[9.1] (backport #17765) pq: reduce read contention when caught up #17792
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* pq: reduce read contention when caught up By keeping track of the lock-holding reader's demand (a quantity of events and a deadline), we can avoid waking up the reader for each event that is written to the queue, significantly reducing the time that the lock-holding _writer_ is blocked by synchronous read operations. The lock-holding reader is awoken when its demand is met, has expired, or when the writer has rotated pages. | batch size | unpatched | this patch | | ---------- | --------- | ---------- | | 125 | 260k | 498k | | 2000 | 219k | 516k | * remove superfluous volatile flag (cherry picked from commit ddd519c)
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Release notes
What does this PR do?
Eliminates per-event overhead from workers consuming from a persisted queue.
By keeping track of the lock-holding reader's demand (a quantity of events and a deadline), we can avoid waking up the reader for each event that is written to the queue, significantly reducing the time that the lock-holding writer is blocked by synchronous read operations. The lock-holding reader is awoken when its demand is met, has expired, or when the writer has rotated pages.
Why is it important/What is the impact to the user?
Significantly improves PQ throughput.
From my local tests (ARM / M3 Pro 12 CPU / SSD) with
input { java_generator {} } output { sink {} }:Checklist
[ ] I have made corresponding changes to the documentation[ ] I have made corresponding change to the default configuration files (and/or docker env variables)[ ] I have added tests that prove my fix is effective or that my feature worksAuthor's Checklist
How to test this PR locally
Related issues
This is an automatic backport of pull request #17765 done by [Mergify](https://mergify.com).