Intel AS/400 RISC Server - Manual (Page 27)

If customers modify an IBM-supplied class description, they are responsible for ensuring the priority
value is 35 or less after each new release or cumulative PTF package has been installed. One way to do
this is to include the Change Class (CHGCLS) command in the system Start Up program.
NOTE: Several IBM-supplied class descriptions already have RUNPTY values of 35 or less. In these
cases no user action is required. One example of this is class description QPWFSERVER with
RUNPTY(20). This class description is used by Client Access database server jobs QZDAINIT (APPC)
and QZDASOINIT (TCP/IP).
The system deprioritizes jobs according to groups or "bands" of RUNPTY values. For example, 10-16 is
band 1, 17-22 is band 2, 23-35 is band 3, and so on.
Interactive jobs with priorities 10-16 are an exception case with V4R1. Their priorities will not be
adjusted by SDT. These jobs will always run at their specified 10-16 priority.
When only a single interactive job is running, it will not be dynamically reprioritized.
When the interactive workload exceeds the knee of the curve, the priority of all interactive jobs is
decreased one priority band, as defined by the Dynamic Priority Scheduler, every 15 seconds. If needed,
the priority will be decreased to the 52-89 band. Then, if/when the interactive CPW work load falls
below the knee, each interactive job's priority will gradually be reset to its starting value when the job is
dispatched.
If the priority of non-interactive jobs are not set to 35 or lower, SDT stills works, but its effectiveness is
greatly reduced, resulting in server behavior more like V3R6 and V3R7. That is, once the knee is
exceeded, interactive priority is automatically decreased. Assuming non-interactive is set at priority 50,
interactive could eventually get decreased to the 52-89 priority band. At this point, the processor is
slowed and interactive and non-interactive are running at about the same priority. (There is little priority
difference between 47-51 band and the 52-89 band.) If the Dynamic Priority Scheduler is turned off,
SDT is also turned off.
Note that even with SDT, the underlying server behavior is unchanged. Customers get no more CPU
cycles for either interactive or non-interactive jobs. SDT simply tries to regulate interactive jobs once
they exceed the knee of the curve.
Obviously systems can still easily exceed the knee and stay above it, by having a large number of
interactive jobs, by setting the priority of interactive jobs in the 10-16 range, by having a small
client/server workload with a modest interactive workload, etc. The entire server behavior is a partnership
with customers to give non-interactive jobs the bulk of the CPU while not entirely shutting out
interactive.
To enable the Server Dynamic Tuning enhancement ensure the following system values are on:
(the shipped defaults are that they are set on)
y
QDYNPTYSCD - this improves the job scheduling based on job impact on the system.
y
QDYNPTYADJ - this uses the scheduling tool to shift interactive priorities after the threshold is
reached.
The Server Dynamic Tuning enhancement is most effective if the batch and client/server priorities are in
the range of 20 to 35.
IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
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Copyright IBM Corp. 2008 Chapter 2 - Server Performance Behavior 27