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https://issues.apache.org/jira/browse/KUDU-3197?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17203521#comment-17203521
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Andrew Wong commented on KUDU-3197:
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Yeah, if I understand the concerns about old schemas here, the proposed
approach seems pretty unsafe. If a scan lasts longer than the time it takes to
update the schema 20 times, we might hit a segfault.
After a brief look around for how the tablet metadatas' schemas are used on the
tablet servers, for a ref-counting solution, it's probably worth identifying
the top-level "owners" of the current schema pointers, i.e. the current callers
of {{TabletMetadata::schema()}}, and ensuring that any references that those
owners pass around either outlive the owners themselves or are also ref-counted.
> Tablet keeps all history schemas in memory may result in high memory
> consumption
> --------------------------------------------------------------------------------
>
> Key: KUDU-3197
> URL: https://issues.apache.org/jira/browse/KUDU-3197
> Project: Kudu
> Issue Type: Improvement
> Components: tablet
> Affects Versions: 1.12.0
> Reporter: wangningito
> Assignee: wangningito
> Priority: Minor
> Attachments: image-2020-09-25-14-45-33-402.png,
> image-2020-09-25-14-49-30-913.png, image-2020-09-25-15-05-44-948.png
>
>
> In case of high frequency of updating table, memory consumption of
> kudu-tserver may be very high, and the memory in not tracked in the memory
> page.
> This is the memory usage of a tablet, the memory consumption of tablet-xxx‘s
> peak is 3.6G， but none of its' childrens' memory can reach.
> !image-2020-09-25-14-45-33-402.png!
> So I use pprof to get the heap sampling. The tserver started for long but the
> memory is still consuming by TabletBootstrap:PlayAlterSchemaRequest.
> !image-2020-09-25-14-49-30-913.png!
> I change the `old_schemas_` in tablet_metadata.h to a fixed size vector,
> // Previous values of 'schema_'.
> // These are currently kept alive forever, under the assumption that
> // a given tablet won't have thousands of "alter table" calls.
> // They are kept alive so that callers of schema() don't need to
> // worry about reference counting or locking.
> std::vector<Schema*> old_schemas_;
> The heap sampling then becomes
> !image-2020-09-25-15-05-44-948.png!
> So, to make application layer more flexible, it could be better to make the
> size of the old_schemas configurable.
>
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