Re: Flink Checkpoint runs slow for low load stream

Posted by Stephan Ewen on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Flink-Checkpoint-runs-slow-for-low-load-stream-tp9147p9180.html

Thanks, the logs were very helpful!

TL:DR - The offset committing to ZooKeeper is very slow and prevents proper starting of checkpoints.

Here is what is happening in detail:

  - Between the point when the TaskManager receives the "trigger checkpoint" message and when the point when the KafkaSource actually starts the checkpoint is a long time (many seconds) - for one of the Kafka Inputs (the other is fine).

  - The only way this delayed can be introduced is if another checkpoint related operation (such as trigger() or notifyComplete() ) is still in progress when the checkpoint is started. Flink does not perform concurrent checkpoint operations on a single operator, to ease the concurrency model for users.

  - The operation that is still in progress must be the committing of the offsets (to ZooKeeper or Kafka). That also explains why this only happens once one side receives the first record. Before that, there is nothing to commit.

What Flink should fix:

  - The KafkaConsumer should run the commit operations asynchronously, to not block the "notifyCheckpointComplete()" method.

What you can fix:

  - Have a look at your Kafka/ZooKeeper setup. One Kafka Input works well, the other does not. Do they go against different sets of brokers, or different ZooKeepers? Is the metadata for one input bad?

  - In the next Flink version, you may opt-out of committing offsets to Kafka/ZooKeeper all together. It is not important for Flink's checkpoints anyways.

Greetings,

Stephan

On Mon, Sep 26, 2016 at 5:13 PM, Chakravarthy varaga <[hidden email]> wrote:

Hi Stefan,

    Please find my responses below.

    - What source are you using for the slow input?

     [CVP] - Both stream as pointed out in my first mail, are Kafka Streams

  - How large is the state that you are checkpointing?

     [CVP] - I have enabled checkpointing on the StreamEnvironment as below.
         final StreamExecutionEnvironment streamEnv = StreamExecutionEnvironment.getExecutionEnvironment();
         streamEnv.setStateBackend(new FsStateBackend("file:///tmp/flink/checkpoints"));
         streamEnv.enableCheckpointing(10000);

      In terms of the state stored, the KS1 stream has payload of 100K events/second, while KS2 have about 1 event / 10 minutes... basically the operators perform flatmaps on 8 fields of tuple (all fields are primitives). If you look at the states' sizes in dashboard they are in Kb...

  - Can you try to see in the log if actually the state snapshot takes that long, or if it simply takes long for the checkpoint barriers to travel through the stream due to a lot of backpressure?
    [CVP] -There are no back pressure atleast from the sample computation in the flink dashboard. 100K/second is low load for flink's benchmarks. I could not quite get the barriers vs snapshot state. I have attached the Task Manager log (DEBUG) info if that will interest you.
 

     I have attached the checkpoints times' as .png from the dashboard. Basically if you look at checkpoint IDs 28 & 29 &30- you'd see that the checkpoints take more than a minute in each case. Before these checkpoints, the KS2 stream did not have any events. As soon as an event(should be in bytes) was generated, the checkpoints went slow and subsequently a minute more for every checkpoint thereafter.

   This log was collected from the standalone flink cluster with 1 job manager & 2 TMs. 1 TM was running this application with checkpointing (parallelism=1)
 

    Please let me know if you need further info.,

    

On Fri, Sep 23, 2016 at 6:26 PM, Stephan Ewen <[hidden email]> wrote:

Hi!

Let's try to figure that one out. Can you give us a bit more information?

  - What source are you using for the slow input?

  - How large is the state that you are checkpointing?

  - Can you try to see in the log if actually the state snapshot takes that long, or if it simply takes long for the checkpoint barriers to travel through the stream due to a lot of backpressure?

Greetings,

Stephan

On Fri, Sep 23, 2016 at 3:35 PM, Fabian Hueske <[hidden email]> wrote:

Hi CVP,

I'm not so much familiar with the internals of the checkpointing system, but maybe Stephan (in CC) has an idea what's going on here.

Best, Fabian

2016-09-23 11:33 GMT+02:00 Chakravarthy varaga <[hidden email]>:

Hi Aljoscha & Fabian,

    I have a stream application that has 2 stream source as below.

     KeyedStream<String, String> ks1 = ds1.keyBy("*") ;
     KeyedStream<Tuple2<String, V>, String> ks2 = ds2.flatMap(split T into k-v pairs).keyBy(0);

     ks1.connect(ks2).flatMap(X);
     //X is a CoFlatMapFunction that inserts and removes elements from ks2 into a key-value state member. Elements from ks1 are matched against that state. the CoFlatMapFunction operator maintains ValueState<Tuple2<Long, Long>>;

     //ks1 is streaming about 100K events/sec from kafka topic

     //ks2 is streaming about 1 event every 10 minutes... Precisely when the 1st event is consumed from this stream, checkpoint takes 2 minutes straight away.

    The version of flink is 1.1.2.

I tried to use checkpoint every 10 Secs using a FsStateBackend... What I notice is that the checkpoint duration is almost 2 minutes for many cases, while for the other cases it varies from 100 ms to 1.5 minutes frequently. I'm attaching the snapshot of the dashboard for your reference.

     Is this an issue with flink checkpointing?

 Best Regards

CVP