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https://issues.apache.org/jira/browse/ARROW-14354?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17429533#comment-17429533
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Antoine Pitrou commented on ARROW-14354:
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Does this stem from an actual experimentation?
> [C++] Investigate reducing I/O thread pool size to avoid CPU wastage.
> ---------------------------------------------------------------------
>
> Key: ARROW-14354
> URL: https://issues.apache.org/jira/browse/ARROW-14354
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Weston Pace
> Priority: Major
>
> If we are reading over HTTP (e.g. S3) we generally want high parallelism in
> the I/O thread pool.
> If we are reading from disk then high parallelism is usually harmless but
> ineffective. Most of the I/O threads will spend their time in a waiting
> state and the cores can be used for other work.
> However, it appears that when we are reading locally, and the data is cached
> in memory, then having too much parallelism will be harmful, but some
> parallelism is beneficial. Once the DRAM <-> CPU bandwidth limit is hit then
> all reading threads will experience high DRAM latency. Unlike an I/O
> bottleneck a RAM bottleneck will waste cycles on the physical core.
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