Open Stack

Mathieu Gagné wrote:
> Hi,
>
> For those that attended the OpenStack Ops meetup, you probably heard
> me complaining about a serious performance issue we had with Nova
> scheduler (Kilo) with Ironic.

BTW, thanks for helping push this and complaining about it and ...

It's a tough and thankless job but it's needed IMHO :)

Without further ado,

>
> Thanks to Sean Dague and Matt Riedemann, we found the root cause.
>
> It was caused by this block of code [1] which is hitting the database
> for each node loaded by the scheduler. This block of code is called if
> no instance info is found in the scheduler cache.
>
> I found that this instance info is only populated if the
> scheduler_tracks_instance_changes config [2] is enabled which it is by
> default. But being a good operator (wink wink), I followed the Ironic
> install guide which recommends disabling it [3], unknowingly getting
> myself into deep troubles.
>
> There isn't much information about the purpose of this config in the
> kilo branch. Fortunately, you can find more info in the master branch
> [4], thanks to the config documentation effort. This instance info
> cache is used by filters which rely on instance location to perform
> affinity/anti-affinity placement or anything that cares about the
> instances running on the destination node.
>
> Enabling this option will make it so Nova scheduler loads instance
> info asynchronously at start up. Depending on the number of
> hypervisors and instances, it can take several minutes. (we are
> talking about 10-15 minutes with 600+ Ironic nodes, or ~1s per node in
> our case)

This feels like a classic thing that could just be made better by a 
scatter/gather (in threads or other?) to the database or other service. 
1s per node seems ummm, sorta bad and/or non-optimal (I wonder if this 
is low hanging fruit to improve this). I can travel around the world 7.5 
times in that amount of time (if I was a light beam, haha).

>
> So Jim Roll jumped into the discussion on IRC and found a bug [5] he
> opened and fixed in Liberty. It makes it so Nova scheduler never
> populates the instance info cache if Ironic host manager is loaded.
> For those running Nova with Ironic, you will agree that there is no
> known use case where affinity/anti-affinity is used. (please reply if
> you know of one)
>
> To summarize, the poor performance of Nova scheduler will only show if
> you are running the Kilo version of Nova and you disable
> scheduler_tracks_instance_changes which might be the case if you are
> running Ironic too.
>
> For those curious about our Nova scheduler + Ironic setup, we have
> done the following to get nova scheduler to ludicrous speed:
>
> 1) Use CachingScheduler
>
> There was a great talk at the OpenStack Summit about why you would
> want to use it. [6]
>
> By default, the Nova scheduler will load ALL nodes (hypervisors) from
> database to memory before each scheduling. If you have A LOT of
> hypervisors, this process can take a while. This means scheduling
> won't happen until this step is completed. It could also mean that
> scheduling will always fail if you don't tweak service_down_time (see
> 3 below) if you have lot of hypervisors.
>
> This driver will make it so nodes (hypervisors) are loaded in memory
> every ~60 seconds. Since information is now pre-cached, the scheduling
> process can happen right away, it is super fast.
>
> There is a lot of side-effects to using it though. For example:
> - you can only run ONE nova-scheduler process since cache state won't
> be shared between processes and you don't want instances to be
> scheduled twice to the same node/hypervisor.

Out of curiosity, do you have only one scheduler process active and 
passive scheduler process(es) idle waiting to become active if the other 
schedule dies? (pretty simply done via something like 
https://kazoo.readthedocs.io/en/latest/api/recipe/election.html) Or do 
you have some manual/other process that kicks off a new scheduler if the 
'main' one dies?

> - It can take ~1m before new capacity is recognized by the scheduler.
> (new or freed nodes) The cache is refreshed every 60 seconds with a
> periodic task. (this can be changed with scheduler_driver_task_period)
>
> In the context of Ironic, it is a compromise we are willing to accept.
> We are not adding Ironic nodes that often and nodes aren't
> created/deleting as often as virtual machines.
>
> 2) Run a single nova-compute service
>
> I strongly suggest you DO NOT run multiple nova-compute services. If
> you do, you will have duplicated hypervisors loaded by the scheduler
> and you could end up with conflicting scheduling. You will also have
> twice as much hypervisors to load in the scheduler.

This seems scary (whenever I hear run a single of anything in a *cloud* 
platform, that makes me shiver). It'd be nice if we at least recommended 
people run 
https://kazoo.readthedocs.io/en/latest/api/recipe/election.html or have 
some active/passive automatic election process to handle that single 
thing dying (which they usually do, at odd times of the night). Honestly 
I'd (personally) really like to get to the bottom of how we as a group 
of developers ever got to the place where software was released (and/or 
even recommended to be used) in a *cloud* platform that ever required 
only one of anything to be ran (that's crazy bonkers, and yes there is 
history here, but damn, it just feels rotten as all hell, for lack of 
better words).

>
> Note: I heard about multiple compute host support in Nova for Ironic
> with use of an hash ring but I don't have much details about it. So
> this recommendation might not apply to you if you are using a recent
> version of Nova.
>
> 3) Increase service_down_time
>
> If you have a lot of nodes, you might have to increase this value
> which is set to 60 seconds by default. This value is used by the
> ComputeFilter filter to exclude nodes it hasn't heard from. If it
> takes more than 60 seconds to list the list of nodes, you might guess
> what we will happen, the scheduler will reject all of them since node
> info is already outdated when it finally hits the filtering steps. I
> strongly suggest you tweak this setting, regardless of the use of
> CachingScheduler.

Same kind of feeling I had above also applies, something feels broken if 
such things have to be found by operators (I'm pretty sure yahoo when I 
was there saw something similar) and not by the developers making the 
software. If I could (and I know I really can't due to the community we 
work in) I'd very much have an equivalent of a retrospective around how 
these kinds of solutions got built and how they ended up getting 
released to the wider public with such flaws....

>
> 4) Tweak scheduler to only load empty nodes/hypervisors
>
> So this is a hack [7] we did before finding out about the bug [5] we
> described and identified earlier. When investigating our performance
> issue, we enabled debug logging and saw that periodic task was taking
> forever to complete (10-15m) with CachingScheduler driver.
>
> We knew (strongly suspected) Nova scheduler was spending a huge amount
> of time loading nodes/hypervisors. We (unfortunately) didn't push
> further our investigation and jumped right away to optimization phase.
>
> So we came up with the idea of only loading empty nodes/hypervisors.
> Remember, we are still in the context of Ironic, not cloud and virtual
> machines. So it made perfect sense for us to stop spending time
> loading nodes/hypervisors we would discard anyway.
>
> Thanks to all that help us debugging our scheduling performance
> issues, it is now crazy fast. =)
>
> [1] https://github.com/openstack/nova/blob/kilo-eol/nova/scheduler/host_manager.py#L589-L592
> [2] https://github.com/openstack/nova/blob/kilo-eol/nova/scheduler/host_manager.py#L65-L68
> [3] http://docs.openstack.org/developer/ironic/deploy/install-guide.html#configure-compute-to-use-the-bare-metal-service
> [4] https://github.com/openstack/nova/blob/282c257aff6b53a1b6bb4b4b034a670c450d19d8/nova/conf/scheduler.py#L166-L185
> [5] https://bugs.launchpad.net/nova/+bug/1479124
> [6] https://www.youtube.com/watch?v=BcHyiOdme2s
> [7] https://gist.github.com/mgagne/1fbeca4c0b60af73f019bc2e21eb4a80
>
> --
> Mathieu
>
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