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- You may have run it many times before you do the test. This may mean that parts of the program or the other objects it accesses are still resident in the computer's main memory, thus reducing the time it takes for the program to be accessed. For example, take the following simple program which does no initial processing, just puts up a screen panel:
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REQUEST FIELDS(#PRODNO)
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You may test this program and find that the panel appears in 1 second on average. However, you may find that if you sign onto the computer at the start of the day and then attempt to access the program initially it takes 4 seconds.
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How can this be, when you know that the program is doing exactly the same amount of work every time it is accessed (assuming that it is not ana *HEAVYUSAGE program) ?
The reason is simple. When you access and re-access the program very quickly, the program and the objects that it accesses tend to remain in the computer's memory. However, if it is the very first access they are not in memory. Likewise, if you take a long break between interactions, it is likely that the objects are "rolled out" of memory back onto disk. If you wish to re-access them, they must be "rolled in" to memory again, causing you to wait while this is done.
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- You may be on a totally different type of machine or using a very different type of environment. If you do your test on an IBM i 270 at 9.00pm at night, when everybody else has gone home, and the actual target machine is a very busy 170, then the results you obtain will be totally misleading.
- The system may not be busy enough. When the IBM i begins to become loaded up with work, its performance characteristics change.
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