z/VM: CP Commands and Utilities Reference - z/VM - IBM

Usage Notes
MAXWSS OFF
sets the MAXWSS percentage to 9999%, effectively turning the mechanism off.
(In other words, with the limit set high enough, it is in effect no longer a limit.)
This is the initial setting.
XSTORE xstpct[%]
sets the percentage of XSTORE that the scheduler will use when determining
the amount of storage available for dispatching purposes. Valid values are from
0% through 9999%. The default value is 0%, and has the effect of excluding
XSTORE from the scheduler/dispatcher calculations.
1. In using SET SRM DSPSLICE, the initial value of the dispatching minor time
slice is determined by the model of the real machine that VM/ESA is IPLed on.
Making the time slice smaller increases system overhead, but it also gives
better sorting of the dispatch list and more accurate share control by the
scheduler. Making the time slice larger decreases system overhead at the
expense of the accuracy of the scheduler share controls. Making the time slice
larger may also interfere with the scheduler’s ability to quickly sort the dispatch
list to get the most I/O-bound users near the top. This in turn may reduce the
system’s ability to drive I/O and maintain performance.
Remember that when the dispatch slice is changed, the IABIAS duration no
longer represents the same amount of processing as before. If the size of the
DSPLICE is changed significantly, you may want to adjust the IABIAS duration
to compensate so that it still represents roughly the same amount of processing
as before.
2. An installation can use the SET SRM STORBUF command to undercommit or
overcommit storage. Note that Q0 users are not restricted by their storage
requirement. By setting the first percentage value (aaa) at less than 100%, you
can reserve a storage buffer for Q0 users. It is also sometimes desirable to do
this to reserve a storage buffer for active users who are temporarily “dormant”
(between transactions). When aaa is set to less than 100%, the storage above
aaa is protected from the (non-Q0) users who are currently running. This means
the running users are prevented from crowding all the pages of dormant users
out of storage. This is useful when many dormant users’ pages are needed
sooner than the system can page them out and then in again.
By increasing or decreasing the differences in the three percentage values of
STORBUF, an installation can affect the response times and expansion factors
for short-running (generally interactive), medium-running, and long-running work.
3. SET SRM LDUBUF is similar to SET SRM STORBUF, except that the resource
buffered is the paging configuration capacity, rather than storage. An installation
may use SET SRM LDUBUF to undercommit or overcommit the access to
paging, or to favor one class of transaction over another.
The values set for SET SRM LDUBUF are usually not critical. However, in most
situations, the default values are too low and a better choice would be:
SET SRM LDUBUF 300% 200% 100%
SET SRM
This represents an overcommitment. If there is a need to do a lot of paging, an
overcommitment insures that enough loading users are allowed in the dispatch
list to keep the DASD paging devices running at or near their full capacity.
4. SET SRM XSTORE causes the scheduler to include a percentage of XSTORE
in the amount of real storage it considers to be available for dispatching
Chapter 2. CP Commands 1395