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Nova-scheduler: when are filters applied? [closed]

asked 2018-08-23 07:38:06 -0500

eblock gravatar image

Hi *,

just for clarification, when does Nova apply its filters (Ram, CPU, etc.)?
Of course at instance creation and (live-)migration of existing instances. But what about existing instances that have been shutdown and in the meantime more instances on the same hypervisor have been launched?

When you start one of the pre-existing instances and even with RAM overcommitment you can end up with an OOM-Killer resulting in forceful shutdowns if you reach the ratio limit. Is there something I've been missing or maybe a bad configuration of my scheduler filters? Or is it the admin's task to keep an eye on the load?

I'd appreciate any insights or pointers what I'm missing.

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Closed for the following reason the question is answered, right answer was accepted by eblock
close date 2018-09-03 07:56:00.571083

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answered 2018-09-03 07:55:22 -0500

eblock gravatar image

Adding an answer to my own question, I got a response on the mailing list.

This is the scenario:

Let's say I have a ratio of 1.0 on my hypervisor, and let it have 24 GB of RAM available, ignoring the OS for a moment. Now I launch 6 instances, each with a flavor requesting 4 GB of RAM, that would leave no space for further instances. Then I shutdown two instances (freeing 8 GB RAM) and create a new one with 8 GB of RAM, the compute node is full again (assuming all instances actually consume all of their RAM).


When you shutdown the two instances the phyisical RAM will be deallocated BUT nova will not remove the resource allocation in placement. Therefore your new instance which requires 8GB RAM will not be placed to the host in question because on that host all the 24G RAM is still allocated even if physically not consumed at the moment.

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Asked: 2018-08-23 07:38:06 -0500

Seen: 148 times

Last updated: Sep 03 '18