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Reasons for Elapsed time being greater than CPU time

 
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mubs55

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Joined: 19 Apr 2006
Posts: 12

PostPosted: Wed Dec 27, 2006 3:37 pm    Post subject: Reasons for Elapsed time being greater than CPU time
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Hi,

Can anyone let me know what might be the reason behind the Elapsed time being greater than CPU time.

One reason i know could be due to database problem. I would like to if it could result from other problems.

Regards,
Mubeena
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UmeySan

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Joined: 22 Aug 2006
Posts: 743
Location: Germany

PostPosted: Wed Dec 27, 2006 5:15 pm    Post subject:
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Hi !

Normaly, in the step accounting data, the elapsed (wall clock) time for a job step represents the period between the beginning of the actual execution of the job step and the completion of all termination processing for it. Elapsed time is influenced by all I/O-activities. In BatchJobs using
large seq. datasets or VSAM you could reduce elapsed time by adding BUFFERS to the Dataset. Please have a look at BUFNO & BUFFND/BUFFNI statements in JCL.

Regards, UmeySan
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DavidatK

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Joined: 22 Nov 2005
Posts: 700
Location: Troy, Michigan USA

PostPosted: Wed Dec 27, 2006 9:09 pm    Post subject: Re: Reasons for Elapsed time being greater than CPU time
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Mubeena,

Unless you are running dedicated on the processor, which is seldom if ever, the elapsed time will always be greater than the CPU time.

As ?UmeySan? said, elapsed time is from the start of your job/program to the end (Clock time)

CPU time, on the other hand, is allocated and accumulated by your job/program only when it is actually executing instructions. So, if there are 5 other jobs/programs vying for time on the processor, the CPU time must be shared between the other jobs/programs.

Depending on the priority you job is running in you may get more, or less of the shared CPU time than the other jobs running.
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sundar28

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Joined: 30 Jul 2005
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Location: Mysore

PostPosted: Thu Dec 28, 2006 12:02 am    Post subject:
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Hi Mubeena,

Elapsed Time = Time taken by the JES to submit and re-submit the job till its completion
+ CPU time
+ Time taken on waiting for the allotment of resources*
+ Time taken for response of the resources
( + User response time for interactive processes)


*resources - Files, Memory and Printers (and CICS or DB2 or MVS system call services like DATE, TIME etc)

Elapsed time can never be equal to the CPU time for an application program.

Please correct me if wrong.
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sundar28

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Joined: 30 Jul 2005
Posts: 12
Location: Mysore

PostPosted: Thu Dec 28, 2006 12:12 am    Post subject: Re: Reasons for Elapsed time being greater than CPU time
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mubs55 wrote:


One reason i know could be due to database problem.



icon_razz.gif Yes.

On tuning the database to optimize for the minimal response time, the time difference can be minimized.
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mmwife

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Joined: 30 May 2003
Posts: 1592

PostPosted: Thu Dec 28, 2006 4:35 am    Post subject:
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You forgot my favorite:

1) Your pgm calls for a tape to be mounted.
2) The tape pool operator decides to go to lunch. icon_evil.gif
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