[FLINK-15171] fix issue with netty shuffle buffer allocation skewing benchmark results

src/main/java/org/apache/flink/benchmark/FlinkEnvironmentContext.java

@@ -41,6 +41,8 @@ public void execute() throws Exception {
     protected Configuration createConfiguration() {
         final Configuration configuration = new Configuration();
         configuration.setInteger(NettyShuffleEnvironmentOptions.NETWORK_NUM_BUFFERS, NUM_NETWORK_BUFFERS);
+        configuration.setString("taskmanager.network.memory.max", "8m");
+        configuration.setString("taskmanager.network.memory.min", "4m");
         return configuration;
     }