Memory Management of the SAP
Gateway 
Overflow Area
The gateway stores
its data in the overflow area.
This area in the heap memory is retrieved by the gateway during initialization
with malloc()(default 1 MByte). The size of the overflow area is
specified by the profile parameter gw/max_overflow_size.
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. An 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 5th request.
CPIC Requests
A specific number of requests can be temporarily stored per CPIC connection. The number of requests is determined by the profile parameter gw/req_stack_size. The default is 30. When the number of requests stored at a time exceeds the threshold value (max(stack_size-6,90% stack_size)), the sender is slowed down by SYNC calls (see above).