Change default for cassandra.set_sep_thread_name to false to reduce CPU usage

Add an extra optimization if it is enabled: invoke Thread.setName only if a phase changed

patch by Dmitry Konstantinov; reviewed by Francisco Guerrero, Jyothsna Konisa for CASSANDRA-21089

Author: netudima

CHANGES.txt

@@ -1,4 +1,5 @@
 6.0-alpha2
+ * Change default for cassandra.set_sep_thread_name to false to reduce CPU usage (CASSANDRA-21089)
  * Avoid permission checks for masked columns when the table doesn't have any (CASSANDRA-21299)
  * Reduce allocations and array copies due to buffer resizing in LocalDataResponse during row serialization (CASSANDRA-21285)
  * Implement a guardrail for client driver versions (CASSANDRA-21146)

src/java/org/apache/cassandra/concurrent/SEPWorker.java

@@ -53,6 +53,8 @@ final class SEPWorker extends AtomicReference<SEPWorker.Work> implements Runnabl
 
     private final AtomicReference<Runnable> currentTask = new AtomicReference<>();
 
+    private String lastUsedExecutorName;
+
     SEPWorker(ThreadGroup threadGroup, Long workerId, Work initialState, SharedExecutorPool pool)
     {
         this.pool = pool;
@@ -129,8 +131,11 @@ public void run()
                 assigned = get().assigned;
                 if (assigned == null)
                     continue;
-                if (SET_THREAD_NAME)
+                if (SET_THREAD_NAME && assigned.name != lastUsedExecutorName) // .equals is not used intentionally
+                {
                     Thread.currentThread().setName(assigned.name + workerIdThreadSuffix);
+                    lastUsedExecutorName = assigned.name;
+                }
 
                 task = assigned.tasks.poll();
                 currentTask.lazySet(task);

src/java/org/apache/cassandra/config/CassandraRelevantProperties.java

@@ -536,7 +536,7 @@ public enum CassandraRelevantProperties
      */
     SEED_COUNT_WARN_THRESHOLD("cassandra.seed_count_warn_threshold"),
     SERIALIZATION_EMPTY_TYPE_NONEMPTY_BEHAVIOR("cassandra.serialization.emptytype.nonempty_behavior"),
-    SET_SEP_THREAD_NAME("cassandra.set_sep_thread_name", "true"),
+    SET_SEP_THREAD_NAME("cassandra.set_sep_thread_name", "false"),
     SHUTDOWN_ANNOUNCE_DELAY_IN_MS("cassandra.shutdown_announce_in_ms", "2000"),
     SIMULATOR_ITERATIONS("simulator.iterations", "3"),
     SIMULATOR_SEED("cassandra.simulator.seed"),