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From Markus Thömmes <markusthoem...@me.com>
Subject Loadbalancer Improvements
Date Mon, 12 Jun 2017 18:20:30 GMT
Hey folks,

it's me again with the latest news on performance :).

As some of you probably now: Our current loadbalancer strategy is quite "simple" and doesn't
take load in the system into account at all. It hops to the next available invoker after you've
invoked an action X times (where X is a fixed value defined at deployment time). For many
many cases that's suboptimal behavior and induces lots of cold-starts, even in a fairly unused
system. 

To improve on this here is a proposal to take the loadbalancer state we already have and make
something out of it.

In a nutshell, the plan is: Before you schedule to an invoker, take into account how much
load is on the invoker you want to schedule to. If it seems full already (determined by outstanding
active-ack responses) search for another invoker.
Via hashing, we define a home invoker to for every subject/action combination. That is the
invoker with the highest probability of having a warm container for that action. If that invoker
is already busy, choose another invoker. "Stepping" through the invokers should be stable
as well, as in: For a given subject/action it should always try the invokers in the same order.
That way, the probability of getting a warm container is higher than if we chose randomly,
but of course it gets lower the more "hops" you need to make.
The step-width is determined via hashing into a series of coprime numbers to the amount of
invokers in the system to minimize collisions and chasing.

The proposal is expected to lead to a more stable warm-container rate and lead to a better
utilization of the system as a whole.

I already took a stab at implementing the proposal above. The pull-request can be found here: https://github.com/apache/incubator-openwhisk/pull/2360

As always, comments, objections, praise. All feedback is very welcome :)

Cheers,
Markus
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