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From Chao Wang <chaow...@wustl.edu>
Subject Re: Experiencing long latency while using sockets
Date Mon, 07 Aug 2017 23:06:19 GMT
Following the original post, I've tried stripping down my Flink app to 
only the following, and then it still exhibits long latencies: after the 
second source socket write, it took 90+ milliseconds from data source to 
the socket-front in Flink. I would like to ask for pointers about how to 
investigate the latency issue like this, and in general how to properly 
benchmark Flink latencies. Thank you very much!


The main method:


   public static void main(String[] args) throws Exception {
     final StreamExecutionEnvironment env = 
StreamExecutionEnvironment.getExecutionEnvironment();
     DataStream<EventGroup> inEventGroupStream = env.addSource(new 
SocketEventGroupStreamFunction(6065, 512));
     inEventGroupStream.writeToSocket("DestHost", 6066, new 
MySeGroup<EventGroup>());
     env.execute("event processing");
  }


where all the custom classes are as follows (for 
serialization/deserialization and socket server functionality):


   public static class MySeGroup<T> implements 
SerializationSchema<EventGroup> {

     @Override
     public byte[] serialize(EventGroup arg0) {
       int tLength = EKFFFTAES.getSizeTimepoint();
       //Note: report error if tLength != arg0.getT().length
       if (tLength != arg0.getT().length) {
         System.out.println ("Serialization error: Timepoint size 
discrepancy.");
         System.out.println ("tLength = " + tLength);
         System.out.println ("arg0.getT().length = " + arg0.getT().length);
       }
       byte[] buffer = new byte[1 + arg0.getT().length + 
arg0.getP().length];
       buffer[0] = arg0.type;
       System.arraycopy(arg0.getT(), 0, buffer, 1, tLength);
       System.arraycopy(arg0.getP(), 0, buffer, 1 + tLength, 
arg0.getP().length);
       return buffer;
     }
   }

   public static class Event extends SimpleImmutableEntry<byte[],byte[]> {

     Event(byte[] timestamp, byte[] payload){
       super(timestamp, payload);
     }
     public byte[] getT() { // get the timestamp
       return getKey();
     }
     public byte[] getP() { // get the payload
       return getValue();
     }
   }

   public static class EventGroup extends Event {
     public byte type;
     EventGroup(byte type, byte[] timestamp, byte[] payload){
       super(timestamp, payload);
       this.type = type;
     }
   }


   public static class SocketEventGroupStreamFunction implements 
SourceFunction<EventGroup> {

     private transient ServerSocket serverSocket;
     private int serverPort;
     private int dataLength;
     private byte[] inbuf;
     private byte[] timestamp;
     private byte[] payload;
     private int tLength = EKFFFTAES.getSizeTimepoint();
     private volatile boolean isRunning = true;

     public SocketEventGroupStreamFunction(int port, int length) {
       serverPort = port;
       dataLength = length;
       inbuf = new byte[1 + dataLength + tLength];
       timestamp = new byte[tLength];
       payload = new byte[dataLength];
     }

     @Override
     public void run(SourceContext<EventGroup> ctx) throws Exception {
       while(isRunning) {
         serverSocket = new ServerSocket(serverPort, 100, 
InetAddress.getByName("192.168.1.13"));
         serverSocket.setSoTimeout(1000000);
         System.out.println("Waiting for incoming connections on port " +
           serverSocket.getLocalPort() + "...");
         Socket server = serverSocket.accept();

         System.out.println("Just connected to " + 
server.getRemoteSocketAddress());
         DataInputStream in = new DataInputStream(server.getInputStream());

         while(isRunning) {
           in.readFully(inbuf, 0, inbuf.length);
           System.arraycopy(inbuf, 1, timestamp, 0, tLength);
           System.arraycopy(inbuf, 1+tLength, payload, 0, dataLength);

           System.out.print("Got an event " + inbuf[0] + ": ");
           displayElapsedTime(timestamp);

           ctx.collect(new EventGroup(inbuf[0], timestamp, payload));
         }
       }
     }

     @Override
     public void cancel() {
       isRunning = false;
       ServerSocket theSocket = this.serverSocket;
       if (theSocket != null) {
         try {
           theSocket.close();
         }catch(SocketTimeoutException s) {
           System.out.println("Socket timed out!");
         }catch(IOException e) {
           e.printStackTrace();
         }
       }
     }
   }


and finally, EKFFFTAES is my cpp library implementing the timestamping 
facility:


int timePointLength = sizeof(std::chrono::system_clock::time_point);

JNIEXPORT jint JNICALL Java_eventProcessing_EKFFFTAES_getSizeTimepoint
   (JNIEnv *, jclass)
{
   return ::timePointLength;
}

JNIEXPORT void JNICALL Java_eventProcessing_EKFFFTAES_displayElapsedTime
   (JNIEnv *env, jclass, jbyteArray inArray)
{
   std::chrono::system_clock::time_point end =
     std::chrono::system_clock::now();
   jbyte *inCArray = env->GetByteArrayElements(inArray, NULL);
   std::chrono::system_clock::time_point start;
   std::memcpy (&start, inCArray, ::timePointLength);
   std::cout << 
std::chrono::duration_cast<std::chrono::microseconds>(end - 
start).count() << std::endl;
}


Thank you,

Chao

On 08/07/2017 03:20 PM, Chao Wang wrote:
> Hi,
>
> I have been trying to benchmark the end-to-end latency of a Flink 
> 1.3.1 application, but got confused regarding the amount of time spent 
> in Flink. In my setting, data source and data sink dwell in separated 
> machines, like the following topology:
>
> Machine 1                                            Machine 2      
> Machine 3
> data source (via a socket client)   ->      Flink ->    data sink (via 
> a socket server)
>
> I observed 200-400 milliseconds end-to-end latency, while the 
> execution time of my stream transformations took no more than two 
> milliseconds, and the socket-only networking latency between machines 
> is no more than one millisecond, and I used ptpd so that the clock 
> offset between machines were also no more than one millisecond.
>
> Question: What took those hundreds of milliseconds?
>
> Here are the details of my setting and my observation so far:
>
> On Machine 2, I implemented a socket server as a data source to Flink 
> (by implementing SourceFunction), and I splited the incoming stream 
> into several streams (by SplitStream) for some transformations 
> (implementing MapFuction and CoFlatMapFunction), where the results 
> were fed to socket (using writeToSocket). I used c++11's chrono time 
> library (through JNI) to take timestamps and determine the elapsed 
> time, and I have verified that the overhead of timestamping this way 
> is no more than one millisecond.
>
> I observed that for the four consecutive writes from Machine 1, with 
> the time between two writes no more than 0.3 milliseconds, on Machine 
> 2 Flink got the first write in 0.2 milliseconds, but then it took 90 
> milliseconds for Flink to get the next write, and another 4 
> milliseconds for the third write, and yet another 4 milliseconds for 
> the fourth write.
>
> And then it took more than 70 milliseconds before Flink started 
> processing my plan's first stream transformation. And after my last 
> transformation, it took more than 70 milliseconds before the result 
> was received at Machine 3.
>
>
> Thank you,
>
> Chao
>
>


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