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Forcing an instance to a specific host is very useful for the operator - it fulfills a valid use case for monitoring and testing purposes. I am not defending a particular way of doing this, just bringing up that it has to be handled. The effect on limits is purely implementation - no limits get set so it by-passes any resource constraints, which is deliberate.

-----Original Message-----
From: Jay Pipes [mailto:jaypipes at gmail.com] 
Sent: 04 June 2014 19:17
To: openstack-dev at lists.openstack.org
Subject: Re: [openstack-dev] [nova] Proposal: Move CPU and memory allocation ratio out of scheduler

On 06/04/2014 06:10 AM, Murray, Paul (HP Cloud) wrote:
> Hi Jay,
>
> This sounds good to me. You left out the part of limits from the 
> discussion - these filters set the limits used at the resource tracker.

Yes, and that is, IMO, bad design. Allocation ratios are the domain of the compute node and the resource tracker. Not the scheduler. The allocation ratios simply adjust the amount of resources that the compute node advertises to others. Allocation ratios are *not* scheduler policy, and they aren't related to flavours.

> You also left out the force-to-host and its effect on limits.

force-to-host is definitively non-cloudy. It was a bad idea that should never have been added to Nova in the first place.

That said, I don't see how force-to-host has any affect on limits. 
Limits should not be output from the scheduler. In fact, they shouldn't be anything other than an *input* to the scheduler, provided in each host state struct that gets built from records updated in the resource tracker and the Nova database.

 > Yes, I
> would agree with doing this at the resource tracker too.
>
> And of course the extensible resource tracker is the right way to do 
> it J

:) Yes, clearly this is something that I ran into while brainstorming around the extensible resource tracker patches.

Best,
-jay

> Paul.
>
> *From:*Jay Lau [mailto:jay.lau.513 at gmail.com]
> *Sent:* 04 June 2014 10:04
> *To:* OpenStack Development Mailing List (not for usage questions)
> *Subject:* Re: [openstack-dev] [nova] Proposal: Move CPU and memory 
> allocation ratio out of scheduler
>
> Does there is any blueprint related to this? Thanks.
>
> 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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>
>
>
> --
>
> Thanks,
>
> Jay
>
>
>
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