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From Gwenhael Pasquiers <gwenhael.pasqui...@ericsson.com>
Subject RE: Distribution of sinks among the nodes
Date Thu, 04 Feb 2016 15:25:04 GMT
Okay ;

Then I guess that the best we can do is to disable chaining (we really want one thread per
operator since they are doing long operations) and have the same parallelism for sinks as
mapping : that way each map will have it’s own sink and there will be no exchanges between
flink instances.

From: ewenstephan@gmail.com [mailto:ewenstephan@gmail.com] On Behalf Of Stephan Ewen
Sent: jeudi 4 février 2016 15:13
To: user@flink.apache.org
Subject: Re: Distribution of sinks among the nodes

To your other question, there are two things in Flink:

(1) Chaining. Tasks are folded together into one task, run by one thread.

(2) Resource groups: Tasks stay separate, have separate threads, but share a slot (which means
share memory resources). See the link in my previous mail for an explanation concerning those.

Greetings,
Stephan


On Thu, Feb 4, 2016 at 3:10 PM, Stephan Ewen <sewen@apache.org<mailto:sewen@apache.org>>
wrote:
Hi Gwen!

You actually need not 24 slots, but only as many as the highest parallelism is (16). Slots
do not hold individual tasks, but "pipelines".

Here is an illustration how that works.
https://ci.apache.org/projects/flink/flink-docs-release-0.10/setup/config.html#configuring-taskmanager-processing-slots

You can control whether a task can share the slot with the previous task with the function
"startNewResourceGroup()" in the streaming API. Sharing lots makes a few things easier to
reason about, especially when adding operators to a program, you need not immediately add
new machines.


How to solve your program case
--------------------------------------------

We can actually make a pretty simple addition to Flink that will cause the tasks to be locally
connected, which in turn will cause the scheduler to distribute them like you intend.
Rather than let the 4 sources rebalance across all 16 mappers, each one should redistribute
to 4 local mappers, and these 4 mappers should send data to one local sink each.

We'll try and add that today and ping you once it is in.

The following would be sample code to use this:

env.setParallelism(4);

env
    .addSource(kafkaSource)
    .partitionFan()
    .map(mapper).setParallelism(16);
    .partitionFan()
    .addSink(kafkaSink);



A bit of background why the mechanism is the way that it is right now
----------------------------------------------------------------------------------------------

You can think of a slot as a slice of resources. In particular, an amount of memory from the
memory manager, but also memory in the network stack.

What we want to do quite soon is to make streaming programs more elastic. Consider for example
the case that you have 16 slots on 4 machines, a machine fails, and you have no spare resources.
In that case Flink should recognize that no spare resource can be acquired, and scale the
job in. Since you have only 12 slots left, the parallelism of the mappers is reduced to 12,
and the source task that was on the failed machine is moved to a slot on another machine.

It is important that the guaranteed resources for each task do not change when scaling in,
to keep behavior predictable. In this case, each slot will still at most host 1 source, 1
mapper, and 1 sink, as did some of the slots before. That is also the reason why the slots
are per TaskManager, and not global, to associate them with a constant set of resources (mainly
memory).


Greetings,
Stephan



On Thu, Feb 4, 2016 at 9:54 AM, Gwenhael Pasquiers <gwenhael.pasquiers@ericsson.com<mailto:gwenhael.pasquiers@ericsson.com>>
wrote:
Don’t we need to set the number of slots to 24 (4 sources + 16 mappers + 4 sinks) ?

Or is there a way not to set the number of slots per TaskManager instead of globally so that
they are at least equally dispatched among the nodes ?

As for the sink deployment : that’s not good news ; I mean we will have a non-negligible
overhead : all the data generated by 3 of the 4 nodes will be sent to a third node instead
of being sent to the “local” sink. Network I/O have a price.

Do you have some sort of “topology” feature coming in the roadmap ? Maybe a listener on
the JobManager / env that would be trigerred, asking usk on which node we would prefer each
node to be deployed. That way you keep the standard behavior, don’t have to make a complicated
generic-optimized algorithm, and let the user make it’s choices. Should I create a JIRA
?

For the time being we could start the application 4 time : one time per node, put that’s
not pretty at all ☺

B.R.

From: Till Rohrmann [mailto:trohrmann@apache.org<mailto:trohrmann@apache.org>]
Sent: mercredi 3 février 2016 17:58

To: user@flink.apache.org<mailto:user@flink.apache.org>
Subject: Re: Distribution of sinks among the nodes


Hi Gwenhäel,

if you set the number of slots for each TaskManager to 4, then all of your mapper will be
evenly spread out. The sources should also be evenly spread out. However, for the sinks since
they depend on all mappers, it will be most likely random where they are deployed. So you
might end up with 4 sink tasks on one machine.

Cheers,
Till
​

On Wed, Feb 3, 2016 at 4:31 PM, Gwenhael Pasquiers <gwenhael.pasquiers@ericsson.com<mailto:gwenhael.pasquiers@ericsson.com>>
wrote:
It is one type of mapper with a parallelism of 16
It's the same for the sinks and sources (parallelism of 4)

The settings are
Env.setParallelism(4)
Mapper.setPrallelism(env.getParallelism() * 4)

We mean to have X mapper tasks per source / sink

The mapper is doing some heavy computation and we have only 4 kafka partitions. That's why
we need more mappers than sources / sinks


-----Original Message-----
From: Aljoscha Krettek [mailto:aljoscha@apache.org<mailto:aljoscha@apache.org>]
Sent: mercredi 3 février 2016 16:26
To: user@flink.apache.org<mailto:user@flink.apache.org>
Subject: Re: Distribution of sinks among the nodes

Hi Gwenhäel,
when you say 16 maps, are we talking about one mapper with parallelism 16 or 16 unique map
operators?

Regards,
Aljoscha
> On 03 Feb 2016, at 15:48, Gwenhael Pasquiers <gwenhael.pasquiers@ericsson.com<mailto:gwenhael.pasquiers@ericsson.com>>
wrote:
>
> Hi,
>
> We try to deploy an application with the following “architecture” :
>
> 4 kafka sources => 16 maps => 4 kafka sinks, on 4 nodes, with 24 slots (we disabled
operator chaining).
>
> So we’d like on each node :
> 1x source => 4x map => 1x sink
>
> That way there are no exchanges between different instances of flink and performances
would be optimal.
>
> But we get (according to the flink GUI and the Host column when looking at the details
of each task) :
>
> Node 1 : 1 source =>  2 map
> Node 2 : 1 source =>  1 map
> Node 3 : 1 source =>  1 map
> Node 4 : 1 source =>  12 maps => 4 sinks
>
> (I think no comments are needed J)
>
> The the Web UI says that there are 24 slots and they are all used but they don’t seem
evenly dispatched …
>
> How could we make Flink deploy the tasks the way we want ?
>
> B.R.
>
> Gwen’



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