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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-4797) Optimise stable ids
Date Mon, 03 Oct 2016 15:07:20 GMT

    [ https://issues.apache.org/jira/browse/OAK-4797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15542630#comment-15542630
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Michael Dürig commented on OAK-4797:
------------------------------------

I couldn't confirm serialisation cost to be significant here. The dominating factor is reading
the stable id from the segment so caching it with the segment might scrape some CPU cycles
off. However, I doubt the overall performance gain would justify the additional memory, so
I would wait for this until we have more evidence and bump the fix version accordingly. 

Optimising stable ids for less memory usage is hard without real structs. We are effectively
limited by Java's capabilities here. 

> Optimise stable ids 
> --------------------
>
>                 Key: OAK-4797
>                 URL: https://issues.apache.org/jira/browse/OAK-4797
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: segment-tar
>            Reporter: Michael Dürig
>            Assignee: Michael Dürig
>              Labels: memory, performance
>             Fix For: Segment Tar 0.0.16
>
>
> Currently {{SegmentNodeState#getStableId()}} returns a string with all its associated
overhead:
> * high memory requirements (42 characters plus the overhead of a {{String}} instance.
The raw requirements are a mere 20 bytes (long msb, long lsb, int offset). The memory overhead
is problematic as the stable id is used as key in the node deduplication cache (See OAK-4635).
> * high serialisation cost. I have seen {{getStableId()}} occurring in stack traces. This
is to be expected as that method is called quite often when comparing node states. 
> This issue is to explore options for reducing both CPU and memory overhead of stable
ids. 



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