[jboss-jira] [JBoss JIRA] Commented: (JGRP-1043) UNICAST: high contention

[Bela Ban (JIRA)]

    [ https://jira.jboss.org/jira/browse/JGRP-1043?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12485782#action_12485782 ] 

Bela Ban commented on JGRP-1043:
--------------------------------

ack() has been changed to:
    public void ack(long seqno) {
        long prev_lowest=0;
        synchronized(this) {
            if(seqno < lowest) return; // not really needed, but we can avoid the max() call altogether...
            prev_lowest=lowest;
            lowest=Math.max(lowest, seqno +1);
        }

        for(long i=prev_lowest; i <= seqno; i++) {
            msgs.remove(i);
            retransmitter.remove(i);
        }
    }

#1 Synchronization is only around setting lowest to max(lowest, seqno+1), so removal from the messages hashmap and the retransmitter is concurrent

#2 'lowest' always goes up, never down, saving us unneeded iterations

> UNICAST: high contention
> ------------------------
>
>                 Key: JGRP-1043
>                 URL: https://jira.jboss.org/jira/browse/JGRP-1043
>             Project: JGroups
>          Issue Type: Task
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 2.6.13, 2.8
>
>
> If UNICAST receives a high number of messages *and* sends a high number of messages concurrently, there will be a lot of retransmissions. Reason is contention on 'connections', with up- and down messages accessing it concurrently. Both up- and down- messages impede each other and thus messages are not received in time, causing retransmissions (ACKs to be resent).
> SOLUTION:
> #1 Reduce contention and turn 'connections' from HashMap into ConcurrentHashMap
> #2 Break 'connections' into a send-table and receive-table: sends and acks access the send-table, receives the receive-table

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