Improve InnoDB purge: reduce context switching, better work distribution

Before this patch, InnoDB purge submitted innodb_purge_threads-1 tasks,
even if there was insufficient work for all. For example,
with innodb_purge_threads at the default (4) and updates on a single table, only one thread could work,
yet three tasks were submitted. This caused unnecessary wakeups and context
switches.

The patch modifies trx_purge_attach_undo_recs() to create as many work
items as necessary. The purge coordinator now processes more tasks itself,
which reduces context switching and improves efficiency.

storage/innobase/include/srv0srv.h

@@ -624,6 +624,15 @@ Complete the shutdown tasks such as background DROP TABLE,
 and optionally change buffer merge (on innodb_fast_shutdown=0). */
 void srv_shutdown(bool ibuf_merge);
 
+/**
+ Fetches and executes tasks from the purge work queue,
+ until this queue is empty.
+ This is main part of purge worker task, but also
+ executed in coordinator.
+ @note needs current_thd to be set beforehand.
+*/
+void srv_purge_worker_task_low();
+
 } /* extern "C" */
 
 #ifdef UNIV_DEBUG

storage/innobase/srv/srv0srv.cc

@@ -1557,7 +1557,6 @@ static bool srv_purge_should_exit(size_t old_history_size)
 
 /*********************************************************************//**
 Fetch and execute a task from the work queue.
-@param [in,out]	slot	purge worker thread slot
 @return true if a task was executed */
 static bool srv_task_execute()
 {
@@ -1697,6 +1696,13 @@ static void release_thd(THD *thd, void *ctx)
 	set_current_thd(0);
 }
 
+void srv_purge_worker_task_low()
+{
+  ut_ad(current_thd);
+  while (srv_task_execute())
+    ut_ad(purge_sys.running());
+}
+
 static void purge_worker_callback(void*)
 {
   ut_ad(!current_thd);
@@ -1704,8 +1710,7 @@ static void purge_worker_callback(void*)
   ut_ad(srv_force_recovery < SRV_FORCE_NO_BACKGROUND);
   void *ctx;
   THD *thd= acquire_thd(&ctx);
-  while (srv_task_execute())
-    ut_ad(purge_sys.running());
+  srv_purge_worker_task_low();
   release_thd(thd,ctx);
 }