Gerhard Adam wrote:
So,the max size a user actual can get for his private use in a single
address space=2G - the size of shared system area?
Yes. For an address space the shared system area is common to all users
and therefore represents storage that is unavailable to individuals.
Adam
True for the primary address space, but ever since MVS/ESA an
application could also allocate one or more dataspaces (for data storage
only, not executable code), and each of these could potentially be as
large as 2GiB, with none of that dataspace virtual address range
reserved for system/common use.
So even prior to 64-bit mode, applications had ways to put much more
than 2 GiB of data in addressable virtual storage by using dataspaces
and running in Access-Register mode, provided MVS had enough combined
Central Storage, E-Store, and Auxiliary Storage to support the
application's actual page working set and get decent performance.
Since E-Store was addressed by 4KiB block instead of by Byte, it wasn't
subject to the same 2GiB limit as Central Storage and allowed MVS to
utilize all real storage beyond 2GiB as a very high-speed paging device.
What first pushed the transition to 64-bit architecture was not the
limitation on virtual storage to an individual address space (since
dataspaces were available), but the 2GiB Central Storage real storage
constraint on MVS, which introduced an increasing amount of overhead and
performance loss from high page transfer rates between Central Store and
E-Store as the aggregate working set of all address spaces approached
and then significantly exceeded 2GiB. In such cases, one achieves
significantly better performance by running z/OS on z-architecture in
64-bit mode with all real memory configured as Central Storage,
eliminating all CS/ES page transfers.
...
--
Joel C. Ewing, Fort Smith, AR [EMAIL PROTECTED]
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO
Search the archives at http://bama.ua.edu/archives/ibm-main.html
