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  1. JDK
  2. JDK-8303616

DatagramSocket performance regression with concurrent senders




      We noticed a major degradation in throughput in our app after upgrading from JDK 11 to JDK 17/19/21-ea.

      The application uses a shared instance of DatagramSocket in multiple threads. The root cause of the performance drop is JDK-8235674, which re-implemented DatagramSocket using DatagramChannel. The problem with the new implementation is that it acquires a ReentrantLock on every send/receive call [1], thus limiting I/O scalability. The old implementation - AbstractPlainDatagramSocketImpl - did not have a per-socket lock on send/receive path.

      I attached the simple test case that demonstrates the issue: DatagramTest.java

      Here are the sample performance results using 4 threads, measured in packets per second (the higher the better).

      $ java DatagramTest 4
      278309 pps
      336652 pps
      342375 pps
      335843 pps
      327992 pps

      $ java -Djdk.net.usePlainDatagramSocketImpl DatagramTest 4
      1616246 pps
      1692003 pps
      1696878 pps
      1693931 pps
      1710490 pps

      The old implementation (AbstractPlainDatagramSocketImpl) shows 5x higher throughput than the new implementation (DatagramChannel).

      The corresponding CPU profiles:
      Old (faster): https://cr.openjdk.org/~apangin/8303616/old.html
      New (slower): https://cr.openjdk.org/~apangin/8303616/new.html
      (note the large volume of ReentrantLock.lock/unlock on the graph)

      The workaround for JDK 17 is the option -Djdk.net.usePlainDatagramSocketImpl that enables the old implementation. Unfortunately, this no longer works since JDK-8253119.

      send() and recv() calls are thread safe in POSIX and Windows, they should not be guarded by a Java level lock.

      [1] https://github.com/openjdk/jdk/blob/5b2e2e4695768a6bd8090fb9a6c342fcddcbb3fd/src/java.base/share/classes/sun/nio/ch/DatagramChannelImpl.java#L827


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              alanb Alan Bateman
              apangin Andrei Pangin
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