[jboss-jira] [JBoss JIRA] (JGRP-1990) Headers: use 1 array rather than 2

[Bela Ban (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-1990?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13135585#comment-13135585 ] 

Bela Ban commented on JGRP-1990:
--------------------------------

The memory savings are nice:
{noformat}
org.jgroups.util.Headers object internals:
 OFFSET  SIZE     TYPE DESCRIPTION                    VALUE
      0    12          (object header)                N/A
     12     4  short[] Headers.ids                    N/A
     16     4 Header[] Headers.hdrs                   N/A
     20     4          (loss due to the next object alignment)
Instance size: 24 bytes (estimated, the sample instance is not available)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
{noformat}

A message had a ref to a {{Headers}} instance, which had refs to an {{id}} (carrying the prot-ids) and a {{headers}} array (carrying the actual headers). A {{Headers}} instances takes roughly 24 bytes of memory as shown above. Half of that is the cost of the _object header_ (12 bytes)!

The new message has a ref to a {{Header[]}} array, in which all headers are stored. The {{Header}} abstract superclass now also carries a short which adds 2 bytes to each header.

The advantages of this new approach, which stores headers directly in an array of {{Message}} are:
* Less memory need: the overall cost of maintaining headers (without the cost of the actual headers) is now ~ 24 bytes - 2 bytes = *22 bytes lower* (per message) !
* If the headers array needs to be grown, we only have to grow 1 rather than 2 arrays


> Headers: use 1 array rather than 2
> ----------------------------------
>
>                 Key: JGRP-1990
>                 URL: https://issues.jboss.org/browse/JGRP-1990
>             Project: JGroups
>          Issue Type: Enhancement
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 3.6.7
>
>
> Currently, {{Headers}} uses 1 array for ids and 1 for the actual headers. Experiment whether joining them into one array makes sense, e.g.:
> {noformat}
> | id-1 | hdr-1 | id-2 | hdr-2 | ... | id-n | hdr-n |
> {noformat}
> This saves 4 bytes (compressed OOPs) for an array ref, plus the overhead of the array itself. Plus, resizing only has to be applied to 1 array.



--
This message was sent by Atlassian JIRA
(v6.4.11#64026)