refactor(rust): Add tracking of async task wait time statistics #19373
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A profiler is useless in an async engine if something is a blocking bottleneck - all threads will just show spending a lot of time in condvar waiting inside the executor loop.
With this PR if you set
POLARS_TRACK_WAIT_STATS=1
we track the time between right before we park a thread, and when that thread finds a new task, and adds the time spent blocked to that tasks metadata. After a new streaming query we print these stats, which can look like this:In this case you know that waiting for the parquet source was the bottleneck for the execution engine.