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Which time to consider when comparing steps

 
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somunote

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Joined: 23 Jan 2010
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PostPosted: Tue Mar 02, 2010 8:28 am    Post subject: Which time to consider when comparing steps
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When comparing two versions of a program to determine which one is faster, I focus on the elapsed time of the step only.
Is it correct?
This time is affected by pauses and stops during the execution and may not reflect the reality.
Should I pay attention to other variables (EXCP, TCB, etc)?
Thanks.
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Robert Sample

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PostPosted: Tue Mar 02, 2010 8:40 am    Post subject:
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The actual CPU time usage is the TCB time. SRB time is the system overhead involved in your task and is usually considered a part of the overall CPU usage. Using elapsed time to measure which program is faster is not a good thing to do -- if the system is heavily loaded during one run and lightly loaded during another run, there can be 2 or 3 orders of magnitude difference in the results (yes, one may run 1000+ times faster than the other) even though they use exactly the same amount of CPU time. EXCP count measurement is important if you're looking at different I/O schemes but otherwise doesn't typically have a major impact (although it can if there's several million EXCP being done for your programs).

While it can be instructive looking at elapsed time, you must ensure yourself that the system load is equivalent for the two runs since even minor differences in system load could have major impacts on elapsed time. If you do not do so, you are wrong to use elapsed time as a comparative measure.
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dick scherrer

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PostPosted: Tue Mar 02, 2010 9:40 am    Post subject:
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Hello and welcome to the forum,

While the goal is often to reduce elapsed time (it is elapsed time that is most visable to managers and users<g>), elapsed time is nearly worthless to measure how one process compares to another. As Robert mentioned, if is quite diffficult to ensure the exact same load for both (or more) tests.

For the most part, i've had the best results in reducing elapsed time by reducing the amount of i/o a process requires and/or improving the way the i/o that is needed is accomplished.

Once upon a time organizations sometimes invested in creating some benchmark jobs to be run in a completely empty mainframe. I have not seen this done for a long time. . .
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somunote

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PostPosted: Tue Mar 02, 2010 10:18 pm    Post subject:
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dick scherrer wrote:

For the most part, i've had the best results in reducing elapsed time by reducing the amount of i/o a process requires and/or improving the way the i/o that is need is accomplished.


Hi,
could you pls let me know how to improve the i/o?
do you mean adjusting the bufno? avoiding vb records?
thank you very much.
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Robert Sample

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PostPosted: Tue Mar 02, 2010 10:25 pm    Post subject:
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The quick and easy fixes are to change block size to half-track blocking and make sure there's enough buffers in the JCL. Long term, looking at the program to determine if there's any way to cut down the reads and writes can help but that's not a fast solution.
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dick scherrer

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PostPosted: Tue Mar 02, 2010 11:02 pm    Post subject:
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Hello,

Quote:
could you pls let me know how to improve the i/o?
Other than what Robert mentioned, there is little "generic" to do. Typically, some amount of study is needed to determine which i/o might be reduced or eliminated. One such example is the case where 10 processes read several hundred million records to produce multiple small reports and output files. Changing these to pass the huge volume only once and create some extract files to use in the subsequent processes reduced total i/o by over 90%.

Quote:
avoiding vb records?
No (and how did this thought originate?), often variable data saves significant amounts of storage space and reduces the number of i/o's required.
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