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Posted to oak-issues@jackrabbit.apache.org by "Michael Dürig (JIRA)" <ji...@apache.org> on 2016/10/26 15:41:58 UTC
[jira] [Commented] (OAK-4987) Segment objects under-report their
size for the cache
[ https://issues.apache.org/jira/browse/OAK-4987?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15608814#comment-15608814 ]
Michael Dürig commented on OAK-4987:
------------------------------------
It is generally hard to correctly estimate the memory requirements of Java instances. This will probably even differ between JREs and Java versions. Furthermore such estimates are problematic to maintain: simple changes i.e. in the way we implement segment references might make them obsolete again.
One thing we could try doing is instead of caching {{Segment}} instances is to just cache the raw data {{byte[]}} and instantiate the segment on the fly when fetched from the cache.
> Segment objects under-report their size for the cache
> -----------------------------------------------------
>
> Key: OAK-4987
> URL: https://issues.apache.org/jira/browse/OAK-4987
> Project: Jackrabbit Oak
> Issue Type: Bug
> Components: segment-tar
> Reporter: Alex Parvulescu
> Fix For: 1.5.14
>
>
> The {{SegmentCache}} expects each {{Segment}} object to report its own size, which will then be used to properly keep the cache within the configured bounds [0]. If a {{Segment}} object is unable to accurately account for its own size the global cache will have a size that is a lot bigger than the expected value:
> {noformat}
> Class Name | Shallow Heap | Retained Heap | Percentage
> -------------------------------------------------------------------------------------------------------------------------
> org.apache.jackrabbit.oak.segment.Segment @ 0x6cd906df0 | 48 | 1 439 728 | 0,09%
> |- org.apache.jackrabbit.oak.segment.ImmutableRecordNumbers @ 0x6cd906e50 | 16 | 1 177 432 | 0,07%
> |- java.nio.HeapByteBuffer @ 0x6cd906e20 | 48 | 262 208 | 0,02%
> |- org.apache.jackrabbit.oak.segment.ImmutableSegmentReferences @ 0x6ce0a0880| 16 | 40 | 0,00%
> -------------------------------------------------------------------------------------------------------------------------
> {noformat}
> This is pre OAK-4872, so the objects won't match anymore, but the core issue is still there (segment size needs to account for {{segmentReferences}} and {{recordNumbers}}).
> [0] https://github.com/apache/jackrabbit-oak/blob/trunk/oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/SegmentCache.java#L53
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