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From "Francesco Mari (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (OAK-4088) CacheLIRS prevents cleanup from being effective
Date Fri, 04 Mar 2016 10:05:40 GMT

    [ https://issues.apache.org/jira/browse/OAK-4088?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15179661#comment-15179661
] 

Francesco Mari commented on OAK-4088:
-------------------------------------

As suggested by [~tmueller] offline, I will try to fix the {{SegmentTracker}} by using a surrogate
key for {{CacheLIRS}}, thus avoiding to put {{SegmentId}} instances in the cache directly.

> CacheLIRS prevents cleanup from being effective
> -----------------------------------------------
>
>                 Key: OAK-4088
>                 URL: https://issues.apache.org/jira/browse/OAK-4088
>             Project: Jackrabbit Oak
>          Issue Type: Bug
>          Components: core, segmentmk, tarmk-standby
>            Reporter: Francesco Mari
>             Fix For: 1.6
>
>
> While performing some tests on compaction in the context of the cold standby, I spotted
an issue involving {{CacheLIRS}} and the {{SegmentTracker}}.
> Cleanup on the standby store is inefficient due to many references to {{SegmentId}} instances.
These references prevent the system to identify unused segments, thus making the cleanup process
less effective. To minimise the amount of references to {{SegmentId}} instances, the current
implementation of the cleanup process forces every cache to be invalidated, including {{CacheLIRS}}.
Unluckily, even if {{CacheLIRS}} is invalidated, heap dumps show a lot of references to {{SegmentId}}
instances held by {{CacheLIRS}} and its inner classes. 
> To verify my hypothesis, I tried to run the system with an implementation of the {{SegmentTracker}}
that doesn't populate {{CacheLIRS}} - the cache is always empty. This greatly improved the
effectiveness of compaction, to the point that the standby instance is able to cleanup almost
the whole amount of unused segments.
> It should be investigated if this behaviour is caused by a bug in {{CacheLIRS}} or by
an incorrect usage of {{CacheLIRS}} by the {{SegmentTracker}}.



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