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When I run ‘top –d 0.5’ on my machine and I press ‘1’ to enable the SMP View, I notice that all of the CPU’s keep toggling between 100%idle and 0% idle.
The odd part is that when a CPU goes to 0.0% idle, I would expect to see one or more of the other columns peek, but they are not.
Distribution: Cinnamon Mint 17.3 and 18 at present.
Welcome to Linux Questions!
It's probably something to do with how the kernel "shares" your one running task over eight processors.
In the bad old days you used to time-slice access to the single processor to allow the multitude of tasks to run (Er... that'll be multi tasking) I'm not sure how it works in reverse though?
top (re-)reads /proc "files" a lot. This causes skewing in the numbers it generates. The faster is does this, the worse the effect
While this statement makes sense, I cant see this being the root cause of what I am seeing. I have run on plenty of machines that work perfectly fine with refresh rates of 0.5 seconds. And this is a pretty new machine with pretty beefy processors. As a matter of fact, on this particular system, the problem doesn't go away until my refresh rate is set to 2.0 seconds. It is just unbearable to watch at this pace.
Originally Posted by syg00
why are you using fraction(s) of a second ?.
Only because I wanted a closer to real-time picture of what is going on in my system. I want to watch my applications run and see how they react while processing different pieces of data (in terms of CPU and memory usage).
Are you running natively, or as a guest ?.
I run a i7 (4 core, hiperthreaded), so it also looks as 8 processors. I see similar at -d 0.5 - but the all zero line only appears on one cpu.
Looking at /proc/stat for the cpu data, it appears to be valid (for all cpus) even for small iterations. Guess that might mean "top" can't keep up, and just tosses the data out when it gets the next timer pop. All guesswork, but matches the evidence.
For "good" detail data, have a look at collectl in daemon mode.