[jboss-jira] [JBoss JIRA] Commented: (JGRP-1133) NakReceiverWindow: make faster

[Bela Ban (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-1133?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12571997#comment-12571997 ] 

Bela Ban commented on JGRP-1133:
--------------------------------

Let's take a step back from making NRW concurrent: it is not as simple as it sounds (e.g. add() needs to make sure the seqno to add is >= highest_delivered, but removeMany() also changes highest_delivered ! This requires lockinng of some sort. Also, if we made NRW concurren, we'd also have to make the Retransmitter and timer classes concurrent, and they currently aren't.

So the following, less dramatic, changes were made:
- NRW already acquires a lock for add(), removeMany() etc. Therefore, we don't need ConcurrentHashMap for xmit_table, and - in a first step - replaced it with an unsynchronized HashMap

- In a second step, we replaced HashMap with RetransmitTable. This is a matrix (N rows of M message arrays). If we have many keys (seqnos), then a HashMap deteriorates and when many keys hash to the same bucket, we have to do a linear walk to find an element. The memory overhead of HashMap is also much bigger than for RetransmitTable: every Hashmap.Entry has 4 pointers...

- RetransmitTable requires 2 lookups (row and element within the row), whereas HashMap requires more if we have many seqnos in the map (1 to get the bucket from the hash and then N to find the element in the linked list of the bucket).

This change made perf go from 132MByte/sec/node on my local box to 148MByte/sec/node ! 

> NakReceiverWindow: make faster
> ------------------------------
>
>                 Key: JGRP-1133
>                 URL: https://issues.jboss.org/browse/JGRP-1133
>             Project: JGroups
>          Issue Type: Task
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 2.12
>
>
> JProfiler shows that NRW is a bit of a locking hotspot

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