* for this segment.
*/
#define DELAY_ACK(tp) \
#define DELAY_ACK(tp, tlen) \
((!tcp_timer_active(tp, TT_DELACK) && \
(tp->t_flags & TF_RXWIN0SENT) == 0) && \
(tlen <= tp->t_maxopd) && \
(V_tcp_delack_enabled || (tp->t_flags & TF_NEEDSYN)))
/*
@ -1863,7 +1866,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
}
/* NB: sorwakeup_locked() does an implicit unlock. */
sorwakeup_locked(so);
if (DELAY_ACK(tp)) {
if (DELAY_ACK(tp, tlen)) {
tp->t_flags |= TF_DELACK;
} else {
tp->t_flags |= TF_ACKNOW;
@ -1954,7 +1957,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
* If there's data, delay ACK; if there's also a FIN
* ACKNOW will be turned on later.
*/
if (DELAY_ACK(tp) && tlen != 0)
if (DELAY_ACK(tp, tlen) && tlen != 0)
tcp_timer_activate(tp, TT_DELACK,
tcp_delacktime);
else
@ -2926,7 +2929,7 @@ dodata: /* XXX */
if (th->th_seq == tp->rcv_nxt &&
LIST_EMPTY(&tp->t_segq) &&
TCPS_HAVEESTABLISHED(tp->t_state)) {
if (DELAY_ACK(tp))
if (DELAY_ACK(tp, tlen))
tp->t_flags |= TF_DELACK;
else
tp->t_flags |= TF_ACKNOW;