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https://issues.apache.org/jira/browse/KUDU-3197?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17210744#comment-17210744
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wangningito commented on KUDU-3197:
-----------------------------------
With deeper look into existing code, I get a little doubt with the scanners.
I saw two methods in tablet_service.cc, `HandleNewScanRequest` and
`HandleContinueScanRequest` charges almost all scan requests. And it use
projection schema to serialize the scan result.
In `HandleNewScanRequest` it create projection schema of scanner in scanner
manager, and serialize the result by the projection.
In `HandleContinueScanRequest`, it get scanner with scanner_id from scanner
manager, the projection schema is kept in scanner, so it may be irrelevant to
the schema kept by tablet_metadata.
So I'm wondering it may not have to much impact on deleting old old_schemas_.
BTW, it took me some time to thinking about ref-counted approach, as I
understand, I considered two approach,
1. Change change schema_ kept by tablet_metadata to shared_ptr wrapped one, and
add a lock in SetSchema() and Schema() cause previously atomic swap is broken.
It may hurts the perform a lot.
2. Add a atomic counter as Schema field, and decrease ref in scanner dtor, it
may also hurt the performance when getting Schema().
> 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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