virtual memory 4509

previous post mentioning 370s shipping with both 2k and 4k virtual page support and comparison of operating systems in different environments that used 2k virtual pages with operating systems that used 4k virtual pages ... sometimes on the same 370 hardware.

in fact, there was an infamous customer situation that involved upgrading from a 370-168-1 (with 16kbyte cache) to 370-168-3 (with 32kbyte cache).

the customer ran vs1 (with 2k virtual pages) under vm-370. vm-370 nominally ran with 4k virtual pages ... except doing shadow page tables for virtual machine guest operating system .. it used the same page mode (2k or 4k) for the shadow page tables as the virtual machine guest operating system used for their virtual page tables.

so for the transition for 370-168-3 with twice the cache size ... they choose to use the "2k" bit to address the additional cache line entries (under the buttumption that this was a high-end machine only used by MVS or some vm). if the 370-168-3 ran with 2k virtual pages it would switch to 16k cache size mode (not using the additional 16k). if there was a switch between 4k virtual page mode and 2k virtual page mode, it would first completely flush the cache.

so for this particular customer, the upgrade from the 370-168-1 to the faster 370-168-3 actually resulted in degraded performance (because of the large number of cache flushes).

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