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


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. 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
>