Jukka Zitting created OAK-629:
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Summary: SegmentMK: Reverse writing of segments
Key: OAK-629
URL: https://issues.apache.org/jira/browse/OAK-629
Project: Jackrabbit Oak
Issue Type: Sub-task
Components: segmentmk
Reporter: Jukka Zitting
Priority: Minor
By design changes to a content tree get written to a segment using a
depth-first traversal. As a consequence the top-down access pattern used for
reading content ends up parsing the content from the end to the beginning. This
is somewhat troublesome as many memory and disk caches are optimized for the
opposite direction of access.
To better leverage cache prefetching features (like readahead in Linux), it
would be useful to reverse the order in which segments get written. In other
words, instead of filling the segment buffer from the beginning to the end as
the depth-first traversal proceeds, we could fill the buffer from the end to
the beginning and count segment offsets from the end of a segment instead of
its beginning.
While at it, it might also make sense to align record offsets at four or eight
byte boundaries to further (micro-)optimize access. It might also be worth it
to reduce the segment size limit to something like 512kB so we could squeeze
record references down from four to three bytes.
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