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Thanks, that is a very good explanation, I get it now.

Thank you very much for your answers!


Zitat von Balázs Gibizer <balazs.gibizer at ericsson.com>:

> 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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