Bug#17310878 PFS OVERHEAD ON FREQUENT CONNECT/DISCONNECT

This fix is for MySQL 5.7

This fix is a performance improvement.

The issue was that the call to PSI_THREAD_CALL(delete_current_thread)(),
which is executed each time a thread disconnects,
was placed inside a critical section involving LOCK_thread_count.

The fix is to perform the same call sooner,
before entering the LOCK_thread_count critical section.

Serializing all calls to pfs_delete_current_thread_v1() was the major
cause of performance overhead, for frequent connect/disconnect.

Author: marcalff

sql/conn_handler/connection_handler_per_thread.cc

@@ -74,14 +74,6 @@ Channel_info* Per_thread_connection_handler::block_until_new_connection()
     DBUG_POP();
     DBUG_ASSERT( ! _db_is_pushed_());
 
-#ifdef HAVE_PSI_THREAD_INTERFACE
-    /*
-      Delete the instrumentation for the job that just completed,
-      before blocking this pthread (blocked on COND_thread_cache).
-    */
-    PSI_THREAD_CALL(delete_current_thread)();
-#endif
-
     // Block pthread
     blocked_pthread_count++;
     while (!abort_loop && !wake_pthread && !kill_blocked_pthreads_flag)
@@ -236,6 +228,13 @@ pthread_handler_t handle_connection(void *arg)
     remove_global_thread(thd);
     delete thd;
 
+#ifdef HAVE_PSI_THREAD_INTERFACE
+    /*
+      Delete the instrumentation for the job that just completed.
+    */
+    PSI_THREAD_CALL(delete_current_thread)();
+#endif
+
     if (abort_loop) // Server is shutting down so end the pthread.
       break;