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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6591) un-deprecate cache recentHitRate and expose in o.a.c.metrics
Date Mon, 23 Jun 2014 22:12:25 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6591?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14041400#comment-14041400
] 

Benedict commented on CASSANDRA-6591:
-------------------------------------

Apologies for the delay on this. I think this ticket does slightly fit into a wider discussion,
though here we're using a Meter instead of a Histogram/Timer (where codahale is much more
broken by default). The Meters *are* still costlier to keep around and maintain than we might
otherwise like: each meter maintains a LongAdder (which is in principle to avoid contention,
but for our use case just means more cpu-cache misses), but more importantly maintains *four*
LongAdders per meter, so each time we attempt to hit the cache we must update at least 8 LongAdders
(since we update two meters here). I would prefer to see us moving to a scheme that uses only
two AtomicLong for this data, but I'm not sure we need to hold up this ticket for that.

However I would say we only need to maintain *either* hits *or* misses, since even for a rate
property, misses = total - hits, so we can define our misses as a simple Gauge returning requests
- hits



> un-deprecate cache recentHitRate and expose in o.a.c.metrics
> ------------------------------------------------------------
>
>                 Key: CASSANDRA-6591
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6591
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Chris Burroughs
>            Assignee: Chris Burroughs
>            Priority: Minor
>         Attachments: j6591-1.2-v1.txt, j6591-1.2-v2.txt, j6591-1.2-v3.txt
>
>
> recentHitRate metrics were not added as part of CASSANDRA-4009 because there is not an
obvious way to do it with the Metrics library.  Instead hitRate was added as an all time measurement
since node restart.
> This does allow changes in cache rate (aka production performance problems)  to be detected.
 Ideally there would be 1/5/15 moving averages for the hit rate, but I'm not sure how to calculate
that.  Instead I propose updating recentHitRate on a fixed interval and exposing that as a
Gauge.



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