CODEWITHSUNDEEP
windowscpu-usage
G S
G S

Reputation: 36858

How is CPU usage computed?

The Windows Task Manager shows CPU usage in percentage. What's the formula behind this? Is it this:

% CPU usage for process A = (Sum of all time slices given to A till now)/ Total time since the machine booted

Or is it something else?

Upvotes: 7

Views: 8016

Answers (4)

cali
cali

Reputation:

See the source code of Task Manager

Upvotes: -2

Brian Rasmussen
Brian Rasmussen

Reputation: 116481

You may also want to check this article as the way CPU cycles are handled with regards to scheduling was changed as part of Vista. I presume that this also applies to Win7.

Upvotes: 1

Mitchel Sellers
Mitchel Sellers

Reputation: 63136

I am not 100% sure what is uses, but I think you are a bit off on the CPU calculation.

I believe they are doing something like.

Process A CPU Usage = (Cycles for A over last X seconds)/(Total cycles for last X seconds)

I believe it is tied to the "update interval" set in task manager.

While doing a bit of research for you though I found this MSDN article that shows a microsoft recommended way of calculating the CPU time of a set of instructions, this might point you a bit towards their calculation as well.

Upvotes: 10

Jon Skeet
Jon Skeet

Reputation: 1503649

No, it's not "since boot time" - it's far more time-sensitive than that.

It's "proportion of time during which a CPU was actively running a thread in that process since the last refresh". (Where the refresh rate is typically about a second.) In task manager I believe it's then divided by the number of CPUs, so the total ends up being 100% (i.e. on a dual core machine, a single-threaded CPU hog will show as 50%). Other similar programs sometimes don't do this, giving a total of 100% * cores.

Upvotes: 8

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