Why does Broadcom recommend the region size for the TPX proc be set to 0M?
Component: TPX For Z/Os
Reason why it's recommended to change the REGION size to 0M in TPX proc is because
TPX will allocate all of the available storage required then it will return the rest of the storage to the system.
By setting REGION=0M, there is no storage limitations below the 16Mb line.
At startup TPX issues a conditional GETMAIN asking the operating system how much storage is available below the line.
Assuming the operating system returns - available storage is 8.5 MB.
If TPX would do a GETMAIN now for 8.5 MB there would be no storage left and any GETMAIN from
e.g. operating system components would fail with S80A, S878 abends.
TPX will not START.
TPX checks the 'System Storage' set in SMRT option 2, value and subtracts this from the 8.5 MB available.
If 'System Storage' is set to 2048K TPX does a GETMAIN only for 6.5 MB for its modules, slots and DSA below the line.
The 2 MB left remains under the control of the operating system to satisfy GETMAIN requests below the line.
TPX itself does not do any explicit GETMAIN here unless you need to do a VTADD.
For every VT added with VTADD TPX will request about 150 bytes (actually 144) above the 16M line through a GETMAIN.