#At file:///home/dlenev/src/bzr/mysql-5.5-12641342/ based on revid:mayank.prasad@stripped
3459 Dmitry Lenev 2011-06-16
Fix for bug #12641342 - "61401: UPDATE PERFORMANCE DEGRADES
GRADUALLY IF A TRIGGER EXISTS".
This bug manifested itself in two ways:
- Firstly execution of any data-changing statement which
required prelocking (i.e. involved stored function or
trigger) as part of transaction slowed down a bit all
subsequent statements in this transaction. So performance
in transaction which periodically involved such statements
gradually degraded over time.
- Secondly execution of any data-changing statement which
required prelocking as part of transaction prevented
concurrent FLUSH TABLES WITH READ LOCK from proceeding
until the end of transaction instead of end of particular
statement.
The problem was caused by incorrect handling of metadata lock
used in FTWRL implementation for statements requiring prelocked
mode.
Each statement which changes data acquires global IX lock
with STATEMENT duration. This lock is supposed to block
concurrent FTWRL from proceeding until the statement ends.
When entering prelocked mode, durations of all metadata locks
acquired so far were changed to EXPLICIT, to prevent
substatements from releasing these locks. When prelocked mode
was left, durations of metadata locks were changed to
TRANSACTIONAL (with a few exceptions) so they can be properly
released at the end of transaction.
Unfortunately, this meant that the global IX lock blocking
FTWRL with STATEMENT duration was moved to TRANSACTIONAL
duration after execution of statement requiring prelocking.
Since each subsequent statement that required prelocking and
tried to acquire global IX lock with STATEMENT duration got
a new instance of MDL_ticket, which was later moved to
TRANSACTIONAL duration, this led to unwarranted growth of
number of tickets with TRANSACITONAL duration in this
connection's MDL_context. As result searching for other
tickets in it became slow and acquisition of other metadata
locks by this transaction started to hog CPU.
Moreover, this also meant that after execution of statement
requiring prelocking concurrent FTWRL was blocked
until the end of transaction instead of end of statement.
This patch solves this problem by not moving locks to EXPLICIT
duration when thread enters prelocked mode (unless it is a real
LOCK TABLES mode). This step turned out to be not really
necessary as substatements don't try to release metadata locks.
Consequently, the global IX lock blocking FTWRL keeps its
STATEMENT duration and is properly released at the end of
statement and the above issue goes away.
@ mysql-test/r/flush.result
Added test for bug #12641342 - "61401: UPDATE PERFORMANCE
DEGRADES GRADUALLY IF A TRIGGER EXISTS".
@ mysql-test/t/flush.test
Added test for bug #12641342 - "61401: UPDATE PERFORMANCE
DEGRADES GRADUALLY IF A TRIGGER EXISTS".
@ sql/mdl.h
Added comment describing various types of metadata lock
duration.
@ sql/sql_class.cc
Since we no longer change duration of metadata locks to EXPLICIT
when entering prelocked mode (unless it is a real LOCK TABLES)
there is no need to restore proper duration of the locks when
leaving prelocked mode.
@ sql/sql_class.h
Do not change duration of metadata locks to EXPLICIT when
entering prelocking mode (unless it is a real LOCK TABLES).
This allows to avoid problems with restoring correct duration
when leaving this mode. It is possible to do this as
substatements won't release metadata locks in any case.
@ sql/sql_parse.cc
Added assert checking that we won't release metadata locks
when in substatement.
modified:
mysql-test/r/flush.result
mysql-test/t/flush.test
sql/mdl.h
sql/sql_class.cc
sql/sql_class.h
sql/sql_parse.cc
=== modified file 'mysql-test/r/flush.result'
--- a/mysql-test/r/flush.result 2011-03-07 09:08:10 +0000
+++ b/mysql-test/r/flush.result 2011-06-16 15:18:16 +0000
@@ -466,3 +466,26 @@ ALTER TABLE t1 COMMENT 'test';
ERROR HY000: Table 't1' was locked with a READ lock and can't be updated
UNLOCK TABLES;
DROP TABLE t1;
+#
+# Test for bug #12641342 - "61401: UPDATE PERFORMANCE DEGRADES
+# GRADUALLY IF A TRIGGER EXISTS".
+#
+# One of side-effects of this bug was that a transaction which
+# involved DML statements requiring prelocking blocked concurrent
+# FLUSH TABLES WITH READ LOCK for the whole its duration, while
+# correct behavior in this case is to block FTWRL only for duration
+# of individual DML statements.
+DROP TABLE IF EXISTS t1;
+CREATE TABLE t1 (id INT PRIMARY KEY, value INT);
+INSERT INTO t1 VALUES (1, 1);
+CREATE TRIGGER t1_au AFTER UPDATE ON t1 FOR EACH ROW SET @var = "a";
+BEGIN;
+UPDATE t1 SET value= value + 1 WHERE id = 1;
+# Switching to connection 'con1'.
+# The below FLUSH TABLES WITH READ LOCK should succeed and
+# should not be blocked by the transaction in default connection.
+FLUSH TABLES WITH READ LOCK;
+UNLOCK TABLES;
+# Switching to connection 'default'.
+COMMIT;
+DROP TABLE t1;
=== modified file 'mysql-test/t/flush.test'
--- a/mysql-test/t/flush.test 2011-03-07 09:08:10 +0000
+++ b/mysql-test/t/flush.test 2011-06-16 15:18:16 +0000
@@ -668,3 +668,36 @@ ALTER TABLE t1 COMMENT 'test';
UNLOCK TABLES;
DROP TABLE t1;
+
+
+--echo #
+--echo # Test for bug #12641342 - "61401: UPDATE PERFORMANCE DEGRADES
+--echo # GRADUALLY IF A TRIGGER EXISTS".
+--echo #
+--echo # One of side-effects of this bug was that a transaction which
+--echo # involved DML statements requiring prelocking blocked concurrent
+--echo # FLUSH TABLES WITH READ LOCK for the whole its duration, while
+--echo # correct behavior in this case is to block FTWRL only for duration
+--echo # of individual DML statements.
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+CREATE TABLE t1 (id INT PRIMARY KEY, value INT);
+INSERT INTO t1 VALUES (1, 1);
+CREATE TRIGGER t1_au AFTER UPDATE ON t1 FOR EACH ROW SET @var = "a";
+BEGIN;
+UPDATE t1 SET value= value + 1 WHERE id = 1;
+
+--echo # Switching to connection 'con1'.
+connect(con1, localhost, root);
+--echo # The below FLUSH TABLES WITH READ LOCK should succeed and
+--echo # should not be blocked by the transaction in default connection.
+FLUSH TABLES WITH READ LOCK;
+UNLOCK TABLES;
+disconnect con1;
+--source include/wait_until_disconnected.inc
+
+--echo # Switching to connection 'default'.
+connection default;
+COMMIT;
+DROP TABLE t1;
=== modified file 'sql/mdl.h'
--- a/sql/mdl.h 2011-04-11 11:39:15 +0000
+++ b/sql/mdl.h 2011-06-16 15:18:16 +0000
@@ -152,11 +152,24 @@ enum enum_mdl_type {
/** Duration of metadata lock. */
-enum enum_mdl_duration { MDL_STATEMENT= 0,
- MDL_TRANSACTION,
- MDL_EXPLICIT,
- /* This should be the last ! */
- MDL_DURATION_END };
+enum enum_mdl_duration {
+ /**
+ Locks with statement duration are automatically released at the end
+ of statement or transaction.
+ */
+ MDL_STATEMENT= 0,
+ /**
+ Locks with transaction duration are automatically released at the end
+ of transaction.
+ */
+ MDL_TRANSACTION,
+ /**
+ Locks with explicit duration survive the end of statement and transaction.
+ They have to be released explicitly by calling MDL_context::release_lock().
+ */
+ MDL_EXPLICIT,
+ /* This should be the last ! */
+ MDL_DURATION_END };
/** Maximal length of key for metadata locking subsystem. */
=== modified file 'sql/sql_class.cc'
--- a/sql/sql_class.cc 2011-05-21 09:29:10 +0000
+++ b/sql/sql_class.cc 2011-06-16 15:18:16 +0000
@@ -3790,16 +3790,24 @@ void THD::set_mysys_var(struct st_my_thr
void THD::leave_locked_tables_mode()
{
+ if (locked_tables_mode == LTM_LOCK_TABLES)
+ {
+ /*
+ When leaving LOCK TABLES mode we have to change the duration of most
+ of the metadata locks being held, except for HANDLER and GRL locks,
+ to transactional for them to be properly released at UNLOCK TABLES.
+ */
+ mdl_context.set_transaction_duration_for_all_locks();
+ /*
+ Make sure we don't release the global read lock and commit blocker
+ when leaving LTM.
+ */
+ global_read_lock.set_explicit_lock_duration(this);
+ /* Also ensure that we don't release metadata locks for open HANDLERs. */
+ if (handler_tables_hash.records)
+ mysql_ha_set_explicit_lock_duration(this);
+ }
locked_tables_mode= LTM_NONE;
- mdl_context.set_transaction_duration_for_all_locks();
- /*
- Make sure we don't release the global read lock and commit blocker
- when leaving LTM.
- */
- global_read_lock.set_explicit_lock_duration(this);
- /* Also ensure that we don't release metadata locks for open HANDLERs. */
- if (handler_tables_hash.records)
- mysql_ha_set_explicit_lock_duration(this);
}
void THD::get_definer(LEX_USER *definer)
=== modified file 'sql/sql_class.h'
--- a/sql/sql_class.h 2011-05-20 09:18:36 +0000
+++ b/sql/sql_class.h 2011-06-16 15:18:16 +0000
@@ -2795,7 +2795,19 @@ public:
{
DBUG_ASSERT(locked_tables_mode == LTM_NONE);
- mdl_context.set_explicit_duration_for_all_locks();
+ if (mode_arg == LTM_LOCK_TABLES)
+ {
+ /*
+ When entering LOCK TABLES mode we should set explicit duration
+ for all metadata locks acquired so far in order to avoid releasing
+ them till UNLOCK TABLES statement.
+ We don't do this when entering prelocked mode since sub-statements
+ don't release metadata locks and restoring status-quo after leaving
+ prelocking mode gets complicated.
+ */
+ mdl_context.set_explicit_duration_for_all_locks();
+ }
+
locked_tables_mode= mode_arg;
}
void leave_locked_tables_mode();
=== modified file 'sql/sql_parse.cc'
--- a/sql/sql_parse.cc 2011-06-10 07:20:15 +0000
+++ b/sql/sql_parse.cc 2011-06-16 15:18:16 +0000
@@ -2027,6 +2027,11 @@ mysql_execute_command(THD *thd)
*/
if (stmt_causes_implicit_commit(thd, CF_IMPLICT_COMMIT_BEGIN))
{
+ /*
+ Note that this should never happen inside of stored functions
+ or triggers as all such statements prohibited there.
+ */
+ DBUG_ASSERT(! thd->in_sub_stmt);
/* Commit or rollback the statement transaction. */
thd->is_error() ? trans_rollback_stmt(thd) : trans_commit_stmt(thd);
/* Commit the normal transaction if one is active. */
Attachment: [text/bzr-bundle] bzr/dmitry.lenev@oracle.com-20110616151816-ppr6n3bspiktvij0.bundle
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| • bzr commit into mysql-5.5 branch (Dmitry.Lenev:3459) Bug#12641342 | Dmitry Lenev | 16 Jun |
