Performance implications of average blocksize

Important

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

Although consideration of the average blocksize written to the coupling facility can happen only at the level of application design, it is still worth bearing in mind when considering the performance implications of the CICS® log manager.

If the average blocksize of data being written to the coupling facility is less than 4K, the write request is processed synchronously. Not only is the operation synchronous to CICS, but the System/390® instruction used to access the coupling facility is also synchronous, in that it executes for as long as it takes to place the data in the structure. For this reason, it is unwise to mix fast CPUs with slow coupling facilities. If the access time to a particular coupling facility remains constant, then, for synchronous accesses, the faster the CPU the more CPU cycles are consumed by the request.

If the average blocksize of data being written to the coupling facility is greater than 4K bytes, the write request is processed asynchronously; the CICS task gives up control and the MVS™ system logger posts the ECB when the write request has been satisfied. This can result in an asynchronous request taking longer to complete than a synchronous one. However, there is no System/390 "long instruction" to place data into the coupling facility.

Synchronous requests may be changed into asynchronous requests, if the sub-system decides this to be necessary--for example, if the sub-channel is busy. Changed requests appear on an RMF™ III report as CHNGD. Figure 55 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 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 55. 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
 

Related tasks
Logging and journaling: performance considerations
Monitoring the logger environment
Performance implications of the number of log streams in the coupling facility structure
Setting LOWOFFLOAD and HIGHOFFLOAD parameters on log stream definition
Tuning the size of staging data sets
Setting the activity keypoint frequency (AKPFREQ)
Specifying the log defer interval (LGDFINT)
Tuning for DASD-only logging
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