4 ms·

13% cpu usage at the lowest c-state is a also very different than 13% at an elevated c-states. I've recently spent a lot of time analyzing c-states/p-states and

by mmarks 10y ago

13% cpu usage at the lowest c-state is a also very different than 13% at an elevated c-states. I've recently spent a lot of time analyzing c-states/p-states and the power mgmt modes of the GPU. After learning more about the complexity behind the clocks, bus speeds, etc. underlying each state, whenever I hear someone quote a utilization number of a minor workload, I want to know at what power state. Not to take away but the author's point, just an aside that utilization numbers can be a lot more complicated when there are dozens of energy states and the utilization might be utilization at a particular state rather than utilization at maximum power

aarongolliver 10y ago
You mean p-states in your first sentence, right? Anything but c0 represents different levels of 'retiring no instructions' (totally idle). The rest of your comment seems accurate though.
- mmarks 10y ago
  You're correct that I just meant a lower power state. The author mentioned blinking cursor, so it reminded me of graphics issues. A more efficient CPU state has the possibility of slowing an app due to CPU-GPU sync points. A blocking CPU in an energy efficient state can reawaken slower from GPU done notifications, so FPS is lower. So both c-state and p-states can affect performance. General point was just utilization may not be utilization at max power. I've worked on problems where utilization was 15% at lower power and it was a problem. But to compare different workloads, it'd be < 1% at max power.
rasz 10y ago
even the best case scenario is 13% of ~900MHz, 100MHz to blink 20 pixels.
gpderetta 10y ago
That's actually even worse: it will prevent the cpu to enter deep sleep states and save more power. A while ago I was trying to minimize the power usage of my laptop (yeah, slow day) to maximize my battery time. Armed with powertop, I removed any undesired process until I was left with an otherwise idle emacs (less than 1<% cpu) as the last major source of wakeups. Sure enough, disabling the blinking cursor brought that down to nothing, allowing the cpu to stay into deep sleep state much longer.

by mmarks 10y ago

aarongolliver 10y ago

You mean p-states in your first sentence, right? Anything but c0 represents different levels of 'retiring no instructions' (totally idle). The rest of your comment seems accurate though.

mmarks 10y ago
You're correct that I just meant a lower power state. The author mentioned blinking cursor, so it reminded me of graphics issues. A more efficient CPU state has the possibility of slowing an app due to CPU-GPU sync points. A blocking CPU in an energy efficient state can reawaken slower from GPU done notifications, so FPS is lower. So both c-state and p-states can affect performance. General point was just utilization may not be utilization at max power. I've worked on problems where utilization was 15% at lower power and it was a problem. But to compare different workloads, it'd be < 1% at max power.

rasz 10y ago

even the best case scenario is 13% of ~900MHz, 100MHz to blink 20 pixels.

gpderetta 10y ago

That's actually even worse: it will prevent the cpu to enter deep sleep states and save more power. A while ago I was trying to minimize the power usage of my laptop (yeah, slow day) to maximize my battery time. Armed with powertop, I removed any undesired process until I was left with an otherwise idle emacs (less than 1<% cpu) as the last major source of wakeups. Sure enough, disabling the blinking cursor brought that down to nothing, allowing the cpu to stay into deep sleep state much longer.