Writing data to the coupling facility: Performance considerations

At the application design level you must consider that the average block size written to the coupling facility affects the performance of the CICS® log manager.

When the average block size of data being written to the coupling facility is less than 4 KB, the write request is processed synchronously. The operation is synchronous to CICS, as is the instruction used to access the coupling facility, in that it runs for as long as it takes to place the data in the structure. For this reason, it is unwise to mix fast processors with slow coupling facilities. If the access time to a particular coupling facility remains constant, then for synchronous accesses, the faster the processor the more processor cycles are used by the request.

When the average block size of data being written to the coupling facility is greater than 4 KB, the write request is processed asynchronously; the CICS task gives up control and the MVS™ system logger posts the event control block (ECB) when the write request has been satisfied. This can result in an asynchronous request taking longer to complete than a synchronous one.

Synchronous requests might be changed by the subsystem into asynchronous requests if necessary—for example, if the subchannel is busy. Changed requests show on an RMF III report as CHNGD. Figure 1 shows an extract from an RMF report showing the numbers of synchronous and asynchronous writes to a coupling facility structure. The report gives the system name, the total number of requests, and the average number of requests per second. For each type of request, it gives the number of requests, the percentage of all requests that this number represents, the average service time, and the standard deviation.

Figure 1. RMF report showing numbers of synchronous and asynchronous writes to a coupling facility

STRUCTURE NAME = LOG_FV_001        TYPE = LIST
          # REQ    -------------- REQUESTS -------------
SYSTEM    TOTAL             #     % OF  -SERV TIME(MIC)-
NAME      AVG/SEC           REQ    ALL     AVG   STD_DEV
 
MV2A      15549    SYNC     15K  95.3%   476.1    339.6
          27.87    ASYNC   721    4.6%  3839.0   1307.3
                   CHNGD    12    0.1%  INCLUDED IN ASYNC

Note: This applies only to log streams that use coupling facility structures.