Memory setting on IBM® z/OS®

The memory requirements vary based on environmental factors or the characteristics of the workload.

In this example, a workload of 250 concurrent clients each sending ECI requests with 2 MB containers to CICS® TS, requires a Gateway daemon defined with 250 connection managers, 250 worker threads, and 250 IPIC send sessions. To avoid running out of memory you would require a maximum heap size (-Xmx), of at least 1536 MB. Better performance is obtained with a maximum heap size of 2048 MB, to avoid excessive garbage collection.

The maximum heap size directly affects the value that must be used for MEMLIMIT, and the Gateway daemon threads still utilize 31-bit storage. In this example, the REGION size was set to 300 MB, tuned by reviewing the SE_CELOAL statistic, and MEMLIMIT set to 3.3 GB.

To tune the heap size, you should review the SE_CHEAPGCMIN statistic as a percentage of SE_SHEAPMAX to ensure the value is in the range 40-70%. If the percentage goes above 70% you can increase the maximum heap size. If the percentage is below 40% then you can reduce the maximum heap size.

Increasing the ECI payload size to 4 MB containers, the heap size needs to be increased to avoid excessive garbage collection. In the example of 250 clients, -Xmx was set to 2048 MB. Increasing the heap size requires MEMLIMIT to be increased. In this example, 3.73 GB was used, so MEMLIMIT was set to 4 GB.

Table 1Table 1 contains some example scenarios with increasing channel sizes: All scenarios use 200 clients, with 250 connection managers, 250 worker threads and 250 IPIC Send Sessions defined in the configuration file.

Table 1. Scenarios with increasing channel sizes
Channel Size	MEMLIMIT	REGION	SE_CELOAL	SE_C31MAX	Max Heap	SE_CHEAPGCMIN	SE_SHEAPMAX	Heap Occupancy
1 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	320010016	2147483648	15%
2 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	315898512	2147483648	15%
3 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	492042016	2147483648	23%
4 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	451839424	2147483648	21%

Table 2. Scenarios with increasing channel sizes
Channel Size	MEM LIMIT	REGIO N	SE_CEL OAL	SE_C31 MAX	Max Heap	SE_CHE APGC MIN	SE_S HEAP MAX	Heap Occu pancy
1 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	320010016	2147483648	15%
2 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	315898512	2147483648	15%
3 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	492042016	2147483648	23%
4 MB	4 GB	300 MB	262 MB	1688 MB	2048 MB	451839424	2147483648	21%

For optimum performance you should aim for a heap occupancy of between 30% and 70%.