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CPU usage percentage - a definition?

post #1 of 11
5/22/10 at 3:21am
Thread Starter 
We've all seen the 'CPU load', be it in an overclocking software context, or even just simply Windows task manager or Linux's top. But what is it actually a measure of? CPUs are digital devices (a switch is on or off), but the scales of CPU load are analog (run from 0-100 in steps). My theories are:
  • How many of the registers are full, and how full
  • How many 'lost cycles' there are, based on latencies, probably as a moving average
  • How full the L1/L2/L3 cache is
  • A measure of percentage of IPS, as a defined maximum for a given CPU

With memory, the percentage is much easy to determine, as it's a percentage measure of capacity, but where CPU is a measure of work done/load, it could be calculated in many ways.

The list is just my theories, I'm not CPU engineer! Does anyone actually know the answer here?
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post #2 of 11
5/22/10 at 5:17am
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CPU load or usage is basically the percentage of time the CPU is actually executing instructions vs. in a wait state.
See:
http://en.wikipedia.org/wiki/CPU_usage
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post #3 of 11
5/23/10 at 6:18am
Thread Starter 
That makes it sound like the second option, but there is the ambiguous statement "percentage of the CPU capacity". In order to have a percentage, the maximum value needs to be defined (and time has no maximum value, it is infinite)
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post #4 of 11
5/23/10 at 7:12am
Think of it as a number of cycles in a given amount of time. OC the cpu and you get more cycles but the same amount of time so more work gets done. And they are both defined as the same % of load.

Another way to think of it is rpm's.

It would be nice to get a more detailed listing of cpu performance, but at the same time getting that information puts a load on the cpu thus lowering the performance.
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post #5 of 11
5/23/10 at 7:55am
Thread Starter 
Hmm, that kind of suggests that CPU load of a given application, is down to optimisation of code and efficiency of the compiler (discounting user interaction) - a well written low-level app with a compiler in top form, would produce higher CPU loading, regardless of the work done (in that, the CPU would be idle for less cycles)
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post #6 of 11
5/23/10 at 10:46am
CPU load (as far as task manager and most things report) is purely how many cycles are being used. It says next to nothing about how much work is actually being done, or how many instructions are being retired in a given time.

Try CPUID's Perfmonitor if you want a more useful measure of load: http://www.cpuid.com/perfmonitor.php
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post #7 of 11
5/23/10 at 4:29pm
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Quote:
Originally Posted by chemicalfan View Post
That makes it sound like the second option, but there is the ambiguous statement "percentage of the CPU capacity". In order to have a percentage, the maximum value needs to be defined (and time has no maximum value, it is infinite)
Time may be infinite, but our sampling of time is not. For example: if, during the last second we noted every microsec whether the CPU was executing an instruction or in a wait then that execution count divided by 1 million becomes the ratio of usage and times 100 becomes the usage percentage.

A crude measure sure - but useful on a macro scale.

Of course by invoking software to measure CPU usage we rapidly get into the observer effect but the results still provide a useful measure.
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post #8 of 11
5/24/10 at 1:39am
Thread Starter 
That makes sense, it's a bit of a strange way to measure it though. Thinking in a kind-of pseudocode way, 1+1 in a constant loop would show higher CPU usage than X=X+1 (as it would eventually run out of space in the registers for X, so would have to shuffle it out into cache, causing latency, and even more so than a complex FP operation which may have multiple data dependecies on other parts of the pipeline
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post #9 of 11
5/25/10 at 12:28am
If you mean "CPU usage percentage" as shown by the task manager in Windows then it's not really showing a CPU load but a task load. For instance we may have a task that is showing 50% CPU usage in the task manager but only utilizing the CPU 2% percent of the time.

My own definition of CPU load would be the time the CPU spends in c-state C0, same as Almogavar really, but perhaps this is a bit too simplistic for what your looking for in your own definition since you seem to be looking for a power load rather than a time/usage load.
Edited by ucode - 5/25/10 at 12:32am
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post #10 of 11
5/25/10 at 1:13am
Thread Starter 
No, you're right - I was referring to Windows/Linux built-in measure of CPU % load. This seems to be the same figure as reported by OCCT, Speedfan, and the other tools we use.

It's a crappy measure if you consider SIMD instruction, as a CPU executing a simple integer operation every cycle, will have a higher load % than a CPU executing a SIMD instruction every 2/3 cycles, despite the SIMD instruction having higher throughput (by definition of SIMD)
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