[openstack-dev] [nova] Proposal: Move CPU and memory allocation ratio out of scheduler

Jay Pipes jaypipes at gmail.com
Wed Jun 4 18:02:23 UTC 2014

On 06/03/2014 10:40 PM, ChangBo Guo wrote:
> Jay, thanks for raising this up .
> +1 for this .
>   A related question about the CPU and RAM allocation ratio, shall we
> apply them when get hypervisor information with command "nova
> hypervisor-show ${hypervisor-name}"
> The output  shows like
> | memory_mb                 | 15824 |
> | memory_mb_used            | 1024 |
> | running_vms               | 1 |
> | service_host              | node-6 |
> | service_id                | 39 |
> | vcpus                     | 4 |
> | vcpus_used                | 1
> vcpus is showing the number of physical CPU, I think that's not
> correct.  Any thoughts ?

Yes, I believe it would be appropriate to return the adjusted total of 
vCPU and memory. This would be trivial if we actually stored the 
allocation ratios in each compute node record, where they naturally 
belong (as the ratios describe an attribute of the compute node, not any 
scheduling policy), instead of in the scheduler filters.


> 2014-06-03 21:29 GMT+08:00 Jay Pipes <jaypipes at gmail.com
> <mailto:jaypipes at gmail.com>>:
>     Hi Stackers,
>     tl;dr
>     =====
>     Move CPU and RAM allocation ratio definition out of the Nova
>     scheduler and into the resource tracker. Remove the calculations for
>     overcommit out of the core_filter and ram_filter scheduler pieces.
>     Details
>     =======
>     Currently, in the Nova code base, the thing that controls whether or
>     not the scheduler places an instance on a compute host that is
>     already "full" (in terms of memory or vCPU usage) is a pair of
>     configuration options* called cpu_allocation_ratio and
>     ram_allocation_ratio.
>     These configuration options are defined in, respectively,
>     nova/scheduler/filters/core___filter.py and
>     nova/scheduler/filters/ram___filter.py.
>     Every time an instance is launched, the scheduler loops through a
>     collection of host state structures that contain resource
>     consumption figures for each compute node. For each compute host,
>     the core_filter and ram_filter's host_passes() method is called. In
>     the host_passes() method, the host's reported total amount of CPU or
>     RAM is multiplied by this configuration option, and the product is
>     then subtracted from the reported used amount of CPU or RAM. If the
>     result is greater than or equal to the number of vCPUs needed by the
>     instance being launched, True is returned and the host continues to
>     be considered during scheduling decisions.
>     I propose we move the definition of the allocation ratios out of the
>     scheduler entirely, as well as the calculation of the total amount
>     of resources each compute node contains. The resource tracker is the
>     most appropriate place to define these configuration options, as the
>     resource tracker is what is responsible for keeping track of total
>     and used resource amounts for all compute nodes.
>     Benefits:
>       * Allocation ratios determine the amount of resources that a
>     compute node advertises. The resource tracker is what determines the
>     amount of resources that each compute node has, and how much of a
>     particular type of resource have been used on a compute node. It
>     therefore makes sense to put calculations and definition of
>     allocation ratios where they naturally belong.
>       * The scheduler currently needlessly re-calculates total resource
>     amounts on every call to the scheduler. This isn't necessary. The
>     total resource amounts don't change unless either a configuration
>     option is changed on a compute node (or host aggregate), and this
>     calculation can be done more efficiently once in the resource tracker.
>       * Move more logic out of the scheduler
>       * With the move to an extensible resource tracker, we can more
>     easily evolve to defining all resource-related options in the same
>     place (instead of in different filter files in the scheduler...)
>     Thoughts?
>     Best,
>     -jay
>     * Host aggregates may also have a separate allocation ratio that
>     overrides any configuration setting that a particular host may have
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> --
> ChangBo Guo(gcb)
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