[ARROW-17299] [C++] [Python] Expose the Scanner kDefaultBatchReadahead and kDefaultFragmentReadahead parameters

• Type: New Feature
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 10.0.0
• Component/s: C++, Python
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/32577
• Assignee: Ziheng Wang
• Reporter: Ziheng Wang
• Created: 03/Aug/22 23:23
• Updated: 11/Jan/23 11:49
• Resolved: 01/Sep/22 07:30

Description

In the Scanner there are parameters kDefaultFragmentReadahead and kDefaultBatchReadahead that are currently set to fixed numbers that cannot be changed.

This is not great because tuning these numbers is the key to tradeoff RAM usage and network IO utilization during reading. For example on an i3.2xlarge instance on AWS you can get peak throughput only by quadrupling kDefaultFragmentReadahead from the default. 

The current settings are very conservative and assume a < 1Gbps network. Exposing them allow people to tune the Scanner behavior to their own hardware. 

Issue Links

• links to: GitHub Pull Request #13799