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From Dominic Kim <style9...@gmail.com>
Subject Re: New scheduling algorithm proposal.
Date Fri, 18 May 2018 09:44:45 GMT
Hi Tyson
Thank you for comments.

First, total data size in Kafka would not be changed, since the number of
activation requests will be same though we activate more topics.
Same amount of data will be just split into different topics, so there
would be no need for more disk space in Kafka.
But it will increase parallel processing at Kafka layer, more number of
consumers will access to Kafka at the same time.

The reason why active/inactive is meaningful in the scene is, the size of
parallel processing will be dependent on it.
Though we have 100k and more topics(actions), if only 10 actions are being
invoked, there will be only 10 parallel processing.
The number of inactive topics does not affect the performance of active
consumers.
If we really need to support 100k parallel action invocation, surely we
need more nodes to handle them not only for just Kafka.
Kafka can horizontally scale out and number of active topics at some point
will always be lesser than the total number of topics,  Based on my
benchmark results, I expect it is enough to take with scale-out of Kafka.

Regarding topic cleanup, we don't need to clean them up by ourselves, Kafka
will clean up expired data based on retention configuration.
So if the topic is no more activated(the action is no more invoked), there
would be no actual data though the topic exists.
And as I said, even if data exists for some topics, they won't affect other
active consumers if they are not activated.

Regarding concurrent activation PR, it is very worthy change. And I
recognize it is orthogonal to my PR.
It will not only alleviate current performance issue but can be used with
my changes as well.

In current logics, controller schedule activations based on hash, your PR
would be much effective if some changes are made on scheduling logic.

Regarding bypassing kafka, I am inclined to use Kafka because it can act as
a kind of buffer.
If some activations are not processed due to some reasons such as lack of
resources or invoker failure and so on, Kafka can keep them up for some
times and guarantee `at most once` invocation though invocation might be a
bit delayed.

With regard to combined approach, I think that is great idea.
For that, container states should be shared among invokers and they can
send activation request to any containers(local, or remote).
As so invokers will utilize warmed resources which are not located in its
local.

But it will also introduce some synchronization issue among controllers and
invokers or it needs segregation between resources based scheduling at
controller and real invocation.
In the earlier case, since controller will schedule activations based on
resources status, it is required to synchronize them in realtime.
Invokers can send requests to any remote containers, there will be mismatch
in resource status between controllers and invokers.

In the later case, controller should be able to send requests to any
invokers then invoker will schedule the activations.
In this case also, invokers need to synchronize their container status
among them.

Under the situation all invokers have same resources status, if two
invokers received same action invocation requests, it's not easy to control
the traffic among them, because they will schedule requests to same
containers. And if we take similar approach with what you suggested, to
send intent to use the containers first, it will introduce increasing
latency overhead as more and more invokers joined the cluster.
I couldn't find any good way to handle this yet. And this is why I proposed
autonomous containerProxy to enable location free scheduling.

Finally regarding SPI, yes you are correct, ContainerProxy is highly
dependent on ContainerPool, I will update my PR as you guided.

Thanks
Regards
Dominic.


2018-05-18 2:22 GMT+09:00 Tyson Norris <tnorris@adobe.com.invalid>:

> Hi Dominic -
>
> I share similar concerns about an unbounded number of topics, despite
> testing with 10k topics. I’m not sure a topic being considered active vs
> inactive makes a difference from broker/consumer perspective? I think there
> would minimally have to be some topic cleanup that happens, and I’m not
> sure the impact of deleting topics in bulk will have on the system either.
>
> A couple of tangent notes related to container reuse to improve
> performance:
> - I’m putting together the concurrent activation PR[1] (to allow reuse of
> an warm container for multiple activations concurrently); this can improve
> throughput for those actions that can tolerate it (FYI per-action config is
> not implemented yet). It suffers from similar inaccuracy of kafka message
> ingestion at invoker “how many messages should I read”? But I think we can
> tune this a bit by adding some intelligence to Invoker/MessageFeed like “if
> I never see ContainerPool indicate it is busy, read more next time” - that
> is, allow ContainerPool to backpressure MessageFeed based on ability to
> consume, and not (as today) strictly on consume+process.
>
> - Another variant we are investigating is putting a ContainerPool into
> Controller. This will prevent container reuse across controllers (bad!),
> but will bypass kafka(good!). I think this will be plausible for actions
> that support concurrency, and may be useful for anything that runs as
> blocking to improve a few ms of latency, but I’m not sure of all the
> ramifications yet.
>
>
> Another (more far out) approach combines some of these is changing the
> “scheduling” concept to be more resource reservation and garbage
> collection. Specifically that the ContainerPool could be a combination of
> self-managed resources AND remote managed resources. If no proper (warm)
> container exists locally or remotely, a self-managed one is created, and
> advertised. Other ContainerPool instances can leverage the remote resources
> (containers). To pause or remove a container requires advertising intent to
> change state, and giving clients time to veto. So there is some added
> complication in the start/reserver/pause/rm container lifecycle, but the
> case for reuse is maximized in best case scenario (concurrency tolerant
> actions) and concurrency intolerant actions have a chance to leverage a
> broader pool of containers (iff the ability to reserve a shared available
> container is fast enough, compared to starting a new cold one). There is a
> lot wrapped in there (how are resources advertised, what are the new states
> of lifecycle, etc), so take this idea with a grain of salt.
>
>
> Specific to your PR: do you need an SPI for ContainerProxy? Or can it just
> be designated by the ContainerPool impl to use a specific ContainerProxy
> variant? I think these are now and will continue to be tied closely
> together, so would manage them as a single SPI.
>
> Thanks
> Tyson
>
> [1] https://github.com/apache/incubator-openwhisk/pull/2795<
> https://github.com/apache/incubator-openwhisk/pull/2795#
> pullrequestreview-115830270>
>
> On May 16, 2018, at 10:42 PM, Dominic Kim <style9595@gmail.com<mailto:st
> yle9595@gmail.com>> wrote:
>
> Dear all.
>
> Does anyone have any comments on this?
> Any comments or opinions would be greatly welcome : )
>
> I think we need around following changes to take this in.
>
> 1. SPI supports for ContainerProxy and ContainerPool ( I already opened PR
> for this: https://na01.safelinks.protection.outlook.com/?url=
> https%3A%2F%2Fgithub.com%2Fapache%2Fincubator-
> openwhisk%2Fpull%2F3663&data=02%7C01%7Ctnorris%40adobe.com%
> 7Cb2dd7c5686bc466f92f708d5bbb8f43d%7Cfa7b1b5a7b34438794aed2c178de
> cee1%7C0%7C0%7C636621325405375234&sdata=KYHyOGosc%
> 2BqMIFLVVRO2gYCYZRegfQLl%2BfGjSvtGI9k%3D&reserved=0)
> 2. SPI supports for Throttling/Entitlement logics
> 3. New loadbalancer
> 4. New ContainerPool and ContainerProxy
> 5. New notions for limit and logics to hanlde more fine-grained limit.(It
> should be able to coexist with existing limit)
>
> If there's no any comments, I will open a PR based on it one by one.
> Then it would be better for us to discuss on this because we can directly
> discuss over basic implementation.
>
> Thanks
> Regards
> Dominic.
>
>
>
> 2018-05-09 11:42 GMT+09:00 Dominic Kim <style9595@gmail.com<mailto:st
> yle9595@gmail.com>>:
>
> One more thing to clarify is invoker parallelism.
>
> ## Invoker coordinates all requests.
>
> I tend to disagree with the "cannot take advantage of parallel
> processing" bit. Everything in the invoker is parallelized after updating
> its central state (which should take a **very** short amount of time
> relative to actual action runtime). It is not really optimized to scale to
> a lot of containers *yet*.
>
> More precisely, I think it is related to Kafka consumer rather than
> invoker. Invoker logic can run in parallel. But `MessageFeed` seems not.
> Once outstanding message size reaches max size, it will wait for messages
> processed. If few activation messages for an action are not properly
> handled, `MessageFeed` does not consume more message from Kafka(until any
> messages are processed).
> So subsequent messages for other actions cannot be fetched or delayed due
> to unprocessed messages. This is why I mentioned invoker parallelism. I
> think I should rephrase it as `MessageFeed` parallelism.
>
> As you know partition is unit of parallelism in Kafka. If we have multiple
> partitions for activation topic, we can setup multiple consumers and it
> will enable parallel processing for Kafka messages as well.
> Since logics in invoker can already run in parallel, with this change, we
> can process messages entirely in parallel.
>
> In my proposal, I split activation messages from container coordination
> message(action parallelism), assign more partition for activation
> messages(in-topic parallelism) and enable parallel processing with multiple
> consumers(containers).
>
>
> Thanks
> Regards
> Dominic.
>
>
>
>
>
> 2018-05-08 19:34 GMT+09:00 Dominic Kim <style9595@gmail.com<mailto:st
> yle9595@gmail.com>>:
>
> Thank you for the response Markus and Christian.
>
> Yes I agree that we need to discuss this proposal in abstract way instead
> in conjunction it with any specific technology because we can take better
> software stack if possible.
> Let me answer your questions line by line.
>
>
> ## Does not wait for previous run.
>
> Yes it is valid thoughts. If we keep cumulating requests in the queue,
> latency can be spiky especially in case execution time of action is huge.
> So if we want to take this in, we need to find proper way to balance
> creating more containers for latency and making existing containers handle
> requests.
>
>
> ## Not able to accurately control concurrent invocation.
>
> Ok I originally thought this is related to concurrent containers rather
> than concurrent activations.
> But I am still inclined to concurrent containers approach.
> In current logic, it is dependent on factors other than real concurrent
> invocations.
> If RTT between controllers and invokers becomes high for some reasons,
> controller will reject new requests though invokers are actually idle.
>
> ## TPS is not deterministic.
>
> I meant not deterministic TPS for just one user rather I meant
> system-wide deterministic TPS.
> Surely TPS can vary when heterogenous actions(which have different
> execution time) are invoked.
> But currently it's not easy to figure out what the TPS is with only 1
> kind of action because it is changed based on not only heterogeneity of
> actions but the number of users and namespaces.
>
> I think at least we need to be able to have this kind of official spec:
> In case actions with 20 ms execution time are invoked, our system TPS is
> 20,000 TPS(no matter how many users or namespaces are used).
>
>
> Your understanding about my proposal is perfectly correct.
> Small thing to add is, controller sends `ContainerCreation` request based
> on processing speed of containers rather than availability of existing
> containers.
>
> BTW, regarding your concern about Kafka topic, I think we may be fine
> because,
> the number of topics will be unbounded, but the number of active topics
> will be bounded.
>
> If we take this approach, it is mandatory to limit retention bytes and
> duration for each topics.
> So the number of active topics is limited and actual data in them are
> also limited, so I think that would be fine.
>
> But it is necessary to have optimal configurations for retention and many
> benchmark to confirm this.
>
>
> And I didn't get the meaning of eventual consistency of consumer lag.
> You meant that is eventual consistent because it changes very quickly
> even within one second?
>
>
> Thanks
> Regards
> Dominic
>
>
>
> 2018-05-08 17:25 GMT+09:00 Markus Thoemmes <markus.thoemmes@de.ibm.com<
> mailto:markus.thoemmes@de.ibm.com>>:
>
> Hey Dominic,
>
> Thank you for the very detailed writeup. Since there is a lot in here,
> please allow me to rephrase some of your proposals to see if I understood
> correctly. I'll go through point-by-point to try to keep it close to your
> proposal.
>
> **Note:** This is a result of an extensive discussion of Christian
> Bickel (@cbickel) and myself on this proposal. I used "I" throughout the
> writeup for easier readability, but all of it can be read as "we".
>
> # Issues:
>
> ## Interventions of actions.
>
> That's a valid concern when using today's loadbalancer. This is
> noisy-neighbor behavior that can happen today under the circumstances you
> describe.
>
> ## Does not wait for previous run.
>
> True as well today. The algorithms used until today value correctness
> over performance. You're right, that you could track the expected queue
> occupation and schedule accordingly. That does have its own risks though
> (what if your action has very spiky latency behavior?).
>
> I'd generally propose to break this out into a seperate discussion. It
> doesn't really correlate to the other points, WDYT?
>
> ## Invoker coordinates all requests.
>
> I tend to disagree with the "cannot take advantage of parallel
> processing" bit. Everything in the invoker is parallelized after updating
> its central state (which should take a **very** short amount of time
> relative to actual action runtime). It is not really optimized to scale to
> a lot of containers *yet*.
>
> ## Not able to accurately control concurrent invocation.
>
> Well, the limits are "concurrent actions in the system". You should be
> able to get 5 activations on the queue with today's mechanism. You should
> get as many containers as needed to handle your load. For very
> short-running actions, you might not need N containers to handle N messages
> in the queue.
>
> ## TPS is not deterministic.
>
> I'm wondering: Have TPS been deterministic for just one user? I'd argue
> that this is a valid metric on its own kind. I agree that these numbers can
> drop significantly under heterogeneous load.
>
> # Proposal:
>
> I'll try to rephrase and add some bits of abstraction here and there to
> see if I understood this correctly:
>
> The controller should schedule based on individual actions. It should
> not send those to an arbitrary invoker but rather to something that
> identifies those actions themselves (a kafka topic in your example). I'll
> call this *PerActionContainerPool*. Those calls from the controller will be
> handled by each *ContainerProxy* directly rather than being threaded
> through another "centralized" component (the invoker). The *ContainerProxy*
> is responsible for handling the "aftermath": Writing activation records,
> collecting logs etc (like today).
>
> Iff the controller thinks that the existing containers cannot sustain
> the load (i.e. if all containers are currently in use), it advises a
> *ContainerCreationSystem* (all invokers combined in your case) to create a
> new container. This container will be added to the
> *PerActionContainerPool*.
>
> The invoker in your proposal has no scheduling logic at all (which is
> sound with the issues lined out above) other than container creation
> itself.
>
> # Conclusion:
>
> I like the proposal in the abstract way I've tried to phrase above. It
> indeed amplifies warm-container usage and in general should be superior to
> the more statistical approach of today's loadbalancer.
>
> I think we should discuss this proposal in an abstract,
> non-technology-bound way. I do think that having so many kafka topics
> including all the rebalancing needed can become an issue, especially
> because the sheer number of kafka topics is unbounded. I also think that
> the consumer lag is subject to eventual consistency and depending on how
> eventual that is it can turn into queueing in your system, even though that
> wouldn't be necessary from a capacity perspective.
>
> I don't want to ditch the proposal because of those concerns though!
>
> As I said: The proposal itself makes a lot of sense and I like it a lot!
> Let's not trap ourselves in the technology used today though. You're
> proposing a major restructuring so we might as well think more
> green-fieldy. WDYT?
>
> Cheers,
> Christian and Markus
>
>
>
>
>
>

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