The ICFs can be used by coupling facility LPARs only. ICFs are dedicated or shared. ICFs can be dedicated to a CF LPAR, or shared by multiple CF LPARs that run on the same system. However, having an LPAR with dedicated and shared ICFs at the same time is not possible. 

## Coupling Thin Interrupts 

With the introduction of Driver 15F (zEC12 and zBC12), the IBM z/Architecture provides a new thin interrupt class called  Coupling Thin Interrupts . The capabilities that are provided by hardware, firmware, and software support the generation of coupling-related “thin interrupts” when the following situations occur: 

GLYPH<SM590000> On the coupling facility (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. 

– The completion of a CF signal that was prev iously sent by the CF occurs (completion of a CF-to-CF duplexing signal). 

GLYPH<SM590000> On the z/OS side: 

– CF signal is received by a shared-engine z/OS image (arrival of a List Notification signal). 

– An asynchronous CF operation completes. 

The interrupt causes the receiving partition to be dispatched by an LPAR, if it is not dispatched. This process allows the request, signal, or request completion to be recognized and processed in a more timely manner. 

After the image is dispatched, “poll for work” lo gic in CFCC and z/OS can be used largely as is to locate and process the work. The new interrupt expedites 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 relinquish control of the processor when all available pending work is exhausted, or when the LPAR undispatches it off the shared processor, whichever comes first. 

## CF processor combinations 

A CF image can have one of the following combinations that are defined in the image profile: 

GLYPH<SM590000> Dedicated ICFs 

GLYPH<SM590000> Shared ICFs 

GLYPH<SM590000> Dedicated CPs 

GLYPH<SM590000> Shared CPs 

Shared ICFs add flexibility. However, running only with shared coupling facility PUs (ICFs or CPs) is not a preferable production configuration. It is preferable for a production CF to operate by using dedicated ICFs. With CFCC Level 19 (and later; z14 servers run CFCC level 22), Coupling Thin Interrupts are available, and dedicated engines continue to be recommended to obtain the best coupling facility performance. 

