CA MIM/MII and GRS ACCELSYS setting

Document ID : KB000108839
Last Modified Date : 01/08/2018
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Introduction:
CA MIM is the industry standard for sharing DASD, tape, and console resources safely and efficiently in z/OS and z/VM multiple-image environments. The product streamlines and automates many of the procedures involved in sharing resources and enables multiple-system sites to share data center resources across as many as 32 systems.
So, large Sysplex Enviroments can highly benefit of our product. 
Question:
Does the IBM  GRS ACCELSYS setting affect CA MIM performance and how to handle this setting when CA MIM is in place?  
Environment:
Z/OS - CA MIM/MII
Answer:
CA MIM/MII don't  utilize the ACCELSYS parameter, however, depending on its setting, system performance can be affected, as documented in IBM GRS Guides.

The ACCELSYS option in GRSCNFxx is used to specify whether or not the complex is to use ring acceleration and, if so, how many systems must see the RSA message before a system sends the shoulder-tap acknowledgment. 
Without ring acceleration, every system in the ring must see each request for a global resource before it can be granted. While the RSA-message makes a complete cycle around the ring, the task that requested the global resource is suspended to guarantee the integrity of resources; no global resource request is granted until all systems know about it. It does, however, mean that every task that requests a global resource must wait for at least one RSA message cycle. 

Ring acceleration offers an alternative technique, which protects the integrity of resources while potentially providing a significant reduction in global resource request response time (the time a task is suspended while waiting for ring processing). 
 
From IBM Guide : 

ACCELSYS value 

z/OS MVS Planning: Global Resource Serialization 
SA23-1389-00 


The value you specify for ACCELSYS can be between 2 and n, where n is the number of systems in the global resource complex. Specify ACCELSYS(2) to obtain the maximum performance benefits. The default value for ACCELSYS is 99, which turns ACCELSYS off. 

So the better value to use for RSA performance is ACCELSYS(2) , as suggested by IBM documentation. 

Additionally, as a side note, the RESMIL setting can also affect the performance of a Sysplex. 
The default value for RESMIL is 10 milliseconds. The value of 10 tells GRS to keep its RSA buffer on each sysplex system to a minimum of 10 milliseconds on each system. So, if you were for instance running with 12 systems, that means the round-trip time the RSA travels is 120 milliseconds - which is killer slow. This can seriously degrade all performance of all GRS-managed ENQs, including the sysplex-centric PDSE ENQs. 

The recommended setting for RESMIL is 0 or 1, to have a positive effect on  RSA performance. 

To summarize:  both  ACCELSYS and RESMIL, have no direct impact on CA MIM/MII but both can impact overall GRS-managed Enqueue performance.