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[nova][scheduler] - Stack VMs based on RAM


On Thu, 18 Apr 2019 01:38:17 +0100, Sean Mooney <smooney at redhat.com> wrote:
> On Wed, 2019-04-17 at 15:30 -0700, melanie witt wrote:
>> On Thu, 18 Apr 2019 01:13:42 +0300, Georgios Dimitrakakis
>> <giorgis at acmac.uoc.gr> wrote:
>>> Shouldnâ??t that be the correct behavior and place the new VM on the host with the smaller weight? Isnâ??t that what the
>>> negative value for â??ram_weight_multiplierâ?? does ?
>>
>> No, it's the opposite, higher weights win. That's why you have to use a
>> negative value for ram_weight_multiplier if you want hosts with _less_
>> RAM to win over hosts with more RAM (stacking).
>>
>>> Please let me know how I can provide to you more debug info....
>>
>> One thing I noticed from your log is on the second request, 'cpu1' has
>> io_ops: 0 whereas 'cpu2' has io_ops: 1 and the IoOpsWeigher [1] will
>> prefer hosts with fewer io_iops by default. Note that's only one piece
>> of the ending weight -- the weighing process will take a sum of all of
>> the weights each weigher returns. So the weight returned from RamWeigher
>> is added to the weight returned from IoOpsWeigher is added the weight
>> returned from CPUWeigher, and so on.
>>
>> So, as Matt said, we're a bit in the dark now as far as what each
>> weigher is returning and we don't currently have debug logging per
>> weigher the way we do for filters. That would be an enhancement we could
>> make to aid in debugging issues like this one. You could hack something
>> up locally to log/print the returned weight in each weight class under
>> the nova/scheduler/weights/ directory, if you want to dig into that.
>>
>> Another thing I noticed is that there are probably some new weighers
>> available by default that did not exist in the previous version of nova
>> that you were using in the past. By default, the config option for weighers:
>>
>> [filter_scheduler]weight_classes = ["nova.scheduler.weights.all_weighers"]
>>
>> will pick up all weigher classes in the nova/scheduler/weights/ code
>> directory. You might take a look at these and see if any are ones you
>> should exclude in your environment. For example, the CPUWeigher [1] (new
>> in Rocky) will spread VMs based on available CPU by default.
> most of the weighers spread by default so the cpu weigher may be a factor but
> the the disk weigher tends to all hevily impact the choice.
> 
> we do not normalise any of the values retured by the different weighers
> the disk wighter is basically  host_state.free_disk_mb * disk_weight_multiplier

Hm, I thought we do based on this code:

https://github.com/openstack/nova/blob/stable/rocky/nova/weights.py#L135

which normalizes the weight to a value between 0.0 and 1.0.

If the multiplier is large though, that could make the considered value 
 > 1.0 (as is the case with the default build_failure_weight_multiplier:

https://github.com/openstack/nova/blob/stable/rocky/nova/conf/scheduler.py#L501

-melanie

> althoer host_state.free_disk_mb is actully disk_available_least.
> 
> as a result the disk filter will weigh cpu1 19456 height then cpu2
> the dela between the cpu1 and cpu2 based on the ram weigher is only 2048
> 
> if you want the ram filter to take presedence over teh disk filter you will
> need to scale the disk filter down to be in a similar value range
> 
> i woudl suggest setting disk_weight_multiplier=0.001
> 
>> This
>> weigher might be contributing to the VM spreading you're seeing. You
>> might try playing with the '[filter_scheduler]weight_classes' config
>> option to select only the weighers you want or alternatively you could
>> set the weighers multipliers the way you prefer.
>>
>> -melanie
>>
>> [1] https://docs.openstack.org/nova/rocky/user/filter-scheduler.html#weights
>>
>>>>> On 4/17/2019 3:50 PM, Georgios Dimitrakakis wrote:
>>>>> And here is the new log where spawning of 2 VMs can be seen with a few seconds of difference:
>>>>> https://pastebin.com/Xy2FL2KL
>>>>> Initially both hosts are of weight 1.0 then the one with one VM already running has negative weight but the new
>>>>> VM is placed on the other host.
>>>>> Really-really strange why this is happening...
>>>>
>>>> < 2019-04-17 23:26:18.770 157355 DEBUG nova.scheduler.filter_scheduler [req-14c666e4-3ff4-4d88-947e-377b3d37bff9
>>>> 6a4c2e32919e4a6fa5c5d956beb68eef 9f22e9bfa7974e14871d58bbb62242b2 - default default] Filtered [(cpu2, cpu2) ram:
>>>> 30105MB disk: 1887232MB io_ops: 1 instances: 1, (cpu1, cpu1) ram: 32153MB disk: 1906688MB io_ops: 0 instances: 0]
>>>> _get_sorted_hosts /usr/lib/python2.7/site-packages/nova/scheduler/filter_scheduler.py:435
>>>>
>>>> < 2019-04-17 23:26:18.771 157355 DEBUG nova.scheduler.filter_scheduler [req-14c666e4-3ff4-4d88-947e-377b3d37bff9
>>>> 6a4c2e32919e4a6fa5c5d956beb68eef 9f22e9bfa7974e14871d58bbb62242b2 - default default] Weighed [WeighedHost [host:
>>>> (cpu1, cpu1) ram: 32153MB disk: 1906688MB io_ops: 0 instances: 0, weight: 1.0], WeighedHost [host: (cpu2, cpu2)
>>>> ram: 30105MB disk: 1887232MB io_ops: 1 instances: 1, weight: -0.00900862553213]] _get_sorted_hosts
>>>> /usr/lib/python2.7/site-packages/nova/scheduler/filter_scheduler.py:454
>>>>
>>>> cpu1 is definitely getting weighed higher but I'm not sure why. We likely need some debug logging on the result of
>>>> each weigher like we have for each filter to figure out what's going on with the weighers.
>>>>
>>>> -- 
>>>>
>>>> Thanks,
>>>>
>>>> Matt
>>
>>
>>
>>
>>
>