I'm curious that Barton wrote on August 28th:
> VMRM has taken so much storage away from servers that the
> server or application dies. I would HIGHLY recommend against using it.
z/VM has recently documented how to keep VMRM from eating up too much
memory, though the mechansim to do so has been there since at least 5.2.
The z/VM Performance book, Chapter 17 - VMRM Tuning Parameters, talks
about a constant MinRequired. MinRequired is the minimum amount of memory
the Linux guest(s) must have at all times. VMRM will not ask the guest(s)
to shrink beyond that point. Below is the usage note added to Chapter 17
of the z/VM Performance book.
4. Use of VMRM-CMM can potentially reduce the performance of one
or
more of the participating Linux guests because VMRM can ask the
Linux guests to give up too much memory resulting in the Linux
guests using excessive CPU time trying to find more storage to give
up and leaving little CPU time available for useful work. To
prevent this situation from happening, VMRM has a configurable
constant that defines a value below which VMRM will not ask the
Linux guests to shrink. By default, this safety net value is 64MB,
but for certain workloads, the value may be too low, thus causing
poor performance. The safety net value is defined by the constant
MinRequired in the VMRM constants file, IRMCONS COPY, and by
default is set to 16384 pages. To change the safety net value,
update the value of constant MinRequired (set in pages) in file
IRMCONS COPY. The update should be made as a local modification
using the automated local modification procedure documented in
Appendix D in the z/VM Guide for Automated Installation and Service.
For more info, see APAR 64439 for z/VM 5.2 and z/VM 5.3 and informational
APAR II14518 for z/VM 5.4
Xenia Tkatschow
z/VM Performance
Endicott, NY 13760
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