(DEPRECATED) Apache Flink User Mailing List archive.

RE: state access causing segmentation fault

Posted by Colletta, Edward on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/state-access-causing-segmentation-fault-tp38562p38664.html

Thanks Arvid,

I added static to ExecQueue and this did fix the problem. I tested without static on RingBufferExec because it seems that if ExecQueue is static nested, there should be no reference to the MyKeyedProcessFunction object as RingBufferExec is an inner class of ExecQueue.

However, I did that just for the test. For my prod code, going forward, I am following flink’s rules for POJO types, adding static to any inner class, and checking for any POJO warnings in the logs.

From: Arvid Heise <[hidden email]>
Sent: Sunday, October 11, 2020 3:46 PM
To: Colletta, Edward <[hidden email]>
Cc: Dawid Wysakowicz <[hidden email]>; [hidden email]
Subject: Re: state access causing segmentation fault

This email is from an external source - exercise caution regarding links and attachments.

Hi Edward,

could you try adding the static keyword to ExecQueue and RingBufferExec? As is they hold a reference to the MyKeyedProcessFunction, which has unforeseen consequences.

On Sun, Oct 11, 2020 at 5:38 AM Colletta, Edward <[hidden email]> wrote:

Tried to attach tar file but it got blocked.   Resending with files attached individually.

Ok, have minimal reproducible example. Attaching a tar file of the job that crashed.

The crash has nothing to do with the number of state variables. But it does seem to be caused by using a type for the state variable that is a class nested in the KeyedProcessFunction.

Reduced to a single state variable. The type of the state variable was a class (ExecQueue) defined in class implementing KeyedProcessFunction. Moving the ExecQueue definition to its own file fixed the problem.

The attached example always crashes the taskManager in 30 seconds to 5 minutes.

MyKeyedProcessFunction.java and also cut and pasted here:

package crash;

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

import org.apache.flink.api.common.state.ValueStateDescriptor;

import org.apache.flink.api.common.typeinfo.TypeHint;

import org.apache.flink.api.common.typeinfo.TypeInformation;

import org.apache.flink.api.common.state.ValueState;

import org.apache.flink.configuration.Configuration;

import org.apache.flink.streaming.api.functions.KeyedProcessFunction;

import org.apache.flink.streaming.api.functions.KeyedProcessFunction.Context;

import org.apache.flink.streaming.api.functions.KeyedProcessFunction.OnTimerContext;

import org.apache.flink.util.Collector;

public class MyKeyedProcessFunction extends KeyedProcessFunction<String, Exec, Exec> {

    private static final Logger LOG = LoggerFactory.getLogger(MyKeyedProcessFunction.class);

    public TypeInformation<ExecQueue> leftTypeInfo;

    public transient ValueState<ExecQueue> leftState;

    public int initQueueSize;

    public long emitFrequencyMs;

    public MyKeyedProcessFunction() {

        initQueueSize = 10;

        emitFrequencyMs = 1;

    }

    @Override

    public void open(Configuration conf) {

        leftTypeInfo = TypeInformation.of(new TypeHint<ExecQueue>(){});

        leftState = getRuntimeContext().getState(

                    new ValueStateDescriptor<>("left", leftTypeInfo, null));

    }

    @Override

    public void processElement(Exec leftIn, Context ctx, Collector<Exec> out) {

        try {

            ExecQueue eq = leftState.value();

            if (eq == null) {

                eq = new ExecQueue(10);

                ctx.timerService().registerProcessingTimeTimer(ctx.timerService().currentProcessingTime() + emitFrequencyMs);

            }

            leftState.update(eq);

        }

        catch (Exception e) {

            LOG.error("Exception in processElement1. Key: " + ctx.getCurrentKey() + ". " + e + ". trace = " );

            for (java.lang.StackTraceElement s:e.getStackTrace())

                LOG.error(s.toString());

        }

    }

    @Override

    public void onTimer(long timestamp, OnTimerContext ctx, Collector<Exec> out) {

        try {

            ExecQueue eq = leftState.value();

            ctx.timerService().registerProcessingTimeTimer(ctx.timerService().currentProcessingTime() + emitFrequencyMs);

        }

        catch ( Exception e) {

            LOG.error("Exception in onTimer. Key: " + ctx.getCurrentKey() + ". " + e + ". trace = " );

            for (java.lang.StackTraceElement s:e.getStackTrace())

                LOG.error(s.toString());

        }

    }

    public class ExecQueue {

        public RingBufferExec queue;

        public ExecQueue (){}

        public ExecQueue (int initSize) {

            queue = new RingBufferExec(initSize);

        }

        public class RingBufferExec {

            public Integer size;

            public Integer count;

            public RingBufferExec(){ }

            public RingBufferExec(int sizeIn){

                size = sizeIn;

                count = 0;

            }

        }

    }

}

From: Dawid Wysakowicz <[hidden email]>
Sent: Thursday, October 8, 2020 6:26 AM
To: Colletta, Edward <[hidden email]>; [hidden email]
Subject: Re: state access causing segmentation fault

Hi,

It should be absolutely fine to use multiple state objects. I am not aware of any limits to that. A minimal, reproducible example would definitely be helpful. For those kind of exceptions, I'd look into the serializers you use. Other than that I cannot think of an obvious reason for that kind of exceptions.

Best,

Dawid

On 08/10/2020 12:12, Colletta, Edward wrote:

Using Flink 1.9.2, Java, FsStateBackend. Running Session cluster on EC2 instances.

I have a KeyedProcessFunction that is causing a segmentation fault, crashing the flink task manager. The seems to be caused by using 3 State variables in the operator. The crash happens consistently after some load is processed.

This is the second time I have encountered this.   The first time I had 3 ValueState variables, this time I had 2 ValueState variables and a MapState variable. Both times the error was alleviated by removing one of the state variables.

This time I replaced the 2 valueState variables with a Tuple2 of the types of the individual variables.   I can try to put together a minimal example, but I was wondering if anyone has encountered this problem.

Are there any documented limits of the number of state variables 1 operator can use?

For background the reason I use multiple state variables is the operator is processing 2 types of inputs, Left and Right. When Left is received it is put it into a PriorityQueue. When the Right type is received I put that into a ring buffer.

I replaced the PriorityQueue with a queue of Ids and MapState to hold the elements. So I have Left stored in a queue ValueState variable and MapState variable, and Right is stored in the ring buffer ValueState variable.

Arvid Heise | Senior Java Developer

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