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If all jobs in a system are coded with REGION=4M

 
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Prasoon Baral

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PostPosted: Thu Sep 18, 2008 7:17 pm    Post subject: If all jobs in a system are coded with REGION=4M
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What problem can occur if all jobs in a system are coded with REGION=4M irrespective of whether the jobs actually required that region or not? It may happen most of the jobs require less storage than 4M.
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Bill Dennis

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PostPosted: Thu Sep 18, 2008 8:05 pm    Post subject:
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Coding an overly large REGION has no bad effects. No storage is obtained if it's never asked for.
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diwa_thilak

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PostPosted: Thu Sep 18, 2008 8:44 pm    Post subject:
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Hi,

I found the below notes in the internet, thought it might be useful sharing in this forum.

The REGION parameter specifies the maximum size of the execution region for any job step in this job. The value specified is the amount in K (1024 byte units) or M (1024*1024 byte units) which will be the upper limit on the amount of virtual storage that can be obtained via GETMAIN requests by any step in the job.

Quote:
"The region size allowed to users can affect performance of the entire system. When there is no limit on region size and the system uses its default values, users might obtain so much space within a region (by repeated requests for small amounts of storage or a single request for a large amount) that no space would remain in the private area for system use. This situation is likely to occur when a program issues a request for storage and specifies a variable length with such a large maximum value that most or all of the space remaining in the private area is allocated to the request. If this program actively uses this large amount of space (to write tables, for example), it can affect central storage (also known as real storage) and thus impact performance."
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Bill Dennis

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PostPosted: Thu Sep 18, 2008 11:58 pm    Post subject:
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The explanation found by Diwakar shows why it is bad form to routinely code REGION=0M (unlimited), as some folks do.
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Prasoon Baral

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PostPosted: Fri Sep 19, 2008 10:51 am    Post subject: Reply to: If all jobs in a system are coded with REGION=4M
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What I understand is that coding REGION=0M has a detrimental effect on system performance. But if REGION=4M is coded in all jobs then the job does not block the storage until it request for any storage, thus there is no effect on performance.

What will happen if REGION is not coded in the job? Can you suggest the best practice of coding REGION in JCL?
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dick scherrer

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PostPosted: Fri Sep 19, 2008 10:55 am    Post subject:
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Hello,

Your best source for that information (it is rather site-specific) would be your system support (or similar) people. They can tell you what class(es) and region specifications are proper for your jobs.
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Bill Dennis

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PostPosted: Sat Sep 20, 2008 1:25 am    Post subject: Re: Reply to: If all jobs in a system are coded with REGION=
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Prasoon Baral wrote:
What will happen if REGION is not coded in the job?
If you don't have REGION coded, you will get a default based on the JES2 JOBCLASS definition or other system exit. If not specified, the system default is 1M.
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Anuj Dhawan

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PostPosted: Sat Sep 20, 2008 11:05 am    Post subject:
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Hi,

Just for my .02 cents..
Prasoon Baral wrote:
What I understand is that coding REGION=0M has a detrimental effect on system performance.
A value of 0K or 0M will allow the program to request the largest available region size, however this specification should be used with caution. From the OS/390 Initialization and Tuning Guide,

"The region size allowed to users can affect performance of the entire system. When there is no limit on region size and the system uses its default values, users might obtain so much space within a region (by repeated requests for small amounts of storage or a single request for a large amount) that no space would remain in the private area for system use. This situation is likely to occur when a program issues a request for storage and specifies a variable length with such a large maximum value that most or all of the space remaining in the private area is allocated to the request. If this program actively uses this large amount of space (to write tables, for example), it can affect central storage (also known as real storage) and thus impact performance."
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Anuj Dhawan

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PostPosted: Sat Sep 20, 2008 11:12 am    Post subject: Re: Reply to: If all jobs in a system are coded with REGION=
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Hi Bill,
Bill Dennis wrote:
If not specified, the system default is 1M.
Is it a universal concept ? I don't find any reference/s, please assist.
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Bill Dennis

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PostPosted: Tue Sep 23, 2008 1:21 am    Post subject:
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One place for a default REGION to be set is based on the JES2 JOBCLASS you submit into. Check the JES2 Init and Tuning Reference manual for the defaults for JOBCLASS.

If you can issue commands, try $DJOBCLASS(n) ,where n is your job class, to see your site values.
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