[HN Gopher] PID Controller for controlling the number of servers...
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PID Controller for controlling the number of servers in a data-
center
Author : se4u
Score : 27 points
Date : 2021-07-04 20:07 UTC (2 hours ago)
(HTM) web link (gist.github.com)
(TXT) w3m dump (gist.github.com)
| chrisbolt wrote:
| Useful if you've never heard of PID controller:
| https://en.wikipedia.org/wiki/PID_controller
| [deleted]
| lmilcin wrote:
| Why not something more intelligent than PID?
|
| Control theory knows a lot more algorithms. PID is arguably
| simple to implement but is not particularly good algorithm.
|
| It kinda seems to me as if everybody red only the first page on
| control theory and decided they don't need to read further and
| base their solution on it.
|
| PID will basically have you experience either large overshoots
| (which you will experience as overcorrecting to changes in
| demand) or slow adaptation to changes.
|
| There is also possibility that your system changes and your PID
| parameters will cause the whole controller to misbehave.
|
| I have implemented a controller for espresso machine boiler water
| temperature. Replacing PID with moving horizon estimator allowed
| me to cut time from startup until stable temperature by at least
| half and eliminate any measurable over or undershoots.
| haolez wrote:
| PID is often good enough and more robust than ad hoc algorithms
| to do the same.
| lmilcin wrote:
| PID is poor algorithm in this case because there is
| relatively large delay between signal to spin up a server and
| observed effect of it. PID requires a lot of iterations to
| stabilize, which multiplied by delay will require a lot of
| time.
|
| A model predictive controller will need much less iterations
| because it would actually try to predict number of servers
| necessary based on some kind of model of the server farm.
|
| Parameters for that model can even be learned/adjusted over
| time, automatically.
| jameshart wrote:
| PID is a damn sight more sophisticated than most datacenter
| dynamic capacity control algorithms - most autoscalers barely
| even qualify as 'bang bang' controllers - they detect a need
| for more capacity, and add nodes at some artificially
| constrained rate until capacity is reached or they hit a max
| cluster size limit. Even rudimentary control theory is an
| improvement.
|
| Of course the problem with applying PID to server capacity is
| that compute resources come in discrete chunks that are slow to
| bring online ('computers') rather than being a continually
| variable resource.
| lmilcin wrote:
| I guess progress _must_ be made in small steps... sigh...
| whatshisface wrote:
| If you have enough of anything it starts acting continuous.
| keithnz wrote:
| as soon as I saw the title I thought, "huh, that really doesn't
| sound like a good idea". It would likely be over sensitive or
| under sensitive and likely require lots of continual tweaking
| with the tuning. Not to mention the discrete step wouldn't be
| smoothed out till you have quite a lot of server resources in
| play.
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(page generated 2021-07-04 23:00 UTC)