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From "Eduard Tudenhoefner (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13533) ColumnIdentifier object size wrong when tables are not flushed
Date Wed, 17 May 2017 00:00:57 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13533?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16013284#comment-16013284
] 

Eduard Tudenhoefner commented on CASSANDRA-13533:
-------------------------------------------------

[~jjirsa] we were using that flag in some of our integration tests where we found out that,
_very rarely_, simple things (usually taking < 1 second) would take a long amount of time
(> 30 seconds). 

We boiled it then down to the point where we saw that this time was 99% spent in the prepared
stmt cache for adding/evicting elements. And in our case, eviction was happening because a
single prepared stmt would account *1mb - 6mb* (prep stmt cache size was *8mb* for us). After
looking closer why prep stmts would be so large, we eventually saw that the binary representation
of {{ColumnIdentifier#text}} was be wrong and not what we would have expected (see attached
screenshot).

We also only detected all of this because on *cassandra-3.0* we don't use *.omitSharedBufferOverhead()*
with {{MemoryMeter}} object measurements ( [QueryProcessor.java#L67|https://github.com/apache/cassandra/blob/cassandra-3.0/src/java/org/apache/cassandra/cql3/QueryProcessor.java#L67]).

On *cassandra-3.11* things got refactored over time and the prep stmt cache was using {{MemoryMeter}}
stuff from {{ObjectSizes}}, where *.omitSharedBufferOverhead()* is being applied and the issue
wouldn't happen ([ObjectSizes.java#L35|https://github.com/apache/cassandra/blob/cassandra-3.11/src/java/org/apache/cassandra/utils/ObjectSizes.java#L35]).



> ColumnIdentifier object size wrong when tables are not flushed
> --------------------------------------------------------------
>
>                 Key: CASSANDRA-13533
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13533
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Eduard Tudenhoefner
>            Assignee: Eduard Tudenhoefner
>             Fix For: 3.0.14, 3.11.0, 4.0
>
>         Attachments: columnidentifier.png
>
>
> It turns out that the object size of {{ColumnIdentifier}} is wrong when *cassandra.test.flush_local_schema_changes:
false*. This looks like stuff is being wrongly reused when no flush is happening.
> We only noticed this because we were using the prepared stmt cache and noticed that prepared
statements would account for *1-6mb* when *cassandra.test.flush_local_schema_changes: false*.
With *cassandra.test.flush_local_schema_changes: true* (which is the default) those would
be around *5000 bytes*.
> Attached is a test that reproduces the problem and also a fix.
> Also after talking to [~jkni] / [~blerer] we shouldn't probably take {{ColumnDefinition}}
into account when measuring object sizes with {{MemoryMeter}}



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