Hi William,
William Newton wrote:
Hello List,
I have this table that has a single row in it:
CREATE TABLE `quicktable` (
`x` int(11) NOT NULL auto_increment,
`quick_id` int(11) NOT NULL default '0',
PRIMARY KEY (`x`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1
select * from quicktable;
+---+----------+
| x | quick_id |
+---+----------+
| 1 | 0 |
+---+----------+
1 row in set (0.00 sec)
I have a large number of connections executing these queries:
BEGIN:
SELECT quick_id FROM quicktable FOR UPDATE;
COMMIT;
This
works well until I hit a large number of concurrent connections (around
200), when I start getting deadlocks. Despite the fact, that I'm only
selecting a single table. Here is the deadlock section from SHOW INNODB
STATUS;
------------------------
LATEST DETECTED DEADLOCK
------------------------
071015 20:22:35
*** (1) TRANSACTION:
TRANSACTION 0 79790779, ACTIVE 2 sec, process no 7658, OS thread id 1185077584
starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 368
MySQL thread id 3961, query id 2102790 10.1.10.122 bnewton statistics
SELECT x, quick_id FROM quicktable WHERE x=1 FOR UPDATE
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD
LOCKS space id 0 page no 540674 n bits 72 index `PRIMARY` of table
`test/quicktable` trx id 0 79790779 lock_mode X locks rec but not gap
waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000001; asc ;; 1: len 6; hex 000004c14f74; asc Ot;; 2:
len 7; hex 0000098005054d;
asc M;; 3: len 4; hex 8018f9fd; asc ;;
*** (2) TRANSACTION:
TRANSACTION 0 79790775, ACTIVE 2 sec, process no 7658, OS thread id 1191733584
2 lock struct(s), heap size 368
MySQL thread id 4094, query id 2102743 10.1.10.122 bnewton
*** (2) HOLDS THE LOCK(S):
RECORD
LOCKS space id 0 page no 540674 n bits 72 index `PRIMARY` of table
`test/quicktable` trx id 0 79790775 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0:
len 4; hex 80000001; asc ;; 1: len 6; hex 000004c14f74; asc
Ot;; 2: len 7; hex 0000098005054d; asc M;; 3: len 4; hex
8018f9fd; asc ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 540674 n bits 72 index `PRIMARY` of table
`test/quicktable`
trx id 0 79791014 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0:
len 4; hex 80000001; asc ;; 1: len 6; hex 000004c14f74; asc
Ot;; 2: len 7; hex 0000098005054d; asc M;; 3: len 4; hex
8018f9fd; asc ;;
TOO DEEP OR LONG SEARCH IN THE LOCK TABLE WAITS-FOR GRAPH
*** WE ROLL BACK TRANSACTION (2)
Can
anyone explain whats going on? Is there a limit for the number of
concurrent transactions, before looking at the lock graph becomes too
expensive? Is that documented somewhere?
It's not documented, but yes there's both a maximum number of steps to
check for a cycle in the waits-for graph, and a maximum depth of
checking it:
http://dev.mysql.com/sources/doxygen/mysql-5.1/lock0lock_8c-source.html#l00052
00046 /* Restricts the length of search we will do in the waits-for
00047 graph of transactions */
00048 #define LOCK_MAX_N_STEPS_IN_DEADLOCK_CHECK 1000000
00049
00050 /* Restricts the recursion depth of the search we will do in the
waits-for
00051 graph of transactions */
00052 #define LOCK_MAX_DEPTH_IN_DEADLOCK_CHECK 200
This is the price you pay for instant deadlock detection, as opposed to
just lock wait timeout (the method a lot of other transactional systems
take). You could recompile with a higher number if you want. It would
be nice if this were configurable; you could submit a feature request
for that.
Baron
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