Thank you all for the responses. I appreciate it! My question was specific to load modules in a PDS/PDSE on z/OS, so I believe no one has contradicted Jim's "answer is no" below for that use case.
Thanks, Tim -----Original Message----- From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf Of Jim Mulder Sent: Thursday, July 24, 2014 2:23 PM To: IBM-MAIN@LISTSERV.UA.EDU Subject: Re: z/OS physical memory usage with multiple copies of same load module at different virtual addresses Jim Mulder z/OS System Test IBM Corp. Poughkeepsie, NY > I had a question about how z/OS handles physical memory for the same > load module that is loaded at different virtual addresses in different > virtual address spaces. > > For example, let's say on the same z/OS system I have the following: > > CICS region A that has load module B loaded at virtual address x'3F400010'. > CICS region B that has the same load module B loaded at virtual > address x'3D400020'. > > More than likely there will be some differences in that same load > module that has been loaded at different virtual addresses, since > things like ADCONs and VCONs will resolve to different virtual > addresses when the load module is brought into memory. However, there > will probably be many virtual pages that are also identical between > the two modules, if nothing in that virtual page needed to be adjusted > by the loader. > > My question is, does z/OS have any functionality to detect the virtual > pages that would be identical for these 2 load modules and then adjust > the virtual page tables for CICS region A and B to point to the same > physical frame and drop the duplicate frame? > > So for example, if the first 4k virtual page of load module B in CICS > region A at x'3F400010' and the first 4k virtual page of load module B > in CICS region B at x'3D400020' had the same data in it, would z/OS > recognize this and cause the page tables for CICS region A at > x'3F400010' and page tables for CICS region B at x'3D400020' to be > adjusted to point to the same physical frame and then drop the other > duplicate physical frame? > > I was under the impression that the answer was no, but one of the > SYSPROGs here thought that z/OS had functionality to detect identical > physical frames that hold instructions that different address spaces > might be using and then do some physical memory savings like I am > mentioning above. > > I tried doing some internet searches on this topic, and could not find > any hits. If z/OS is doing this and someone has links to where this > documented, I would appreciate a point in the right direction. You are correct. The answer is "no". Jim Mulder z/OS System Test IBM Corp. Poughkeepsie, NY ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
