When looking for the next hole to retransmit from the scoreboard,

or to compute the total retransmitted bytes in this sack recovery
episode, the scoreboard is traversed. While in sack recovery, this
traversal occurs on every call to tcp_output(), every dupack and
every partial ack. The scoreboard could potentially get quite large,
making this traversal expensive.

This change optimizes this by storing hints (for the next hole to
retransmit and the total retransmitted bytes in this sack recovery
episode) reducing the complexity to find these values from O(n) to
constant time.

The debug code that sanity checks the hints against the computed
value will be removed eventually.

Submitted by:   Mohan Srinivasan, Noritoshi Demizu, Raja Mukerji.

1 files changed, 7 insertions, 2 deletions

diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index c6ce9e2..783a00f 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -863,6 +863,7 @@ send:
 	} else {
 		th->th_seq = htonl(p->rxmit);
 		p->rxmit += len;
+		tp->sackhint.sack_bytes_rexmit += len;
 	}
 	th->th_ack = htonl(tp->rcv_nxt);
 	if (optlen) {
@@ -1091,9 +1092,13 @@ timer:
 			 * the TF_SENTFIN flag handles that case.
 			 */
 			if ((flags & TH_SYN) == 0) {
-				if (sack_rxmit)
+				if (sack_rxmit) {
 					p->rxmit -= len;
-				else
+					tp->sackhint.sack_bytes_rexmit -= len;
+					KASSERT(tp->sackhint.sack_bytes_rexmit
+						>= 0,
+						("sackhint bytes rtx >= 0"));
+				} else
 					tp->snd_nxt -= len;
 			}
 		}