IBM z13s Technical Manual page 132

Coupling Thin Interrupts
With the introduction of Driver 15F (zEC12 and IBM zEnterprise BC12 (zBC12)), the
z Systems architecture provides a new thin interrupt class called
The capabilities that are provided by hardware, firmware, and software support the
generation of coupling-related "thin interrupts" when the following situations occur:
On the CF side, a CF command or a CF signal (arrival of a CF-to-CF duplexing signal) is
received by a shared-engine CF image, or when the completion of a CF signal that was
previously sent by the CF occurs (completion of a CF-to-CF duplexing signal).
On the z/OS side, a CF signal is received by a shared-engine z/OS image (arrival of a List
Notification signal) or an asynchronous CF operation completes.
The interrupt causes the receiving partition to be dispatched by an LPAR, if it is not already
dispatched. This action allows the request, signal, or request completion to be recognized
and processed in a more timely manner.
After the image is dispatched, existing "poll for work" logic in both CFCC and z/OS can be
used largely as is to locate and process the work. The new interrupt speeds up the
redispatching of the partition.
LPAR presents these Coupling Thin Interrupts to the guest partition, so CFCC and z/OS both
require interrupt handler support that can deal with them. CFCC also changes to give up
control of the processor as soon as all available pending work is exhausted, or when the
LPAR undispatches it off the shared processor, whichever comes first.
Coupling facility combinations
A coupling facility image can have one of the following combinations that are defined in the
image profile:
Dedicated ICFs
Shared ICFs
Dedicated CPs
Shared CPs
Shared ICFs add flexibility. However, running only with shared coupling facility PUs (either
ICFs or CPs) is not a preferable production configuration. A production CF should generally
operate by using dedicated ICFs. With CFCC Level 19 (and later; z13s servers run CFCC
level 21), Coupling Thin Interrupts are available, and dedicated engines continue to be
recommended to obtain the best coupling facility performance.
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Coupling Thin Interrupts
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