More hydra.nixos.org changes #1464
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It is redundant
My current theory is that running more parallel xz than available CPU cores is reducing our overall throughput by requiring more scheduling overhead and more cache thrashing.
By looking at the ratio of running vs. waiting for the dispatcher and the queue monitor, we should get better visibility into what hydra is currently bottlenecked on. There are other side effects we can try to measure to get to the same result, but having a simple way doesn't cost us much.
Running the query with/without it shows that it makes no difference to postgres, since there's an index on finished=0 already. This allows a few simplifications, but also paves the way towards running multiple parallel monitor threads in the future.
This will induce more DB queries (though these are fairly cheap), but at the benefit of processing bumps within 1m instead of within 10m.
Each output for a given step being ingested is looked up in parallel, which should basically multiply the speed of builds ingestion by the average number of outputs per derivation.
We don't rely on sequential / monotonic build IDs processing anymore, so randomizing actually has the advantage of mixing builds for different systems together, to avoid only one chunk of builds for a single system getting processed while builders for other systems are starved.
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