I've tried wacky things like this before to see if I could run a 250G guest on an lpar with ~140GB of memory and oodles of page space, running z/VM 5.1
It came up, the guest initialized and Linux IPLed fine. It didn't have a problem till I started running a memory thrasher in the Linux guest. It sucked up all available memory and VM started paging, as you'd guess. It kept making progress till it had used about 20% of the paging space, but eventually VM itself started thrashing in its memory management routines. Like a %SY of 500 or so ( 5 processors running memory management stuff?? ) I'd guess that VM itself ran out of space below the 2G bar for page tables or something along that line. It never abended though - it just thrashed itself for days. Admittedly - not 8TB in a 200G box, as Lee tried to do, and it was on z/VM 5.1, so it didn't have the system execution space stuff of later z/VM releases. It did teach the lesson that more page packs can only get you so far. At some point the system data structures needed to support the enormous guest just wont fit. This may be a reasonable calculation to make within CP as a sanity check. -- Jay Brenneman
