Could someone ELI5 for me... it seems like the end result is that it fails to scale down once the spiky load has gone away
Is that intended? Or I'm misinterpreting the graphs?
Is that intended? Or I'm misinterpreting the graphs?
You are right that it's not unloading. In general, you'd expect the response (green line) of a PID controller to a sinusoid load to be another sinusoid. In this case, because the maximum error is bounded on one side(the setpoint is 0, and queue length can never get below 0), it can only unload at a certain rate. It's typically the job of the Ki term to bring the long-term error towards zero. In this case, I think it's tuned small enough + the bounded asymmetric maximum error to result in it never unloading.
The PID controller is finding a reasonable number of workers that handle the peak bursts without allowing the queue to get too long, and without spamming and destroying threads unnecessarily.
The cost is that under peak load the queue does back up a little, but if the queue stayed long for too long, the PID should create new threads.
The other line, which flattens out at 26 (see right y axis) is the number of workers.
Perhaps my naive implementation of the PID controller isn't good enough, maybe:
> you'll probably want to put a maximum on your integral accumulator
is needed as icegreentea2 pointed out.