Locking
This topic shows detailed information about "Accounting - Locking".
This block provides locking information. Locking ensures the integrity of data.
For formatting reasons, OMEGAMON XE for DB2 PE shows different labels for report and trace. The following example shows both layouts, the report on the left, and the trace layout on the right.
Accounting - Locking
The field labels shown in the following sample layout of "Accounting - Locking" are described in the following section.
Report: Trace:
LOCKING AVERAGE TOTAL LOCKING TOTAL
--------------------- -------- -------- ------------------- --------
TIMEOUTS 0.00 0 TIMEOUTS 0
DEADLOCKS 0.00 0 DEADLOCKS 0
ESCAL.(SHARED) 0.00 0 ESCAL.(SHAR) 0
ESCAL.(EXCLUS) 0.00 0 ESCAL.(EXCL) 0
MAX PG/ROW LOCKS HELD 0.00 0 MAX PG/ROW LCK HELD 7
LOCK REQUEST 0.00 0 LOCK REQUEST 48
UNLOCK REQUEST 0.00 0 UNLOCK REQST 34
QUERY REQUEST 0.00 0 QUERY REQST 0
CHANGE REQUEST 0.00 0 CHANGE REQST 32
OTHER REQUEST 0.00 0 OTHER REQST 0
TOTAL SUSPENSIONS 0.00 0 TOTAL SUSPENSIONS 0
LOCK SUSPENSIONS 0.00 0 LOCK SUSPENS 0
IRLM LATCH SUSPENS. 0.00 0 IRLM LATCH SUSPENS 0
OTHER SUSPENS. 0.00 0 OTHER SUSPENS 0- TIMEOUTS
The number of times a unit of work was suspended for a time exceeding the timeout value. This number should be low, ideally 0.
Field Name: QTXATIM
- DEADLOCKS
The number of times deadlocks were detected. This number should be low, ideally 0.
Background and Tuning Information
Deadlocks occur when two or more application processes each hold locks on resources that the others need, without which they cannot proceed. Ensure that all applications accessing the same tables access them in the same order.
To improve concurrency: - Use row level locking instead of page level locking to minimize deadlocks.
- For small tables use page level locking with MAXROWS 1.
- Delay updates to just before commit.
- Use SELECT with the FOR UPDATE clause to use U lock.
- Adjust the deadlock detection cycle parameter DEADLOK in the IRLM procedure.
This field is incremented once for each deadlock encountered. There is no correlation between this field and the deadlock events reported in the Locking report set or the number of IFCID 172 records written. This field reports all deadlocks, regardless of how they were resolved. The locking report and record trace IFCID 172 show only those deadlocks that were resolved by DB2.
Field Name: QTXADEA
This is an exception field.
- ESCAL.(SHARED)/ESCAL.(SHAR)
The number of times the maximum page locks per table space are exceeded, and the table space lock escalates from a page lock (IS) to a table space lock (S) for this thread. You can specify the number of locks allowed per table space with the LOCKS PER TABLE(SPACE) parameter on the DB2 install panel DSNTIPJ.
Background and Tuning Information
Escalations can cause unpredictable response times. Lock escalations should only happen when an application process updates or references (if repeatable read is used) more pages than normal.
Field Name: QTXALES
This is an exception field.
- ESCAL.(EXCLUS)/ESCAL.(EXCL)
The number of times the maximum page locks per table space are exceeded and the table space lock escalates from a page lock (IX) to a table space lock (X).
Background and Tuning Information
Escalations can cause unpredictable response times. Lock escalations should only happen when an application process updates or references (if repeatable read is used) more pages than it normally does.
A useful rule of thumb is to compare the number of escalations (shared and exclusive) to the successful escalations (those that did not cause deadlocks and timeouts). If this value, or the number Lock escalations - shared and if the number of timeouts or deadlocks is also not 0, the timeout or deadlock is probably caused by the escalation.
If many escalations cause deadlocks and timeouts, the recommendation is to change the escalation threshold value. Use of ANY is extremely useful to prevent unnecessary and expensive page locks, for example locking all pages in a tablespace.
Lock escalations, shared or exclusive, should not be expected in a transaction environment.
Field Name: QTXALEX
This is an exception field.
- MAX PG/ROW LOCKS HELD/MAX PG/ROW LCK HELD
The maximum number of page or row locks concurrently held against all table spaces by a single application during its execution. This count is a high-water mark. It cannot exceed the LOCKS PER USER parameter on panel DSNTIPJ.
Field Name: QTXANPL
This is an exception field.
- LOCK REQUEST
The number of requests to lock a resource.
Field Name: QTXALOCK
This is an exception field.
- UNLOCK REQUEST
The number of requests to unlock a resource.
This value can be less than the number of lock requests because DB2 can release several locks with a single unlock request.
Field Name: QTXAUNLK
- QUERY REQUEST
The number of query requests.
Field Name: QTXAQRY
- CHANGE REQUEST
The number of change requests.
Field Name: QTXACHG
- OTHER REQUEST
The number of requests to IRLM to perform a function other than LOCK, UNLOCK, QUERY, or CHANGE.
Field Name: QTXAIRLM
- TOTAL SUSPENSIONS
The number of all types of lock suspensions.
Field Name: ALRSUSP
- LOCK SUSPENSIONS/LOCK SUSPENS
The number of times a lock could not be obtained and the unit of work was suspended.
Background and Tuning Information
This number should be low, ideally 0.
The number of lock suspensions is a function of the lock requests. Lock suspensions (or conflicts) can happen on either LOCK REQUEST or CHANGE REQUEST.
Suspensions are highly dependent on the application and table space locking protocols.
Field Name: QTXASLOC
This is an exception field.
- IRLM LATCH SUSPENS.
The number of latch suspensions.
Field Name: QTXASLAT
This is an exception field.
- OTHER SUSPENS.
The number of suspensions caused by something other than lock or latch.
Field Name: QTXASOTH
This is an exception field.