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Posted to jira@arrow.apache.org by "Vibhatha Lakmal Abeykoon (Jira)" <ji...@apache.org> on 2021/12/16 11:28:00 UTC

[jira] [Assigned] (ARROW-14648) [C++][Dataset] Change scanner readahead limits to be based on bytes instead of number of batches

     [ https://issues.apache.org/jira/browse/ARROW-14648?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Vibhatha Lakmal Abeykoon reassigned ARROW-14648:
------------------------------------------------

    Assignee: Vibhatha Lakmal Abeykoon

> [C++][Dataset] Change scanner readahead limits to be based on bytes instead of number of batches
> ------------------------------------------------------------------------------------------------
>
>                 Key: ARROW-14648
>                 URL: https://issues.apache.org/jira/browse/ARROW-14648
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Weston Pace
>            Assignee: Vibhatha Lakmal Abeykoon
>            Priority: Major
>              Labels: datasets, query-engine
>
> In the scanner readahead is controlled by "batch_readahead" and "fragment_readahead" (both specified in the scan options).  This was mainly motivated on my work with CSV and the defaults of 32 and 8 will cause the scanner to buffer ~256MB of data (given the default block size of 1MB).
> For parquet / IPC this would mean we are buffering 256 row groups which is entirely too high.
> Rather than make users figure out complex parameters we should have a single readahead limit that is specified in bytes.
> This will be "best effort".  I'm not suggest we support partial reads of row groups / record batches so if the limit is set very small we still might end up with more in RAM just because we can only load entire row groups.



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