2. PERFORMANCE REPORTING ANALYSIS › 2.6 PR/SM LPAR Performance Analysis › 2.6.4 Usage Guidelines › 2.6.4.3 Using the Wait State Assist Feature

2.6.4.3 Using the Wait State Assist Feature

A key component of PR/SM is the Wait State Assist (WSA)
feature.  WSA allows an installation to define what happens
in a shared processor environment when an LPAR goes into a
wait state prior to the end of its time slice.  When the
feature is disabled (the usual setting), a CP will remain
allocated to an LPAR until the time slice expires or the SCP
running in the LPAR attempts to place the logical processor
into a wait state; at this time, the CP will be made
available for work elsewhere.

When WSA is enabled, PR/SM will not take control of a CP when
the logical processor goes into a wait state, but leaves the
CP under the control of the LPAR for the entire duration of
the time slice.  Ordinarily, fixed time slicing would be
utilized with WSA enabled.

Since one of the primary objectives of PR/SM is to greatly
reduce (if not eliminate) the time a logical processor is in
a wait state, enabling WSA should be weighed carefully.
Generally speaking, enabling WSA effectively disables one of
PR/SM's key tools for attempting to enhance processor
utilization.  Nevertheless, there may be certain environments
where it is desirable to allow individual SCPs to retain
control of a CP, thereby preventing the overhead of PR/SM
processor switching.  This might be the case in highly CP
intensive workloads.  However, the use of dedicated
processors might be more appropriate than utilizing WSA.

In any case, both the CEC Summary and LPAR Activity reports
discussed in section 2.6.3 should be monitored for the
effects of enabling or disabling WSA.