CPU Usage reporting graph mislabeled?

[ShimadaRiku]

i3-4130T 2 physical core with hyperthreading this is my graph with 1 user transferring file with samba. Since samba is single threaded it should max out at least 1 physical core which shows in the system load avg of ~1.05.

However, in the CPU Usage graph, Y axis is labeled as %CPU. Freenas Doc links collectd Y axis as jiffies not %CPU. Assuming, it actually is %cpu, it should show about 50% not 25%.

Am I mistaken or is it a bug?

 

[Bidule0hm]

Everything looks ok. You have 4 cores and you max out one core --> ~25 % and load is ~1 as it should be.

 

[DrKK]

That graph will take into account four "cores" in the case...whoops, I see @biduleohm probably just answered this as I was typing.

 

[ShimadaRiku]

Everything looks ok. You have 4 cores and you max out one core --> ~25 % and load is ~1 as it should be.

I have 2 physical cores with hyperthreading for a total of 4 logical cores. So because I am only utilizing 25% of my CPU, I should disable hyperthreading to take advantage of my full physical core?

 

[survive]

Hi ShimdaRiku,

I would think the OS might not have any idea that 2 of your cores are HT cores & it's just dividing by 4 when it ideally would divide by 2.

-Will

 

[Bidule0hm]

As survive said a CPU with 2 cores and HT is like a CPU with 4 cores to the OS.

I never thought about disabling HT to get half the number of cores but double the perfs per core. I wonder if it works like that or if you just half the number of cores without any gain in perf per core.

 

[ShimadaRiku]

I never thought about disabling HT to get half the number of cores but double the perfs per core. I wonder if it works like that or if you just half the number of cores without any gain in perf per core.

I think it is the latter; half number of cores without any gain assuming single thread perfs.

OS see's 4 cores, but doesn't know 2 of them are virtual ones. Behind the scenes each physical core is doing the work of two logical at full speed.

So it means I don't have a real way of viewing actual cpu usage?

 

[Ericloewe]

Disabling hyperthreading probably won't help, except in highly specific scenarios.

The thread switching penalty is smaller than the penalty for having to wait for data from system memory (or other pipeline stalls).

 

[jgreco]

Since samba is single threaded it should max out at least 1 physical core which shows in the system load avg of ~1.05.

However, in the CPU Usage graph, Y axis is labeled as %CPU.

Be careful about reading too much into the system load average. It's just a sampled estimation of the number of processes in a runnable state.