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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6271) Replace SnapTree in AtomicSortedColumns
Date Mon, 30 Dec 2013 22:31:51 GMT


Benedict commented on CASSANDRA-6271:

bq. [junit] No runnable methods

Sorry, laziness on my part for not checking the ant for if we actually had a long test runner,
I was just running this through the main() method. I'll refactor into an actual JUnit test
and upload.

bq. This looks like we're second-guessing lower/upper bounds. Would prefer to assert that
lower is actually <= upper.

Actually, IIRC this is to deal with inclusive/exclusive bounds. Since we find these using
floor/ceil/higher/lower, we can actually end up in a position where for empty result ranges
we find the upper/lower bounds in the btree to be either side of each other, even though the
provided ranges were legal. 

> Replace SnapTree in AtomicSortedColumns
> ---------------------------------------
>                 Key: CASSANDRA-6271
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Benedict
>            Assignee: Benedict
>              Labels: performance
>         Attachments: oprate.svg
> On the write path a huge percentage of time is spent in GC (>50% in my tests, if accounting
for slow down due to parallel marking). SnapTrees are both GC unfriendly due to their structure
and also very expensive to keep around - each column name in AtomicSortedColumns uses >
100 bytes on average (excluding the actual ByteBuffer).
> I suggest using a sorted array; changes are supplied at-once, as opposed to one at a
time, and if < 10% of the keys in the array change (and data equal to < 10% of the size
of the key array) we simply overlay a new array of changes only over the top. Otherwise we
rewrite the array. This method should ensure much less GC overhead, and also save approximately
80% of the current memory overhead.
> TreeMap is similarly difficult object for the GC, and a related task might be to remove
it where not strictly necessary, even though we don't keep them hanging around for long. TreeMapBackedSortedColumns,
for instance, seems to be used in a lot of places where we could simply sort the columns.

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