Coupling Thin Interrupts; Dynamic Cf Dispatching - IBM z13s Technical Manual

The CFCC is implemented by using the active wait technique. This technique means that the
CFCC is always running (processing or searching for service) and never enters a wait state. It
also means that the CF Control Code uses all the processor capacity (cycles) that are
available for the coupling facility LPAR. If the LPAR running the CFCC has only dedicated
processors (CPs or ICFs), using all processor capacity (cycles) is not a problem. However,
this configuration can be an issue if the LPAR that is running the CFCC also has shared
processors. Therefore, enable dynamic dispatching on the CF LPAR. With CFCC Level 19
and Coupling Thin Interrupts, shared-processor CF can provide more consistent CF service
time and acceptable usage in a broader range of configurations. For more information, see
3.9.3, "Dynamic CF dispatching" on page 133.
Performance consideration: Dedicated processor CF still provides the best CF image
performance for production environments.
CF structure sizing changes are expected when going from CFCC Level 17 (or earlier) to
CFCC Level 20. Review the CF structure size by using the CFSizer tool, which is available at
this website:
http://www.ibm.com/systems/z/cfsizer/
For latest recommended levels, see the current exception letter on the Resource Link at the
following website:
https://www.ibm.com/servers/resourcelink/lib03020.nsf/pages/exceptionLetters

3.9.2 Coupling Thin Interrupts

CFCC Level 19 introduced Coupling Thin Interrupts to improve performance in environments
that share Coupling Facility engines. Although dedicated engines are preferable to obtain the
best Coupling Facility performance, Coupling Thin Interrupts can help facilitate the use of a
shared pool of engines, helping to lower hardware acquisition costs.
The interrupt causes a shared logical processor coupling facility partition that has not
already been dispatched to be dispatched by PR/SM, allowing the request or signal to be
processed in a more timely manner. The coupling facility gives up control when work is
exhausted or when PR/SM takes the physical processor away from the logical processor.
The use of Coupling Thin Interrupts is controlled by the new DYNDISP specification.
You can experience CF response time improvements or more consistent CF response time
when using Coupling Facilities with shared engines. This configuration can also allow more
environments with multiple CF images to coexist in a server, and share CF engines with
reasonable performance. The response time for asynchronous CF requests can also be
improved as a result of using Coupling Thin Interrupts on the z/OS host system, regardless of
whether the CF is using shared or dedicated engines.

3.9.3 Dynamic CF dispatching

Dynamic CF dispatching provides the following functions on a coupling facility:
1. If there is no work to do, CF enters a wait state (by time).
2. After an elapsed time, CF wakes up to see whether there is any new work to do (that is,
there are requests in the CF Receiver buffer).
Chapter 3. Central processor complex system design
133