[jira] [Commented] (OAK-11934) segment prefetching for segmentstore cache

["Julian Sedding (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OAK-11934?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18028753#comment-18028753 ] 

Julian Sedding commented on OAK-11934:
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I have spent some time experimenting with the approach to add prefetching to the in-memory {{SegmentCache}} (which implicitly also fills the persistent cache), however, this leads to frequent cache evictions, which is undesirable. Also, from within the {{SegmentCache}} it is not possible to probe whether a segment is already cached in a persistent cache. Furthermore, without a persistent cache, prefetching using this approach can be wasteful, due to unwanted cache evictions. It might even lead to a slowdown, as the same segment might be loaded repeatedly without ever being used.

I have also looked into the idea described in OAK-11932, which adds prefetching to the {{CachingSegmentReader}}. This approach is also problematic, because due to the nature of this API, it is impossible to prefetch segments that are in a different archive (a different {{CachingSegmentReader}} instance.

The approach that worked best adds the possibility to add a {{PersistentCache}} directly to an {{AbstractFileStore}} via the {{FileStoreBuilder}}. This {{PersistentCache}} can be decorated internally, by the {{AbstractFileStore}}, with a {{SegmentPreloader}}, if preloading is configured. The {{SegmentPreloader}} has access to the {{TarFiles}} instance, as well as to the {{PersistentCache}}. This allows it
- to probe the cache whether a segment is already present
- load missing segments via {{TarFiles#readSegment}}
- load segment graphs to determine segment references without reading them from the segments via {{TarFiles#getGraph}}

> segment prefetching for segmentstore cache
> ------------------------------------------
>
>                 Key: OAK-11934
>                 URL: https://issues.apache.org/jira/browse/OAK-11934
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: segment-tar
>    Affects Versions: 1.84.0
>            Reporter: Julian Sedding
>            Assignee: Julian Sedding
>            Priority: Major
>
> Particularly for remote segment stores, IO can be a constraining factor. Processes like compaction, that traverse the repository, often alternate between processing segments and loading segments.
> IO could be parallelized by enhancing the {{SegmentCache}} to asynchronously prefetch segments that are referenced by a newly loaded segment. I.e. if the "main" thread requests a segment from the cache, and the segment needs to be loaded from the persistence, then all segments referenced by the newly loaded segment are prefetched, and placed into the cache, asynchronously. When the "main" thread loads the next segment, it is likely already in the cache.
> Prefetching could preload a configurable "depth" of references. Presumably, usually a depth of 1 or 2 strikes a good balance between preloading too aggressively and efficiently parallelizing IO.
> If prefetching of references is only performed for newly loaded segments, the overhead of the prefetch mechanism should be minimal to non-existent while only cached segments are read.
> cc [~miroslav], [~nuno.santos]



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