[jboss-jira] [JBoss JIRA] Commented: (JGRP-1331) Digest: remove low seqno

[Bela Ban (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-1331?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623628#comment-12623628 ] 

Bela Ban commented on JGRP-1331:
--------------------------------

Having 2 arrays: 1 for Addresses and one for byte[] (compressed highest_delivered and highest_received, as discussed above) defies the purpose of the compression: if we have an array of byte[] buffers, then the pointer to each byte[] buffer also takes up 8 bytes (64 bit arch), plus the size of the byte[] buffer (say 6 bytes) ! Therefore it is probably better to store the HD/HR seqnos as long, in a long[] array.
For each address, we store the HRD seqno, followed by the HR seqno. So to get HD for P, it's seqno_array[index *2];

This costs 8 bytes * 2 = 16 bytes versus, the 8 bytes (pointer) plus ~6 bytes. That's 2 bytes more, but having a long[] array might be more cache friendly, plus passing might destroy the 2 byte advantage anyway !

> Digest: remove low seqno
> ------------------------
>
>                 Key: JGRP-1331
>                 URL: https://issues.jboss.org/browse/JGRP-1331
>             Project: JGroups
>          Issue Type: Task
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>            Priority: Minor
>             Fix For: 3.0
>
>
> We don't need Digest.Entry.low_seqno for NakReceiverWindow anymore. We only need it for the Retransmitter implementations. If we can remove that last dependency, e.g. by storing the last seqno removed from a retransmitter, we can remove low_seqno from a digest. This is significant, for example if we have 100 nodes, a digest includes 300 longs:(low+highest_received+highest_delivered) * 100. If we remove low_seqno, we only have 200 longs. (These will still be further compressed in a different JIRA).
> This is important for large clusters, to reduce the size of STABLE messages and views.

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