[VL] Disable background IO threads by default

[zhztheplayer]

Velox's background IO threads (typically used for split preloading) have been causing some locking issues since it's not aligned with Spark's threading model. For example: #7161.

And it also causes slow runtime filter: facebookincubator/velox#10173.

So we are exploring opportunity to disable it by default until these related issues be resolved. Meanwhile would like to here from users for thoughts.

[zhli1142015]

Hello @zhztheplayer ,
May I ask some details about the issue #7161, did this happend recently or it exists from the time when this flag is enabled by default? As I don't recall there is such issue before.

This feature would impact outr perf number, as remote stroage access takes time, this would help reduce it. Do we have any plan to fix the issues and re-enable it?
Thanks.

[zhztheplayer]

Hi @zhli1142015 , as mentioned offline, issue #7161 is not a new one and should exist for long time. I assume your scenario is not memory-bound so IO threads could work stably without spilling.

Although PR #7165 for this issue is filed, we can still be tracking on any possible solutions for #7161 after the PR is merged. And I am also adding one TPC benchmark job for iothreads=on in the PR in case loosing test coverage. So I hope we can finally minimize the overall impacts on your use cases.

[zhli1142015]

Thanks, make sense.