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[Openstack] [nova] Nova-scheduler: when are filters applied?



On Mon, Sep 3, 2018 at 1:27 PM, Eugen Block <eblock at nde.ag> wrote:
> Hi,
> 
>> To echo what cfriesen said, if you set your allocation ratio to 1.0, 
>> the system will not overcommit memory. Shut down instances consume 
>> memory from an inventory management perspective. If you don't want 
>> any danger of an instance causing an OOM, you must set you 
>> ram_allocation_ratio to 1.0.
> 
> let's forget about the scheduler, I'll try to make my question a bit 
> clearer.
> 
> 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, right?
> 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.


> Now I boot one of the shutdown instances again, the compute node 
> would require additional 4 GB of RAM for that instance, and this 
> would lead to OOM, isn't that correct? So a ratio of 1.0 would not 
> prevent that from happening, would it?

Nova did not place the instance require 8G RAM to this host above. 
Therefore you can freely start up the two 4G consuming instances on 
this host later.

> Regards,
> Eugen
> 
> 
> Zitat von Jay Pipes <jaypipes at gmail.com>:
> 
>> On 08/30/2018 10:54 AM, Eugen Block wrote:
>>> Hi Jay,
>>> 
>>>> You need to set your ram_allocation_ratio nova.CONF option to 1.0 
>>>> if you're running into OOM issues. This will prevent overcommit 
>>>> of memory on your compute nodes.
>>> 
>>> I understand that, the overcommitment works quite well most of the 
>>> time.
>>> 
>>> It just has been an issue twice when I booted an instance that had 
>>> been shutdown a while ago. In the meantime there were new 
>>> instances created on that hypervisor, and this old instance 
>>> caused the OOM.
>>> 
>>> I would expect that with a ratio of 1.0 I would experience the same 
>>> issue, wouldn't I? As far as I understand the scheduler only 
>>> checks at instance creation, not when booting existing instances. 
>>> Is that a correct assumption?
>> 
>> To echo what cfriesen said, if you set your allocation ratio to 1.0, 
>> the system will not overcommit memory. Shut down instances consume 
>> memory from an inventory management perspective. If you don't want 
>> any danger of an instance causing an OOM, you must set you 
>> ram_allocation_ratio to 1.0.
>> 
>> The scheduler doesn't really have anything to do with this.
>> 
>> Best,
>> -jay
>> 
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> 
> 
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