>Many moons ago code was implemented in ASM and RSM to slowly reclaim ASM
>page slots for virtual pages that were changed in real storage. I have
>vague memories of this functionality later being disabled for some
>technical concern, but can't remember what it was.
Slot Scavenger was introduced in OS3/390 2.10. It did its thing during
the UIC
Update function, UIC Update was removed when Global Steal was
implemented
(z/OS 1.8, I think), so at that point there was no more Slot Scavenger.
Back when real storage was expensive and scarce, then ratio of aux slots
to
real frames tended to be higher than it is now, and there weren't that
many pages
that were backed in both real and aux because there wasn't much real.
There are some performance advantages to having an unchanged page backed
in real and aux.
1. You can steal the frame synchronously, without waiting for page-out
I/O.
2. When capturing it during SDUMP, you can give one of the copies to SDUMP
with needing to copy it.
But like any design tradeoff, there are drawbacks as well, and with real
storage
less expensive and more plentiful, you can have a lot more virtual pages
consuming both an aux slot and a real frame. There has been some
thought given to changing things so that we always free the aux slot after
we
page-in from it.
Jim Mulder z/OS Diagnosis, Design, Development, Test IBM Corp.
Poughkeepsie NY
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