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

Georgios Dimitrakakis giorgis at acmac.uoc.gr
Wed Apr 17 22:08:27 UTC 2019

OK! I have applied the patch and now weights are shown!
Furthermore as per your suggestion I have removed the "RamFilter" which was the only one present

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...


>>> On Wed, 17 Apr 2019 12:55:45 -0700, Melanie Witt <melwittt at gmail.com> wrote:
>>> On Wed, 17 Apr 2019 22:45:45 +0300, Georgios Dimitrakakis
>>> <giorgis at acmac.uoc.gr> wrote:
>>>>  Hello again Menalie!
>>>>  Exactly this is what I am thinking...something is not working
>>>>  correctly!
>>>>  To answer your questions there is one node acting as controller where
>>>>  the scheduler is running and I have pasted the nova.conf file from
>>>>  there.
>>>>  I have also noticed that I have "ram_weight_multiplier" two times (one
>>>>  in [cells] and one in [filter_scheduler]) therefore I have removed the
>>>>  one in [cells] because I though it might give a problem but the results
>>>>  are still the same.
>>>>  The log for the scheduler has this entry:
>>>>  2019-04-17 22:04:50.045 131723 DEBUG oslo_service.service
>>>>  [req-7e548ecb-f3ed-4a4d-835f-b3a996e32534 - - - - -]
>>>>  filter_scheduler.ram_weight_multiplier = -1.0 log_opt_values
>>>>  /usr/lib/python2.7/site-packages/oslo_config/cfg.py:3032
>>>>  so it seems to be picked up correctly but without any influence.
>>> Agreed, that log shows that the -1.0 value is being picked up properly
>>> by the scheduler service.
>>>>  What also worries me from the scheduler log that I have send to you
>>>>  before is that in there I see an entry like this:
>>>>  2019-04-17 19:53:07.298 98874 DEBUG nova.filters
>>>>  [req-02fb5504-cbdb-4219-9509-d2be9da7bb0e
>>>>  6a4c2e32919e4a6fa5c5d956beb68eef 9f22e9bfa7974e14871d58bbb62242b2 -
>>>>  default default] Filter RamFilter returned 2 host(s)
>>>>  get_filtered_objects
>>>>  /usr/lib/python2.7/site-packages/nova/filters.py:104
>>>>  Shouldn't the RamFilter return 1host and the one with less RAM? Why
>>>>  does it return 2hosts??
>>> No -- the RamFilter will return any hosts that meet the RAM requirement.
>>> Filters do not weigh hosts. The RamFilter returns two hosts because both
>>> hosts have enough RAM to fulfill the request. FYI though, as of Pike
>>> [1], the (Core|Ram|Disk)Filter are redundant, as placement will do the
>>> filtering for those resources before the nova scheduler filters run. So
>>> you can safely remove (Core|Ram|Disk)Filter from your enabled_filters.
>>> [1]
>>> https://docs.openstack.org/releasenotes/nova/pike.html#relnotes-16-0-0-stable-pike-upgrade-notes
>>>>  If you have any other ideas or would like me to do some more checking I
>>>>  am all ears!
>>> At this point, you could take Matt's suggestion from his latest reply on
>>> this thread and patch in the logging regression fix he linked. That
>>> would allow you to see in the debug log what weights nova is giving to
>>> the hosts.
>> OK, so I just searched open nova bugs for "weigh" and found this
>> issue, which isn't necessarily a defect:
>> https://bugs.launchpad.net/nova/+bug/1818239
>> but something that could be affecting the host weighing in your
>> environment. There's something called the BuildFailureWeigher which
>> will apply a low weight multiplier to hosts that have had VMs fail to
>> build on them. And that weight resets when a host experiences a
>> successful VM build.
>> If you apply the patch Matt suggested and take a look at the host
>> weights, we should be able to see whether the BuildFailureWeigher is
>> involved in the behavior you're seeing.
>> -melanie
>>> Aside from that, it's looking like we/I would need to reproduce this
>>> issue locally with a devstack and try to figure out what's causing this
>>> behavior.
>>> -melanie
>>>>>> Thank you both Melanie and Matt for trying to assist me.
>>>>>> I have double checked the nova.conf at the controller and here is
>>>>>> what
>>>>>>   I have (ignored hashed lines and obfuscating sensitive data):
>>>>>>   https://pastebin.com/hW1PE4U7
>>>>>> As you can see I have everything with default values as discussed
>>>>>>   before with Melanie except the filters and the weight that I have
>>>>>>   applied that should lead to VM stacking instead of spreading.
>>>>>> My case scenario is with two compute hosts (let's call them "cpu1"
>>>>>> and
>>>>>>   "cpu2") and when an instance is already placed on "cpu2" I expect
>>>>>> the
>>>>>>   next instance to be placed also there. But instead is placed on
>>>>>> "cpu1"
>>>>>>   as you can see from the scheduler log that can find here:
>>>>>>   https://pastebin.com/sCzB9L2e
>>>>>> Do you see something strange that I fail to recognize?
>>>>> Thanks for providing the helpful data. It appears you have set your
>>>>> nova.conf correctly (this is where your scheduler is running, yes?).
>>>>> I
>>>>> notice you have duplicated the ram_weight_multiplier setting but that
>>>>> shouldn't hurt anything.
>>>>> The relevant scheduler log is this one:
>>>>> 2019-04-17 19:53:07.303 98874 DEBUG nova.scheduler.filter_scheduler
>>>>> [req-02fb5504-cbdb-4219-9509-d2be9da7bb0e
>>>>> 6a4c2e32919e4a6fa5c5d956beb68eef 9f22e9bfa7974e14871d58bbb62242b2 -
>>>>> default default] Weighed [(cpu1, cpu1) ram: 32153MB disk: 1906688MB
>>>>> io_ops: 0 instances: 0, (cpu2, cpu2) ram: 30105MB disk: 1886208MB
>>>>> io_ops: 0 instances: 1] _get_sorted_hosts
>>>>> /usr/lib/python2.7/site-packages/nova/scheduler/filter_scheduler.py:455
>>>>> and here we see that host 'cpu1' is being weighed ahead of host
>>>>> 'cpu2', which is the problem. I don't understand this considering the
>>>>> docs say that setting the ram_weight_multiplier to a negative value
>>>>> should result in the host with the lesser RAM being weighed
>>>>> higher/first. According to your log, the opposite is happening --
>>>>> 'cpu1' with 32153MB RAM is being weighed higher than 'cpu2' with
>>>>> 30105MB RAM.
>>>>> Either your ram_weight_multiplier setting is not being picked up or
>>>>> there's a bug causing weight to be applied with reverse logic?
>>>>> Can you look at the scheduler debug log when the service first
>>>>> started up and verify what value of ram_weight_multiplier the service
>>>>> is using?
>>>>> -melanie
>>>>>>>> On 4/16/2019 7:03 PM, melanie witt wrote:
>>>>>>>> To debug further, you should set debug to True in the nova.conf on
>>>>>>>> your scheduler host and look for which filter is removing the
>>>>>>>> desired
>>>>>>>> host for the second VM. You can find where to start by looking for
>>>>>>>> a
>>>>>>>> message like, "Starting with N host(s)". If you have two hosts
>>>>>>>> with
>>>>>>>> enough RAM, you should see "Starting with 2 host(s)" and then look
>>>>>>>> for
>>>>>>>> the log message where it says "Filter returned 1 host(s)" and that
>>>>>>>> will be the filter that is removing the desired host. Once you
>>>>>>>> know
>>>>>>>> which filter is removing it, you can debug further.
>>>>>>> If the other host isn't getting filtered out, it could be the
>>>>>>> weighers that aren't prioritizing the host you expect, but debug
>>>>>>> logs
>>>>>>> should dump the weighed hosts as well which might give a clue.

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