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0 pointsjcalvinowens4y ago0 comments

You're arguing with a point I didn't make: the bang bang controller was a random counterexample.

> Bang-bangs cannot do the job where you have a low inertia, low damping system that cannot tolerate jerks. PID simulates exactly those properties, inertia and damping, but through the controller.

This is exactly what I'm saying: the control theory should model the actual system being controlled. Most uses I have seen of PID in the software industry do not, and consequently have to be hacked to death and are very sensitive to tuning. Old industrial stuff can be like that too sometimes.

Does datacenter load have analogous properties? Absolutely not. That's my point: it's a bad usecase for PID.

> Bang-bangs are just not smooth enough and does not give enough control over tracking

Well, in fairness it could if the timestep could be made arbitrarily small. But in practice, yes, this obviously wouldn't work.

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srean4y ago

> Well, in fairness it could if the timestep could be made arbitrarily small. But in practice, yes, this obviously wouldn't work.

I agree. This would essentially be pulse width modulation or control. You need sufficient inertia though, and damping that the vibration does not have any adverse effect.

> Does datacenter load have analogous properties? Absolutely not. That's my point: it's a bad usecase for PID.

I am not knowledgeable about that but I would presume there would be inertia like effects, delays. Bringing lots of servers up simultaneously may choke a contended shared resource leading to damping. In such cases one cant boot up too many servers too fast. Jerks might be better tolerated, except perhaps in high frequency trading scenario.

jcalvinowensOP4y ago

> I am not knowledgeable about that but I would presume there would be inertia like effects, delays. Bringing lots of servers up simultaneously may choke a contended shared resource leading to damping. In such cases one cant boot up too many servers too fast. Jerks might be better tolerated, except perhaps in high frequency trading scenario.

Sure, you can model it like that. But I can actually go measure all the physical properties of the car and calculate a tuning that will work for cruise control, because the car follows known physical laws (within the limits of my ability to measure things).

I can't "weigh" the datacenter. I must to some extent observe the system and empirically derive tuning. That tuning is basically overfitting PID to second and higher order effects in the system without taking the time to actually understand the root of them.

Can that work? Sure. That doesn't change the fact that, IMHO, there is almost always a better way.

This is probably more contraversial... but personally, I'd rather have a hacky ad hoc approach than overfit PID, because the former is so much easier to debug and understand.

srean4y ago

I am not sure what you mean -- measuring impulse response in cars good but measuring impulse response of datacenters bad (or impossible). Measuring the weight of a car is an empirical observation as well. I also don't see how it follows that PID will necessarily overfit second and 3rd order effects whereas bang-bang wouldnt

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0 pointsjcalvinowens4y ago0 comments

You're arguing with a point I didn't make: the bang bang controller was a random counterexample.

> Bang-bangs cannot do the job where you have a low inertia, low damping system that cannot tolerate jerks. PID simulates exactly those properties, inertia and damping, but through the controller.

Does datacenter load have analogous properties? Absolutely not. That's my point: it's a bad usecase for PID.

> Bang-bangs are just not smooth enough and does not give enough control over tracking

Well, in fairness it could if the timestep could be made arbitrarily small. But in practice, yes, this obviously wouldn't work.

0 comments

srean4y ago

> Well, in fairness it could if the timestep could be made arbitrarily small. But in practice, yes, this obviously wouldn't work.

I agree. This would essentially be pulse width modulation or control. You need sufficient inertia though, and damping that the vibration does not have any adverse effect.

> Does datacenter load have analogous properties? Absolutely not. That's my point: it's a bad usecase for PID.

jcalvinowensOP4y ago

Can that work? Sure. That doesn't change the fact that, IMHO, there is almost always a better way.

This is probably more contraversial... but personally, I'd rather have a hacky ad hoc approach than overfit PID, because the former is so much easier to debug and understand.

srean4y ago

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