Calculating global contention percentages with the Db2 statistics trace

You can use the Db2 statistics trace to calculate global contention percentages for lock requests.

Procedure

To calculate global contention:

Determine the total number of suspends that are caused by contention.
Determine the total number of requests that went to XES.
Divide the total number of suspends by the total number of requests.
Multiply the result by 100.

Example

Figure 1. Data sharing locking block of IBM® OMEGAMON® for Db2 Performance Expert on z/OS® statistics trace

DATA SHARING LOCKING            QUANTITY  /SECOND  /THREAD  /COMMIT
---------------------------     --------  -------  -------  -------
GLOBAL CONTENTION RATE (%)          0.19
FALSE CONTENTION RATE (%)           0.04
P/L-LOCKS XES RATE (%)             36.47

LOCK REQUESTS (P-LOCKS)           107.4K    44.74     0.93     0.18
UNLOCK REQUESTS (P-LOCKS)         105.2K    43.82     0.91     0.18
CHANGE REQUESTS (P-LOCKS)        1453.00     0.61     0.01     0.00

SYNCH.XES - LOCK REQUESTS        1478.8K   616.16    12.77     2.48
SYNCH.XES - CHANGE REQUESTS     45450.00    18.94     0.39     0.08
SYNCH.XES - UNLOCK REQUESTS       810.0K   337.48     7.00     1.36
BACKGROUND.XES -CHILD LOCKS         0.00     0.00     0.00     0.00
ASYNCH.XES -CONVERTED LOCKS       108.00     0.04     0.00     0.00

SUSPENDS - IRLM GLOBAL CONT      3236.00     1.35     0.03     0.01
SUSPENDS - XES GLOBAL CONT.       334.00     0.14     0.00     0.00
SUSPENDS - FALSE CONT. MBR        863.00     0.36     0.01     0.00
SUSPENDS - FALSE CONT. LPAR          N/A      N/A      N/A      N/A
REJECTED - XES                  31897.00    13.29     0.28     0.05
INCOMPATIBLE RETAINED LOCK          0.00     0.00     0.00     0.00
NOTIFY MESSAGES SENT              344.00     0.14     0.00     0.00
NOTIFY MESSAGES RECEIVED          106.00     0.04     0.00     0.00
P-LOCK/NOTIFY EXITS ENGINES       500.00      N/A      N/A      N/A
P-LCK/NFY EX.ENGINE UNAVAIL         0.00     0.00     0.00     0.00

PSET/PART P-LCK NEGOTIATION        38.00     0.02     0.00     0.00
PAGE P-LOCK NEGOTIATION          3026.00     1.26     0.03     0.01
OTHER P-LOCK NEGOTIATION         1520.00     0.63     0.01     0.00
P-LOCK CHANGE DURING NEG.        3072.00     1.28     0.03     0.01

For this example statistics trace, calculate global contention as follows:

Determine the total number of suspends that are caused by contention (SUSPENDS - IRLM GLOBAL CONT + SUSPENDS - XES GLOBAL CONT. + SUSPENDS - FALSE CONT. LPAR).

3236 + 334 + 863 = 4433

Determine the total number of requests that went to XES (

SYNCH.XES LOCK REQUESTS + CHANGE REQUESTS + UNLOCK REQUESTS + BACKGROUND.XES CHILD LOCKS + ASYNCH.XES CONVERTED LOCKS + SUSPENDS IRLM GLOBAL CONT + XES GLOBAL CONT. + FALSE CONT. LPAR

1478800 + 45450 + 810000 + 0 + 108 + 3236 + 334 + 863 = 2338791

Divide the total number of suspends by the total number of requests.

4433 / 2338791 = .0019

Multiply the result by 100.

.0019 x 100 = 0.19

The result indicates that the global contention rate is approximately 0.19%( A ). Because this is such a low rate of contention, it is probably not necessary to determine the amount of false contention.

What to do next

If the global contention rate is greater than 5%, investigate whether the following types of contention are contributing to the global contention:

IRLM contention: Lock resource contention. Use the lock trace to identify the objects. Some object definitions might need to be changed. For example, row-level locking might be needed for random access.
XES contention: XES understands only S or X locks. This is usually not an problem
False contention: If the size of the lock facility coupling size is too small, you might need to increase the size of the coupling facility lock table.