[jira] [Updated] (CASSANDRA-6465) DES scores fluctuate too much for cache pinning

["Chris Burroughs (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-6465?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Chris Burroughs updated CASSANDRA-6465:
---------------------------------------

    Attachment: des-score-graph.png
                des.sample.15min.csv
                get-scores.py

> DES scores fluctuate too much for cache pinning
> -----------------------------------------------
>
>                 Key: CASSANDRA-6465
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6465
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: 1.2.11, 2 DC cluster
>            Reporter: Chris Burroughs
>         Attachments: des-score-graph.png, des.sample.15min.csv, get-scores.py
>
>
> To quote the conf:
> {noformat}
> # if set greater than zero and read_repair_chance is < 1.0, this will allow
> # 'pinning' of replicas to hosts in order to increase cache capacity.
> # The badness threshold will control how much worse the pinned host has to be
> # before the dynamic snitch will prefer other replicas over it.  This is
> # expressed as a double which represents a percentage.  Thus, a value of
> # 0.2 means Cassandra would continue to prefer the static snitch values
> # until the pinned host was 20% worse than the fastest.
> dynamic_snitch_badness_threshold: 0.1
> {noformat}
> An assumption of this feature is that scores will vary by less than dynamic_snitch_badness_threshold during normal operations.  Attached is the result of polling a node for the scores of 6 different endpoints at 1 Hz for 15 minutes.  The endpoints to sample were chosen with `nodetool getendpoints` for row that is known to get reads.  The node was acting as a coordinator for a few hundred req/second, so it should have sufficient data to work with.  Other traces on a second cluster have produced similar results.
>  * The scores vary by far more than I would expect, as show by the difficulty of seeing anything useful in that graph.
>  * The difference between the best and next-best score is usually > 10% (default dynamic_snitch_badness_threshold).
> Neither ClientRequest nor ColumFamily metrics showed wild changes during the data gathering period.
> Attachments:
>  * jython script cobbled together to gather the data (based on work on the mailing list from Maki Watanabe a while back)
>  * csv of DES scores for 6 endpoints, polled about once a second
>  * Attempt at making a graph



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