Difference between CPU time and Unused CPU time.

Difference between CPU time and Unused CPU time.

Abhay Hegde's picture
Abhay Hegde
Sat, 11/07/2009 - 19:27

Hello everyone. I am trying to implement the Pi program in Intel Parllel studio and these two terms namely CPU time and unused CPU time has been bothering me.So thought of asking with the deveopers itself :)

As per the Intel Amplifier manual ,
CPU time - CPU Time is the sum of CPU time for all threads --ok fine i get it.!
Unused CPU time - Unused CPU Time is the total time for each core when it was either waiting or not utilized by the application... < little confusion arises over this one !

I have attached a screen shot of my program.
According to the screen shot CPU time= 13.063 seconds.
CPU unused time=13.23 seconds << My confusion becomes quad pumped over here !!

Why is the CPU unused time more than the CPU time ?
Could some one in simple terms explain the difference between these two terms.

Please have a look at the screenshot.

Thanks in advance.
Abhay

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Peter Wang (Intel)'s picture
Peter Wang (Intel)
Mon, 11/09/2009 - 00:53

(Sorry that I can't see you screen-shot, I don't know why)

Hi Abhay,

Itcould beright result (normal case)- "Unused CPU time" is more than "CPU time".

I can give a simpleexample to explain:
I assumeyou have 4 cores system, and computing Pi with two threads. Thus, CPU0 and CPU1 are busy - itspent10s CPU time respectively; CPU2 and CPU3 arenot used; Elapsed time is 12s (I assumed)
So "CPU time" is 10s+10s = 20s, "Unused CPU time"are12sx2 (for CPU2+CPU3) + 2sx2 (for CPU0+CPU1, maybe free since IO task ran) = 28s.

Actually the real situation could be complicated, "CPU time" is to accumulate all CPU time spent from each core for the application, and"Unused CPU time" is to calculate all of the CPU time of cores which did not spend for this application. This may happen, for example, not used, blocked, by other application (e.g. Parallel Amplifier's overhead), etc.

Hope it helps.

Regards, Peter

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vladimir-tsymbal (Intel)'s picture
vladimir-tsymbal (Intel)
Mon, 11/09/2009 - 01:59
Quoting - Abhay Hello everyone. I am trying to implement the Pi program in Intel Parllel studio and these two terms namely CPU time and unused CPU time has been bothering me.So thought of asking with the deveopers itself :)

As per the Intel Amplifier manual ,
CPU time - CPU Time is the sum of CPU time for all threads --ok fine i get it.!
Unused CPU time - Unused CPU Time is the total time for each core when it was either waiting or not utilized by the application... < little confusion arises over this one !

I have attached a screen shot of my program.
According to the screen shot CPU time= 13.063 seconds.
CPU unused time=13.23 seconds << My confusion becomes quad pumped over here !!

Why is the CPU unused time more than the CPU time ?
Could some one in simple terms explain the difference between these two terms.

Please have a look at the screenshot.

Thanks in advance.
Abhay

According to the screen shot, you were running single threaded application on the 2 core machine.

Elapsed time 13.149 time of executing the process

CPU Time 13.063 time the app was running on one core (can be less then Elapsed time due to blocking on I/O)

Unused CPU Time 13.236 the sum of time of unused core (equal to elapsed, i.e. 13.149) plus blocked time on the used core (13.149 13.063)

-Vladimir
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