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Is there any way to tell the kernel that I don't need the full CPU power?

Basically, I want to do some calculation while waiting for another process. But I don't need the full CPU power for that. As the CPU load during the computation is still 100%, the frequency is high. I want to tell the kernel that I am satisfied with a lower CPU frequency in order to save energy.

Instead of calculating using the full frequency and then suspend to wait for the other process, I want to try calculating with lower frequency so that the CPU is not in a lower C state when the other process has finished and the frequency can scale back again.

Lundin
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ttm02
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5 Answers5

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This doesn't make any sense on a multi-process system, particularly not in Linux. The CPU frequency is a very basic parameter, which affects everything running on the computer - including other processes and the OS itself.

If your program would fiddle with the CPU frequency, it would not only dictate the priority of itself, but also the priority of everything in the computer, including the OS. This isn't possible to do on a desktop system, simply because it doesn't make any sense to have a single application process dictate things that not even the OS dares to meddle with.

If saving power is a priority, you should probably look for completely different alternatives than some desktop Linux solution. PC computers only care about 1) speed, 2) speed and also 3) speed.

The kind of things you ask for are common in real-time embedded systems, where CPUs have a "sleep mode", from which it can wake up to execute something, then go back to sleep. It would usually also be possible for such systems to fiddle with the internal PLL to adjust their own frequency, but such solutions are rare. The industry standard way of doing things is to perform all calculations at max speed, then revert to power-saving sleep mode.

Lundin
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  • thx, i was just wondering why I cant tell the OS "Hey, I don't need the full power, if you want to scale down: go ahead!" – ttm02 Sep 19 '16 at 12:03
  • @ttm02 well, you can alter the scheduling priority of a program, which in a demanding system may result in less CPU time for said program. https://linux.die.net/man/2/setpriority – yeyo Sep 19 '16 at 14:03
  • @lundin The mechanisms you say that are rare in your last paragraph are called "Intel SpeedStep" and "AMD Power now"" and are built into practically any modern x86-based CPU. What you say about Desktop Systems is not necessarily true for Laptops and even power-hungry desktop systems benefit from being able to step down (and up) in CPU frequency – tofro Sep 19 '16 at 15:40
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In case of multiple cores - there is a way to specify a certain cpu core to an interrupt. In this way you can save a certain CPU to a certain process: to find the irq number of the task use:

cat /proc/interrupts

look for your irq number. lets say that the irp number is 99, so in order to set core #2 to handle this irq, do:

echo 2 > /proc/irq/99/smp_affinity

in this way you can save a certain core to handle your special process.

stzahi
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  • thx, but thats not what I am looking for. I look for a solution to scale down the reserved CPU without suspend it. – ttm02 Sep 19 '16 at 09:22
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You can actually use nice() to tell the kernel that your process can live with a lower-than-normal scheduling priority. This effectively reduces the amount of time slices your process will get to use the CPU (typically in favor of other processes running at the same time).

On some more modern systems, if this will reduce the overall CPU load significantly, the CPU might eventually even decide to run on lower frequency. But you typically don't have direct influence on that decision.

Note: Depending on the system, you might be having problems to restore the original nice value (i.e. to scale up on priority again) without running with appropriate permissions.

In case your application is I/O bound and is not doing stupid things wasting CPU cycles like busy-waiting, it shouldn't be necessary to revert to reducing your nice value - Modern CPUs and operating systems should be able to detect themselves when the system is mainly idling around and step down autonomously.

tofro
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  • What kind of CPU that runs Linux will do this? Assuming you mean this stuff: https://en.wikipedia.org/wiki/Dynamic_frequency_scaling – Lundin Sep 19 '16 at 14:46
  • @Lundin I was rather referring to this and similar tricks in the Centrino: https://en.wikipedia.org/wiki/Intel_Turbo_Boost, or AMD PowerNow! and Intel's *SpeedStep* – tofro Sep 19 '16 at 15:17
  • To answer your question in detail: All modern Intel CPUs past the Centrino support some sort of EIST, and thus can step down in CPU frequency "on the fly", and EIST support is built into Linux kernels from 2.6 – tofro Sep 19 '16 at 15:26
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modify scalling_goverance accordingly. The "scaling_governor" feature enables setting a static frequency to the CPU.

Frequency value must be between scaling_min_freq and scaling_max_freq.

When CPU frequency governor is set to "powersave" mode, CPU is set to the lowest static frequency (within the borders of scaling_min_freq and scaling_max_freq).

Check in below path on the target cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_governors and select the required scalling governanceby writing to echo "powersaving"/sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

charan
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To tune the performance, few of the files can be updated which will make changes in CPU and the frequency and also the scheduler policies.

Based on the performance analysis, and the load balancing. Modifications can be adopted. Check /sys/devices/system/cpu/

Example:- root:/sys/devices/system/cpu/cpu0/cpufreq# cat scaling_available_governo rs interactive performance root@:/sys/devices/system/cpu/cpu0/cpufreq# root@:/sys/devices/system/cpu/cpu0/cpufreq# cat scaling_governor interactive root@:/sys/devices/system/cpu/cpu0/cpufreq# cat scaling_available_frequen cies 400000 800000 998400 1094400 1190400 1248000 1305600

##:/sys/devices/system/cpu/cpu0/cpufreq# cat scaling_available_frequen cies 400000 800000 998400 1094400 1190400 1248000 1305600

charan
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