Has something changed in last 5 years? This has been in my standard
SHARE presentations for the last 5 years, with CMM drop 20,000 pages,
and there are 20,000 page reads. Drop another 20,000 pages, and there
are 40,000 page reads (and 20,000 page writes becuase it was a
constrained storage experiment). End result was 40,000 less pages on
disk space and a LOT of page reads. Too bad no one from Endicott sees
my presentations.
On 12/11/2015 4:33 AM, Bill Holder wrote:
On Thu, 10 Dec 2015 14:05:26 -0800, Barton Robinson wrote:
If anybody in endicott is listening, ONE REQUEST. The current diagnose
used by the CMM function to release a page actually causes z/VM to read
the page in from the paging subsystem before releasing it. So release
100,000 pages that are on the paging subsystem means 100,000 page reads
from disk. VERY BAD. If you discard a page that is on paging disk,
discard it, don't read it and then discard it.
Sorry, Barton, but this is simply factually incorrect, and would certainly
be APARable if it were so.
We do not read data pages from DASD during any virtual storage release
operation. However,
it is true that releasing them may require paging I/O reads to read the
PGMBKs which represent
the megabytes containing the released pages. The ratio of PGMBKs to pages
represented is
8K to 1M (256 pages), but the actual ratio we need to read can be
substantially higher depending
on the distribution - if only one page per megabyte is being released, we
may have to read one
8K PGMBK for each 4K page being released. If you have evidence that actual
data pages are
being read during any release operation (diag x'10' or otherwise), please
open a PMR and send
in your evidence.
(re-sending with corrected subject, sorry)
- Bill Holder, z/VM Development, IBM
"Much that once was is lost, for none now live who remember it." -
Galadriel
"He that breaks a thing to find out what it is has left the path of wisdom"
- Gandalf
(JRR Tolkein)
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