[jira] [Commented] (OAK-5042) Improve caching of segments

["Michael Dürig (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OAK-5042?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15982681#comment-15982681 ] 

Michael Dürig commented on OAK-5042:
------------------------------------

While investigating on OAK-6106 I discovered a problem with the current {{SegmentCache}}'s invalidation logic: as it turns out the {{onRemoval}} callback might get called for the item currently being put into the cache (I discovered this when I set the cache size to 0). In this case the current segment will be invalidated *before* it is actually connected to its id via {{SegmentId.loaded()}}. This causes such segments to never ever by evicted. 

> Improve caching of segments
> ---------------------------
>
>                 Key: OAK-5042
>                 URL: https://issues.apache.org/jira/browse/OAK-5042
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: segment-tar
>            Reporter: Michael Dürig
>            Assignee: Michael Dürig
>              Labels: performance, scalability
>             Fix For: 1.8, 1.7.1
>
>         Attachments: OAK-5042.png
>
>
> Various aspects of how Segment Tar caches segments could possibly improved. The current cache is a result of replacing the former ad-hoc cache with a proper one in OAK-3055. While the former was prone to contention under concurrent load the current cache is too oblivious about reads: read accesses are always served through {{SegmentId.segment}} and never actually hit the cache. This results in frequently accessed segments not to be seen as such by the cache and potentially being prematurely evicted. 
> Possibly approaches to address this problem include: 
> * Reinstantiating the cache we had pre OAK-3055 but making in fully concurrent. 
> * Convey the information about read accesses to the current cache. 
> * In either of the above cases avoid bulk segments from being placed into the cache. 



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