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Windows 7: CPU usage question

09 Apr 2019   #1
F22 Simpilot

Windows 7 Ultimate x64
 
 
CPU usage question

When I run a program that should be ideally using 100% of the CPU, it doesn't and I'm wondering why. Take for example a hash calculation of a 15 GB file. It took about 20 minutes to complete, yet the CPU was only utilized at about 25%. Why is that? I would think with such a high mathematical commutation the CPU would be at 100%.


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09 Apr 2019   #2
LMiller7

Windows 7 Pro 64 bit
 
 

This is not surprising. Doing a hash calculation is largely a disk bound operation. For a modern CPU such a calculation is quite a simple thing and most of the time is taken waiting for the slower disk drive to read the file. In some situations even access to system memory can't keep up to the CPU and a disk drive is much slower
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11 Apr 2019   #3
F22 Simpilot

Windows 7 Ultimate x64
 
 

I guess that makes sense then.
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12 Apr 2019   #4
Alejandro85

Windows 7 Ultimate x64
 
 

LMiller7 is correct in that disk read speed is MUCH slower than CPU, hence the most likely limiting factor. You can even measure CPU and disk utilization, seeing which one reaches 100% before.

But there is another aspect to consider. Current (and not-so-current) CPUs are all multicore, and software must be explictly designed to make use of multiple cores at once, and the algorithms involved must also be paralelizable, otherwise it'll only use one full core, not the full CPU.
And the vast majority of sofware doesn't bothers with multithreading and the like (it's much harder to get right), so it isn't surprinsing to see that limitation all over the place, even discarding the disk aspect of hashing a file.

A simple infinite loop would collapse 100% of a 1990 era, single core CPU, but the same program today would only use 25 or even 12% of it (for a 4 or 8 core CPU, respectively).

There isn't much you can do to influence that, except to get better software designed to take advantage of today's CPUs.
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12 Apr 2019   #5
iko22

Windows 7 x64, Vista x64, 8.1 smartphone
 
 

1. The CPU utilisation can be throttled in Windows by the Power Plan that you have set.

2. Processor scheduling can be adjusted to give best performance for either (foreground) programs or background services.

3. In Task Manager also, in the Process tab, processes can be set a priority (Realtime, High, Above normal, Normal, Below normal, and Low).

Any one of the three may have a noticeable effect on the task that the computer is trying to perform.

If you really want to test the CPU maxed at 100%, then run Prime95 on all cores.
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