Memory Management of the SAP Gateway 

Shared Memory Segments

The data of a CPIC request can be stored in this segment. This offers the advantage that instead of data having to be copied, only pointers into the shared memory need to be set.
The size of segment 22 is calculated as follows:
The maximum size of a request (40728 bytes) multiplied by the number of shared memory segments (profile parameter gw/max_shm_req). A total of approx. 2.1 MB is created in shared memory.

If data can no longer be stored in the shared memory area, the system attempts to temporarily store the data in the overflow area.
The size of the overflow area is specified by the profile parameter gw/max_overflow_size. This area is retrieved by the gateway during initialization with malloc() (default 1 MB).
The profile parameter gw/max_overflow_usage (specified as a percentage) specifies the usage of the overflow area as of which the gateway slows down the sender. If this value is exceeded, the sender is slowed down by SYNC calls. A SYNC ensures that control is returned to the sender only when the data has been retrieved by the receiver. You must, however, make sure that 5 CPIC requests can still be saved in the gateway, since the sender only executes CMSEND synchronously with every 5^th request.

A specific number of requests can be temporarily stored per CPIC connection. This number is determined by the profile parameter gw/req_stack_size; the default value is 30. The system first attemps to store the individual CPIC requests in shared memory. The number of CPIC requests that can be stored in shared memory is determined by the profile parameter gw/max_shm_req_per_conn. When the shared memory area is 75% filled, no more CPIC requests can be stored in shared memory, and the data must be stored in the overflow area. When the number of requests stored at a time exceeds a threshold value (max(stack_size-6,90% stack_size)), the sender is slowed down by SYNC calls (see Overflow).