Thanks, that is a very good explanation, I get it now.
Thank you very much for your answers!
Zitat von Balázs Gibizer <[email protected]>:
On Mon, Sep 3, 2018 at 1:27 PM, Eugen Block <[email protected]> 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 <[email protected]>:
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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