BUG#29049207: SLAVE PERFORMANCE DEGRADES OVER TIME

ANALYSIS:
=========
If a slave instance use sysvar_session_track_gtids ==
OWN_GTID and with MTS enabled, the slave will lag behind
master and its performance degrades over time.

Each slave worker adds its own gtid into its
Session_consistency_gtids_ctx.m_gtid_set during each
transaction commit. Overtime, the
Session_consistency_gtids_ctx.m_gtid_set may have
gno_interval in millions and thus will take long time to
insert a gtid into Session_consistency_gtids_ctx.m_gtid_set.
This causes the lag/performance degrade.

FIX:
====
Disable tracking GTIDs for slave threads.

Note:
=====
This patch is based on a contribution by Facebook.

RB#25006

Author: karthik-kamath

sql/rpl_context.cc

@@ -1,4 +1,4 @@
-/* Copyright (c) 2014, 2015, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2.0,
@@ -136,6 +136,12 @@ bool Session_consistency_gtids_ctx::notify_after_gtid_executed_update(const THD
   DBUG_RETURN(res);
 }
 
+void Session_consistency_gtids_ctx::
+    update_tracking_activeness_from_session_variable(const THD* thd)
+{
+  m_curr_session_track_gtids= thd->variables.session_track_gtids;
+}
+
 bool Session_consistency_gtids_ctx::notify_after_response_packet(const THD *thd)
 {
   int res= false;
@@ -149,7 +155,7 @@ bool Session_consistency_gtids_ctx::notify_after_response_packet(const THD *thd)
    this value. It may have changed (the previous command may have been
    a SET SESSION session_track_gtids=...;).
    */
-  m_curr_session_track_gtids= thd->variables.session_track_gtids;
+  update_tracking_activeness_from_session_variable(thd);
   DBUG_RETURN(res);
 }
 
@@ -171,7 +177,7 @@ Session_consistency_gtids_ctx::register_ctx_change_listener(
      if the session_track_gtids value is set at startup time to anything 
      different than OFF.
      */
-    m_curr_session_track_gtids= thd->variables.session_track_gtids;
+    update_tracking_activeness_from_session_variable(thd);
   }
 }
 

sql/rpl_context.h

@@ -1,4 +1,4 @@
-/* Copyright (c) 2014, 2018, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2.0,
@@ -202,6 +202,10 @@ class Session_consistency_gtids_ctx
     return notify_after_transaction_commit(thd);
   }
 
+  /**
+    Update session tracker (m_curr_session_track_gtids) from thd.
+  */
+  void update_tracking_activeness_from_session_variable(const THD* thd);
 
 private:
   // not implemented

sql/rpl_slave.cc

@@ -1,4 +1,4 @@
-/* Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2.0,
@@ -4207,6 +4207,11 @@ void set_slave_thread_options(THD* thd)
   thd->variables.option_bits= options;
   thd->variables.completion_type= 0;
 
+  /* Do not track GTIDs for slave threads to avoid performance issues. */
+  thd->variables.session_track_gtids= OFF;
+  thd->rpl_thd_ctx.session_gtids_ctx()
+      .update_tracking_activeness_from_session_variable(thd);
+
   /*
     Set autocommit= 1 when info tables are used and autocommit == 0 to
     avoid trigger asserts on mysql_execute_command(THD *thd) caused by