[jboss-jira] [JBoss JIRA] Commented: (JGRP-1122) Revisit critical path in UNICAST

[Bela Ban (JIRA)]

    [ https://jira.jboss.org/jira/browse/JGRP-1122?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12502884#action_12502884 ] 

Bela Ban commented on JGRP-1122:
--------------------------------

The down code:
               entry.lock(); // threads will only sync if they access the same entry
                try {
                    seqno=entry.sent_msgs_seqno;
                    send_conn_id=entry.send_conn_id;
                    hdr=new UnicastHeader(UnicastHeader.DATA, seqno, send_conn_id, seqno == DEFAULT_FIRST_SEQNO);
                    msg.putHeader(getName(), hdr);

                    // AckSenderWindow.add() is costly as it calls Retransmitter.add() which calls TimeScheduler.schedule(),
                    // which adds the scheduled task to a DelayQueue, which does costly tree rebalancing.
                    // In 2.9 (or 3.0), we'll replace use of ScheduledThreadPoolExecutor in TimeScheduler with
                    // a ConcurrentSkipListMap, which is faster (log(n) access cost for most ops). CSLM requires JDK 1.6
                    // Note that moving the next statement out of the lock scope made for some really ugly code, that's
                    // why this was reverted !
                    entry.sent_msgs.add(seqno, msg);  // add *including* UnicastHeader, adds to retransmitter
                    entry.sent_msgs_seqno++;
                }
                finally {
                    entry.unlock();
                }

could be changed to *not* acquire any lock at all (the lock showed up as a hotsport for thread contention). 

However this would mean that a seqno could be lost, for example if we have a temp OOME !

SOLUTION: 

Keep track of 'lost' seqnos, e.g. +1 +2 -3 +4 +5, where 3 is lost. We cannot send 3 later because this would make it get delivered *before* messages which were sent after 3 !

Piggyback the NIL information for 3 in other UNICAST messages and skip 3 on the receiver.

> Revisit critical path in UNICAST
> --------------------------------
>
>                 Key: JGRP-1122
>                 URL: https://jira.jboss.org/jira/browse/JGRP-1122
>             Project: JGroups
>          Issue Type: Task
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 2.9
>
>
> Same as https://jira.jboss.org/jira/browse/JGRP-1104, but for UNICAST. Reduce contention on the down and up directions

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