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Difference between "ExecutionTime" and "ClockTime"

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Old   August 24, 2009, 06:44
Default Difference between "ExecutionTime" and "ClockTime"
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Daniel WEI (老魏)
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Dear all,

A small question, what is the difference between "ExecutionTime" and "ClockTime"?

I can get these lins from icoFoam.C,
Code:
Info<< "ExecutionTime = " << runTime.elapsedCpuTime() << " s"
<< " ClockTime = " << runTime.elapsedClockTime() << " s"
<< nl << endl;
someone said:
Quote:
elapsedCPUTime() is the elapsed CPU time.
elapsedClockTime() is the elapsed wall clock time.
  1. Could anyone explain this in detail?
  2. If ClockTime means something, why then that we have to care about ExecutionTime?
  3. Is there any meaning concerning the ratio of ClockTime over ExecutionTime? Latency, or what?
  4. How can I change the writing format of these time, eg. show four decimal places in second.?

Thanks in advance for any reply.
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Old   August 24, 2009, 06:56
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Sandy Lee
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Look at hani:
http://www.tfd.chalmers.se/~hani/kur...tyTutorial.pdf
in page 43.
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Old   August 24, 2009, 21:19
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Simon Lapointe
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Hi,

I'm not an expert but here's what I think,

1) There isn't much to add there, the ExecutionTime is the time spent by the processor and the ClockTime is the wall clock time or "real" time if you prefer.

2 & 3) There is an interest because a significative difference between the ExecutionTime and the ClockTime is a sign of a slow calculation. The difference can be explained by the time when a file is being written or when there is communication between processors in a parallel run. Using a computer with a slow interconnect or not enough cells per processor can cause a difference in cpu and clock time.

Hope that helps
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Old   August 24, 2009, 21:58
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Daniel WEI (老魏)
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Quote:
Originally Posted by Simon Lapointe View Post
Hi,

I'm not an expert but here's what I think,

1) There isn't much to add there, the ExecutionTime is the time spent by the processor and the ClockTime is the wall clock time or "real" time if you prefer.

2 & 3) There is an interest because a significative difference between the ExecutionTime and the ClockTime is a sign of a slow calculation. The difference can be explained by the time when a file is being written or when there is communication between processors in a parallel run. Using a computer with a slow interconnect or not enough cells per processor can cause a difference in cpu and clock time.

Hope that helps

I noticed this is your first two posts on the forum, and they are so helpful, thank you.
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Old   September 1, 2014, 10:53
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Quote:
Originally Posted by lakeat View Post
Dear all,

A small question, what is the difference between "ExecutionTime" and "ClockTime"?

I can get these lins from icoFoam.C,
Code:
Info<< "ExecutionTime = " << runTime.elapsedCpuTime() << " s"
<< " ClockTime = " << runTime.elapsedClockTime() << " s"
<< nl << endl;
someone said:

  1. Could anyone explain this in detail?
  2. If ClockTime means something, why then that we have to care about ExecutionTime?
  3. Is there any meaning concerning the ratio of ClockTime over ExecutionTime? Latency, or what?
  4. How can I change the writing format of these time, eg. show four decimal places in second.?

Thanks in advance for any reply.
Hi Daniel,

Have you found the answer to your 4th question? Thanks!

Wei
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