At 02:53 -0400 on 06/02/2011, Jim Mulder wrote about Re: What is the
current feeling for MVC loop vs. MVCL?:
>>> Obviously you can have "fast clear" for objects with lengths that
256-multiple using n*XC - however I have often wished for a
ZERO=YES option on the CPOOL BUILD so that you are guaranteed a
clean cell for each subsequent CPOOL GET. If anyone else agrees, I
might consider raising a Share requirement ....
Such a requirement would likely be rejected. There isn't anything
CPOOL can do to obtain better performance for clearing a cell than
what you can do in your own code. Clearing done by CPOOL would be
guaranteed to be at least marginally slower, and in some cases
considerably slower, than clearing done by your code.
If the CPOOL spanned one or more 4K aligned pages, then a ZERO=YES
could zero the area before and after the 4K block and just set the
virtual page table entries for the block to be a read-only dedicated
pre-zero'ed page. I seem to have the impression that STORAGE does
this for allocated pages in some cases (I think that allocated area
has to be page aligned and a multiple of page length to get
pre-zeroed storage).
There are no cases where STORAGE provides you with
a read-only page.
What you are suggesting would be considerably slower
than clearing done by your own code.
Jim Mulder z/OS System Test IBM Corp. Poughkeepsie, NY
I might have phrased my description incorrectly. What I was saying is
that when you initially allocate an virtual area when that area is
actually mapped to memory the page that gets loaded (paged-in) is
pre-zeroed. I was just suggesting that this would be simplest to do
if there was a single pre-zeroed page to be used for the page-in and
the mapping table entry was marked to not use that DASD copy as the
target when the virtual page needs to be paged out again.
BTW: Is my impression that if I use STORAGE to allocate a
MULTIPLE-PAGE area, then the virtual page will be pre-zeroed? If so,
how is this done?
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