Deadlocks in a DB2 database

Tuning your database to reduce disk I/O contention will help alleviate deadlocks, but it still may not be an uncommon occurrence that the new_edit_state stored procedure call deadlocks the calling application and blocks all other use of the ArcSDE database.

Imagine a scenario where the stored procedure acquires a large number of row locks on the STATE_LINEAGES table, exceeding the threshold for the maximum number of locks and attempting to escalate to an exclusive table lock. Unfortunately, the calling application's query already holds a shared lock on the STATE_LINEAGES table, thus leading to a deadlock. Large numbers of row locks arise from having a deep state lineage. This, along with having a low setting for lock list size, guarantees that there will be problems. Given how lock escalation is handled, other deadlock scenarios are also possible.

The implication here is that deadlocks may not be an uncommon occurrence at certain sites, depending on one's application and the database configuration. Once again, note that the problem may be aggravated with deep states lineages.

Fortunately, IBM DB2 provides tuning parameters to control the size of the lock list (LOCKLIST), the maximum percentage of locks an application can hold (MAXLOCKS), the amount of time a request will wait for a lock to be acquired (LOCKTIMEOUT), frequency interval for deadlock detection (DLCHKTIME) and deadlock rollback behavior (DB2LOCK_TO_RB).

Briefly, to increase the lock list capacity and lock escalation threshold, modify the LOCKLIST and MAXLOCKS parameters, respectively.

The default value for LOCKLIST and MAXLOCKS in DB2 9 is AUTOMATIC, which enables these parameters for self tuning. This allows DB2's memory tuner to dynamically size the memory resources between different memory consumers. Automatic tuning only occurs if self tuning memory is enabled for the database (SELF_TUNING_MEM=ON).

Additionally, you may be able to improve concurrency through lock avoidance by using DB2's Lock Deferral registry variables DB2_EVALUNCOMMITED, DB2_SKIPDELETED, and DB2_SKIPINSERTED. These registry variables permit scans to unconditionally skip uncommitted deletes and inserts.

By default, a lock time-out will roll back the request transaction. To change this behavior to only roll back the statement making the lock request, modify DB2LOCK_TO_RB with db2set DB2LOCK_TO_RB=STATEMENT. The default behavior should be fine for ArcSDE, though.

See the DB2 documentation or performance tuning guides for detailed information on properly setting these parameters. An overview of using these parameters is given below.

Diagnosing lock problems

A few useful tools to diagnose lock problems are detailed below.

Parameter settings that deal with deadlocks

To view lock list settings, issue the following command:

db2 get db cfg

The following is an example of the information returned as a result of issuing this command:

Max storage for lock list (4KB)		(LOCKLIST) = 50
Interval for checking deadlock (ms)	(DLCHKTIME) = 10000
Percent. of lock lists per application	(MAXLOCKS) = 22
Lock time out (sec)			(LOCKTIMEOUT) = -1
Max number of active applications	(MAXAPPLS) = AUTOMATIC

Consult the IBM DB2 Infocenter for details on setting these parameters.