[Pacemaker] placement-strategy=minimal - placing and logging

[Lars Marowsky-Bree]

On 2013-03-07T21:34:47, Vladimir <ml at foomx.de> wrote:

> All resources are only able to run if they are distributed in the right
> combination. A working example could like:

The algorithm is somewhat simplistic, which has the advantage of being
fast. It works "quite well" in scenarios where there's a number of nodes
available and differences between resources are not so large, but it
won't always find the "optimal" solution.

Optimal placement (rucksack problem) is NP-complete, so all realistic
algorithms implement heuristics.

In theory, the case you describe is probably one that can be solved
easily enough, but the current one doesn't. We do accept patches ;-)

(If someone is looking for a master's thesis topic, transforming the
constraints into linear equations and applying an appropriate
optimization function for location scores/utilization and finding a
discrete solution using one of the available libraries is probably the
way to go.)

> Is there a possibility to configure it in another way?

You can influence the placement of resources using resource priorities,
but of course that gets a bit hackish for larger configurations and not
exactly automatic.

If you know the work packaes exactly, you can also use collocation sets.
That's quite feasible for a low node count, which is where the current
algorithm is least effective.

> Furthermore if there is a lack of configured cores and thus a resource
> cannot be started I don't see any log messages or crm_mon output.

This part is normal, yes. You don't get an error message if -inf
colocation constraints prevent a resource from being placed, either.

>    dc-version="1.1.6-9971ebba4494012a93c03b40a2c58ec0eb60f50c" \

There have been some fixes/improvements to utilization based placement
since. But I'm not sure if they'd help you.


Regards,
    Lars

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