2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.4 Expanded Storage Concepts › 2.2.4.4.4 Measurement, Analysis and Management
2.2.4.4.4 Measurement, Analysis and Management
RMF measures and reports on many data elements concerning
paging, swapping, and memory use. Beginning with RMF 3.3.0
and continuing through RMF Version 4 (introduced with ESA),
much information has been added that differentiates between
paging or swapping to expanded versus auxiliary storage, and
between the use of central versus expanded storage for CSA,
LPA, SQA, etc., pages. Expanded storage has added to the
complexity of understanding and managing storage hierarchy.
Even though well-tuned systems or systems with low storage
contention are likely to experience expanded storage-related
delays so small as to be relatively unimportant when compared
to other sources of delay, installations with expanded
storage should now monitor its utilization and activity as
well as auxiliary storage activity. Expanded storage delay
can become substantial to the point of significantly reducing
the benefit from your investment in it. The System Storage
Usage Report described in section 2.2.3.2 can aid in tracking
the utilization of expanded storage.
In a system with expanded storage, the degradation due to
page movement delay from expanded storage to central storage
is not directly measurable. Indeed, the rate of movement
from expanded to central is not measured by RMF in MVS/XA, it
is only measured in MVS/ESA systems. While the system
control program maintains (and RMF records) the average time
that it takes to read a page from auxiliary to central
storage, no such estimate is made of the time it takes to
transfer a page from expanded to central storage.
Page migration should be a primary concern because, if
excessive, it represents ineffective page placement and
ineffective use of expanded storage. If there is contention
for expanded storage, it makes no sense to move a page from
central to expanded storage if that page will eventually be
moved back into central and then paged out to auxiliary. It
would be much better to page it out directly to auxiliary
storage.
Minimizing such ineffective page placement depends on how
accurately an address space's future activity and storage
references can be predicted. RSM makes these predictions
based on the criteria age values specified in the IEAOPTxx
parameters for the various categories of expanded storage
use.
Expanded storage can be used to improve the response time of
online systems, to increase the capacity of the processor
complex, or a combination of the two. The best way to use
expanded storage depends on the business and technical
environment in which it is used.
Unfortunately, the mechanisms used to control expanded
storage allocation do not provide for workload assignments,
such as performance group or domain. It is therefore
necessary to understand the storage utilization
characteristics of the individual workloads and the system as
a whole before attempting to modify the criteria ages that
affect expanded storage control.
There are some general guidelines that apply to all systems,
however. To improve the response times of online systems
running as non-swappable address spaces, set low criteria
ages for the type 0 paging group and high ages for types 1
and 2 paging groups. This favors the onlines over other
work, but only if there is other work in the same system. In
a system used for application development and testing with
ISPF/PDF users submitting batch compiles and executions, you
might want to favor type 2 over types 0 and 1. If you have a
processor complex that is performing a wide variety of tasks
and with changing workload characteristics over time, then to
achieve both somewhat better response time and more
throughput use the default values for all criteria ages.
When making criteria age changes, keep in mind the following:
o Assigning a low criteria age to a particular page type and
status is saying, in effect, that you want RSM to move
such pages to expanded storage before other page
type/status classifications even if the contention for
expanded storage is high. You would do this if
(a) you regard the workload owning the page as important
and deserving of a performance improvement, or
(b) you expect the page to be referenced quickly and want
to lower the load on ASM by making it unnecessary to
page-out and then page-in the same page in a brief
time period.
o Assigning a high criteria age to pages has the opposite
effect: such pages are sent to expanded storage by RSM
after page types with lower criteria ages. In periods of
contention, such pages tend to go to auxiliary storage
instead. You would do this if you either
(a) want to use your expanded storage resources for more
important work, or
(b) expect that such a page will not be referenced for a
relatively long time and would be migrated to
auxiliary storage anyway.
Although IBM has established reasonable default values, it is
ultimately the responsibility of individual installations to
set these values for best use of expanded storage. The
performance analyst must monitor page movement to ensure that
the specified values are optimizing the use of expanded
storage for the installation's requirements. If criteria
ages are set too low, expanded storage will tend to be over-
utilized. This will result in a low migration age, large
values for both central-to-expanded page movement, and page
migration. If the difference between these two measurements
is small, it could indicate that too many pages are being
migrated.
