Lock Management

In GaussDB(DWS), a deadlock may occur when concurrently executed transactions compete for resources. This section describes parameters used for managing transaction lock mechanisms.

deadlock_timeout

Parameter description: Specifies the time, in milliseconds, to wait on a lock before checking whether there is a deadlock condition. When the applied lock exceeds the preset value, the system will check whether a deadlock occurs.

The check for deadlock is relatively expensive. Therefore, the server does not check it when waiting for a lock every time. Deadlocks do not frequently occur when the system is running. Therefore, the system just needs to wait on the lock for a while before checking for a deadlock. Increasing this value reduces the time wasted in needless deadlock checks, but slows down reporting of real deadlock errors. On a heavily loaded server, you may need to raise it. The value you have set needs to exceed the transaction time. By doing this, the possibility that a lock will be released before the waiter decides to check for deadlocks will be reduced.
When log_lock_waits is set, this parameter also determines the duration you need to wait before a log message about the lock wait is issued. If you are trying to investigate locking delays, you need to set this parameter to a value smaller than normal deadlock_timeout.

Type: SUSET

Value range: an integer ranging from 1 to 2147483647. The unit is millisecond (ms).

Default value: 1s

lockwait_timeout

Parameter description: Specifies the longest time to wait before a single lock times out. If the time you wait before acquiring a lock exceeds the specified time, an error is reported.

Type: SUSET

Value range: an integer ranging from 0 to INT_MAX. The unit is millisecond (ms).

Default value: 20 min

update_lockwait_timeout

Parameter description: sets the maximum duration that a lock waits for concurrent updates on a row to complete when the concurrent update feature is enabled. If the time you wait before acquiring a lock exceeds the specified time, an error is reported.

Type: SUSET

Value range: an integer ranging from 0 to INT_MAX. The unit is millisecond (ms).

Default value: 2min

max_locks_per_transaction

Parameter description: Controls the average number of object locks allocated for each transaction.

The size of the shared lock table is calculated under the condition that a maximum of N independent objects need to be locked at any time. N = max_locks_per_transaction x (max_connections + max_prepared_transactions). Objects that do not exceed the preset number can be locked simultaneously at any time. You may need to increase this value when you modify many different tables in a single transaction. This parameter can only be set at database start.
If this parameter is set to a large value, GaussDB(DWS) may require more System V shared memory than the default setting.
When running a standby server, you must set this parameter to a value that is no less than that on the primary server. Otherwise, queries will not be allowed on the standby server.

Type: POSTMASTER

Value range: an integer ranging from 10 to INT_MAX

Default value: 256

max_pred_locks_per_transaction

Parameter description: Controls the average number of predicated locks allocated for each transaction.

The size of the shared and predicated lock table is calculated under the condition that a maximum of N independent objects need to be locked at any time. N = max_pred_locks_per_transaction x (max_connections + max_prepared_transactions). Objects that do not exceed the preset number can be locked simultaneously at any time. You may need to increase this value when you modify many different tables in a single transaction. This parameter can only be set at server start.
If this parameter is set to a large value, GaussDB(DWS) may require more System V shared memory than the default setting.

Type: POSTMASTER

Value range: an integer ranging from 10 to INT_MAX

Default value: 64

partition_lock_upgrade_timeout

Parameter description: Specifies the time to wait before the attempt of a lock upgrade from ExclusiveLock to AccessExclusiveLock times out on partitions.

When you do MERGE PARTITION and CLUSTER PARTITION on a partitioned table, temporary tables are used for data rearrangement and file exchange. To concurrently perform as many operations as possible on the partitions, ExclusiveLock is acquired for the partitions during data rearrangement and AccessExclusiveLock is acquired during file exchange.
Generally, a partition waits until it acquires a lock, or a timeout occurs if the partition waits for a period of time longer than specified by the lockwait_timeout parameter.
When doing MERGE PARTITION or CLUSTER PARTITION on a partitioned table, you need to acquire AccessExclusiveLock during file exchange. If the lock fails to be acquired, the acquisition is retried in 50 ms. This parameter specifies the time to wait before the lock acquisition attempt times out.
If this parameter is set to -1, the lock upgrade never times out. The lock upgrade is continuously retried until it succeeds.

Type: USERSET

Value range: an integer ranging from -1 to 3000. The unit is second (s).

Default value: 1800

enable_online_ddl_waitlock

Parameter description: Specifies whether to block DDL operations to wait for the release of cluster locks, such as pg_advisory_lock and pgxc_lock_for_backup. This parameter is mainly used in online OM operations and you are not advised to modify the settings.

Type: SIGHUP

Value range: Boolean