5 ms·

CPUs already adjust their own frequency based on load and temperature (maybe with some help from kernel drivers). It would be nice if the README explained how t

CPUs already adjust their own frequency based on load and temperature (maybe with some help from kernel drivers). It would be nice if the README explained how this tool is different from that.

signa11 5y ago
its outlined in the readme “ … Using tools like TLP will help in this situation with extending battery life (which is something I did for numerous years now), but it also might come with its own set of problems, like losing turbo boost. With that said, I needed a simple tool which would automatically make "cpufreq" related changes, save battery like TLP, but let Linux kernel do most of the heavy lifting. That's how auto-cpufreq was born. … “
marcan_42 5y ago
As far as I can tell, this just switches the cpufreq governor based on whether the laptop is on AC or not. Tools to do this have existed for at least a decade, and integrated into various desktop environments too, so this isn't really novel in any way. It also suggests using "powersave" on battery, based on the old idea that lower CPU frequencies save energy. This is not a given; lower frequencies use less power but also take longer to get the same amount of work done. While the CPU is active, it is consuming static power regardless of frequency. This has gotten even more significant with newer CPUs. What the optimal power saving config is varies from system to system. Ultimately, what you need is to actually benchmark the performance and energy consumption of your system. It's complicated and you also have to deal with things like switching latencies so the cpufreq governor and scheduler can make optimized decisions. Most PCs don't really do a good job of this at all, which is one reason why battery life can be all over the place. Sadly, switching to the "powersave" governor may not, in fact, end up saving you power, depending on what you're doing :)
- pverghese 5y ago
  Lower frequencies consume less power than higher frequencies
  - saagarjha 5y ago
    Yes, that's mentioned in the parent comment. The problem is that at slower frequencies also keep the core powered up for longer, which is often bad for power consumption. For many mobile devices, it's often much better to ramp up the processor to 100% and get the work done quickly, then rush to idle as quickly as possible.
    - FeepingCreature 5y ago
      Has anyone actually measured this on a laptop? My view is that this works for CPU heavy tasks, but not for low-frequency repeated background tasks like browser window drawing, where if you speed up your CPU you just burn energy to get a higher framerate, something you generally don't care about on powersave. So optimally you'd want to clock up for "burst tasks" and keep freq low for "per-frame tasks". But I don't know of a scheduler that does that, or even how it could do it. But if you assume most of your battery loss will be to frame tasks, powersave makes sense.
      - saagarjha 5y ago
        These kinds of things do get used in laptops–the keyword you're looking for is "race to idle", as the processor quickly clocks up and then rushes back to a lower-frequency state or even goes back to sleep. On the mobile side, I do know that Apple did work a couple years back to scale up to maximum clock rate very quickly to handle user-interactive tasks: https://www.anandtech.com/show/13392/the-iphone-xs-xs-max-review-unveiling-the-silicon-secrets/6 https://www.anandtech.com/show/13392/the-iphone-xs-xs-max-re.... Then they'd ramp it back down to save on battery life.
        FeepingCreature 5y ago
        Yeah but are there actually stats that this saves power in daily use?
        saagarjha 5y ago
- TacticalCoder 5y ago
  > It also suggests using "powersave" on battery, based on the old idea that lower CPU frequencies save energy. This is not a given; lower frequencies use less power but also take longer to get the same amount of work done. Hmmmm... I've always heard from chip engineers that power consumption / heat dissipation was not linear compared to clock speed. I'd be very surprised to find a case where a linear amount of work to be done by the CPU would cause more energy consumption overall at lower clock speed then the same task ran on the same CPU in a shorter timeframe, but at a higher clock rate. I do agree that if the task takes longer to run, there's a longer timeframe where more scheduling/switching and whatnots needs to be done, so it's not a totally linear increase in time, but can that really be sufficient to offset the energy consumption when running the CPU at an higher clock speed? There has to be some numbers out there: it's not hard that hard to test. EDIT: as per the other comments, googling "race to idle" and "race to sleep" which may enlighten my curious self
  - BeeOnRope 5y ago
    > Hmmmm... I've always heard from chip engineers that power consumption / heat dissipation was not linear compared to clock speed. I'd be very surprised to find a case where a linear amount of work to be done by the CPU would cause more energy consumption overall at lower clock speed then the same task ran on the same CPU in a shorter timeframe, but at a higher clock rate. Both cases ("slower is better" and "faster is better") are true at different points on the frequency curve, because the frequency (hereafter "F") and power (hereafter "P") relationship has both linear and superlinear terms. To a first order approximation, dynamic power (roughly the power actually used to do work) scales with F^3, but chips also have a significant static power draw as well: power they draw for just being on. Actually, the relationship is more complicated than that since static power may also depend on V, hence indirectly on frequency, but the bottom line is that as you reduce frequency to zero power use doesn't go to zero but asymptotically approaches some non-zero plateau. So imagine a simplified CPU where we care only about the order-0 and order-3 terms, and it happens that power goes like (P in W, F in MHz): P(F) = 2 + 1e-10*F^3 That is, this chip draws at least 2W at any frequency, plus a cubic term in F. This gives a power curve like this [1] where I've also plotted "work vs energy" which is energy efficiency: how much computation you can do for a given energy input (this is simply F/P). As you can see, power use increases in a cubic way to the right, but has an asymptote at 2W on the left. Work/energy has a maximum in the middle: at too low frequencies, you are doing very little work/time but paying the full 2W cost, while at the right the cubic term kills power: P increases with F^3 but work only with F. The most efficient spot is somewhere in the middle. Let's call this point Feff. So it never really makes sense to run your CPU at less than Feff: you are less efficient and it takes longer. It can definite makes sense, however, to run your CPU at more than Feff: you use somewhat more energy but get your work done faster. People don't buy 5 GHz CPUs because they just want their work done efficiently, after all: they want it done fast too. That's what a lot of the discussion misses: it's not a one dimensional problem that can be solved in terms of joules and MHz: it depends on your time preference too. That's why there are so many tunables, such as Intel's EPP (energy performance preference). Different processes may have different ideal F values as well: if you have a periodic job running in the background that takes 1s of (nominal) CPU every minute, you won't care if it is on your CPU for 0.5s or 2s, as it runs on a fixed schedule anyway which is much less than a full CPU: this should run at Feff. OTOH when you are compiling a source file and twiddling your thumbs there might be a big difference between 5 and 20 seconds. This is really only brushing the surface: there are a lot of additional considerations too: e.g., the whole chip may have a power limit, so it may not be possible to go all the way to right on the F graph, especially if multiple cores are running: even if your "performance preference" is way to the right (prefer a higher work/rate regardless of the power cost) you might get the fastest work rate by running more cores at lower F, or even one core at lower F to avoid throttling (because running at F is generally more efficient than running at F-d and F+d in a 50/50 ratio, due to the cubic term, so throttling is inherently inefficient). --- [1] https://gist.githubusercontent.com/travisdowns/1e685007c9719d68888e0280ca771a00/raw/a8WExX.png https://gist.githubusercontent.com/travisdowns/1e685007c9719...
- orangeoxidation 5y ago
  > Sadly, switching to the "powersave" governor may not, in fact, end up saving you power, depending on what you're doing Independently of whether that's true, isn't most consumer hardware (laptops) running "idle" most of the time? Not setting "powersave" might be better if you want to compile something, transcode a video or do some machine learning and turn the machine off as soon as it's done. But that's hardly how a majority uses their devices.
  - deleted 5y ago
    [deleted]
  - marcan_42 5y ago
    > Independently of whether that's true, isn't most consumer hardware (laptops) running "idle" most of the time? Yes, which is why you want to become idle as fast as possible - and that means running at the fastest clock speed when you need to get some work done. Otherwise you are extending the amount of the time system is not idle, which has a fixed power cost beyond the CPU cores themselves. Here's a Lenovo power optimization guide. It's for servers, but the same principles apply to pretty much all modern systems. Spoiler alert: the "ondemand" governor has the highest energy efficiency, significantly higher than "powersave" (page 14). The newer schedutil governor (page 23) is even better, over 34% more power-efficient than powersave. https://lenovopress.com/lp0870.pdf https://lenovopress.com/lp0870.pdf "Powersave" means saving power, it doesn't mean saving energy. Your battery holds a fixed amount of energy, not a fixed amount of power. Energy is what matters.
    - BeeOnRope 5y ago
      > "Powersave" means saving power, it doesn't mean saving energy. Powersave means saving energy (for a given amount of work), despite the name. Any other behavior is a bug. It usually saves energy by running at a lower frequency, where the frequency/watt and work/joule are better (those aren't the same: there is a region where the first is better but the latter is worse, and you want to avoid that!). "Race to sleep" isn't binary: it's not a choice between the slowest supported speed and the fastest. Rather, you can select any supported speed. The fastest speeds (usually north of 3 GHz) will almost never provide the best work/joule, regardless of whether you sleep earlier.
- christkv 5y ago
  I think it’s more that a lower TDP might mean the compile takes 50% longer but I’m still multitasking and I might get 2h more out of a battery charge than when the cpu is spiking to max TDP
riedel 5y ago
Would be great to have more real world benchmarks (in a climate chamber). To me any difference should show empirically. Does anyone have a good suggestion to "debug" performance/battery/thermal issue as a matter of trade-offs. I feel that laptop performance is so subjective and very hard to track. Particularly thermal throttling on mobile Ryzen CPU makes my new Lenovo sometimes close to unusable. Particularly trying to do work while pandemic video conferencing seems to be more difficult than necessary...
- marcan_42 5y ago
  You don't need a climate chamber; you can just measure energy consumption. Most modern battery management ICs should have a joule counter (or equivalent).
Tenoke 5y ago
For what is worth, whatever adjustments are made on Linux (or at least Ubuntu) seem terrible compared to on Windows. If nothing else I can usually hear my fans working overtime even under minimal load on Linux compared to on Windows so I can see why people would look for more advanced tools.

CPUs already adjust their own frequency based on load and temperature (maybe with some help from kernel drivers). It would be nice if the README explained how this tool is different from that.

signa11 5y ago

its outlined in the readme “ … Using tools like TLP will help in this situation with extending battery life (which is something I did for numerous years now), but it also might come with its own set of problems, like losing turbo boost. With that said, I needed a simple tool which would automatically make "cpufreq" related changes, save battery like TLP, but let Linux kernel do most of the heavy lifting. That's how auto-cpufreq was born. … “

marcan_42 5y ago

As far as I can tell, this just switches the cpufreq governor based on whether the laptop is on AC or not. Tools to do this have existed for at least a decade, and integrated into various desktop environments too, so this isn't really novel in any way. It also suggests using "powersave" on battery, based on the old idea that lower CPU frequencies save energy. This is not a given; lower frequencies use less power but also take longer to get the same amount of work done. While the CPU is active, it is consuming static power regardless of frequency. This has gotten even more significant with newer CPUs. What the optimal power saving config is varies from system to system. Ultimately, what you need is to actually benchmark the performance and energy consumption of your system. It's complicated and you also have to deal with things like switching latencies so the cpufreq governor and scheduler can make optimized decisions. Most PCs don't really do a good job of this at all, which is one reason why battery life can be all over the place. Sadly, switching to the "powersave" governor may not, in fact, end up saving you power, depending on what you're doing :)

pverghese 5y ago
Lower frequencies consume less power than higher frequencies
- saagarjha 5y ago
  Yes, that's mentioned in the parent comment. The problem is that at slower frequencies also keep the core powered up for longer, which is often bad for power consumption. For many mobile devices, it's often much better to ramp up the processor to 100% and get the work done quickly, then rush to idle as quickly as possible.
  - FeepingCreature 5y ago
    Has anyone actually measured this on a laptop? My view is that this works for CPU heavy tasks, but not for low-frequency repeated background tasks like browser window drawing, where if you speed up your CPU you just burn energy to get a higher framerate, something you generally don't care about on powersave. So optimally you'd want to clock up for "burst tasks" and keep freq low for "per-frame tasks". But I don't know of a scheduler that does that, or even how it could do it. But if you assume most of your battery loss will be to frame tasks, powersave makes sense.
    - saagarjha 5y ago
      These kinds of things do get used in laptops–the keyword you're looking for is "race to idle", as the processor quickly clocks up and then rushes back to a lower-frequency state or even goes back to sleep. On the mobile side, I do know that Apple did work a couple years back to scale up to maximum clock rate very quickly to handle user-interactive tasks: https://www.anandtech.com/show/13392/the-iphone-xs-xs-max-review-unveiling-the-silicon-secrets/6 https://www.anandtech.com/show/13392/the-iphone-xs-xs-max-re.... Then they'd ramp it back down to save on battery life.
      - FeepingCreature 5y ago
        Yeah but are there actually stats that this saves power in daily use?
        saagarjha 5y ago
TacticalCoder 5y ago
> It also suggests using "powersave" on battery, based on the old idea that lower CPU frequencies save energy. This is not a given; lower frequencies use less power but also take longer to get the same amount of work done. Hmmmm... I've always heard from chip engineers that power consumption / heat dissipation was not linear compared to clock speed. I'd be very surprised to find a case where a linear amount of work to be done by the CPU would cause more energy consumption overall at lower clock speed then the same task ran on the same CPU in a shorter timeframe, but at a higher clock rate. I do agree that if the task takes longer to run, there's a longer timeframe where more scheduling/switching and whatnots needs to be done, so it's not a totally linear increase in time, but can that really be sufficient to offset the energy consumption when running the CPU at an higher clock speed? There has to be some numbers out there: it's not hard that hard to test. EDIT: as per the other comments, googling "race to idle" and "race to sleep" which may enlighten my curious self
- BeeOnRope 5y ago
  > Hmmmm... I've always heard from chip engineers that power consumption / heat dissipation was not linear compared to clock speed. I'd be very surprised to find a case where a linear amount of work to be done by the CPU would cause more energy consumption overall at lower clock speed then the same task ran on the same CPU in a shorter timeframe, but at a higher clock rate. Both cases ("slower is better" and "faster is better") are true at different points on the frequency curve, because the frequency (hereafter "F") and power (hereafter "P") relationship has both linear and superlinear terms. To a first order approximation, dynamic power (roughly the power actually used to do work) scales with F^3, but chips also have a significant static power draw as well: power they draw for just being on. Actually, the relationship is more complicated than that since static power may also depend on V, hence indirectly on frequency, but the bottom line is that as you reduce frequency to zero power use doesn't go to zero but asymptotically approaches some non-zero plateau. So imagine a simplified CPU where we care only about the order-0 and order-3 terms, and it happens that power goes like (P in W, F in MHz): P(F) = 2 + 1e-10*F^3 That is, this chip draws at least 2W at any frequency, plus a cubic term in F. This gives a power curve like this [1] where I've also plotted "work vs energy" which is energy efficiency: how much computation you can do for a given energy input (this is simply F/P). As you can see, power use increases in a cubic way to the right, but has an asymptote at 2W on the left. Work/energy has a maximum in the middle: at too low frequencies, you are doing very little work/time but paying the full 2W cost, while at the right the cubic term kills power: P increases with F^3 but work only with F. The most efficient spot is somewhere in the middle. Let's call this point Feff. So it never really makes sense to run your CPU at less than Feff: you are less efficient and it takes longer. It can definite makes sense, however, to run your CPU at more than Feff: you use somewhat more energy but get your work done faster. People don't buy 5 GHz CPUs because they just want their work done efficiently, after all: they want it done fast too. That's what a lot of the discussion misses: it's not a one dimensional problem that can be solved in terms of joules and MHz: it depends on your time preference too. That's why there are so many tunables, such as Intel's EPP (energy performance preference). Different processes may have different ideal F values as well: if you have a periodic job running in the background that takes 1s of (nominal) CPU every minute, you won't care if it is on your CPU for 0.5s or 2s, as it runs on a fixed schedule anyway which is much less than a full CPU: this should run at Feff. OTOH when you are compiling a source file and twiddling your thumbs there might be a big difference between 5 and 20 seconds. This is really only brushing the surface: there are a lot of additional considerations too: e.g., the whole chip may have a power limit, so it may not be possible to go all the way to right on the F graph, especially if multiple cores are running: even if your "performance preference" is way to the right (prefer a higher work/rate regardless of the power cost) you might get the fastest work rate by running more cores at lower F, or even one core at lower F to avoid throttling (because running at F is generally more efficient than running at F-d and F+d in a 50/50 ratio, due to the cubic term, so throttling is inherently inefficient). --- [1] https://gist.githubusercontent.com/travisdowns/1e685007c9719d68888e0280ca771a00/raw/a8WExX.png https://gist.githubusercontent.com/travisdowns/1e685007c9719...
orangeoxidation 5y ago
> Sadly, switching to the "powersave" governor may not, in fact, end up saving you power, depending on what you're doing Independently of whether that's true, isn't most consumer hardware (laptops) running "idle" most of the time? Not setting "powersave" might be better if you want to compile something, transcode a video or do some machine learning and turn the machine off as soon as it's done. But that's hardly how a majority uses their devices.
- deleted 5y ago
  [deleted]
- marcan_42 5y ago
  > Independently of whether that's true, isn't most consumer hardware (laptops) running "idle" most of the time? Yes, which is why you want to become idle as fast as possible - and that means running at the fastest clock speed when you need to get some work done. Otherwise you are extending the amount of the time system is not idle, which has a fixed power cost beyond the CPU cores themselves. Here's a Lenovo power optimization guide. It's for servers, but the same principles apply to pretty much all modern systems. Spoiler alert: the "ondemand" governor has the highest energy efficiency, significantly higher than "powersave" (page 14). The newer schedutil governor (page 23) is even better, over 34% more power-efficient than powersave. https://lenovopress.com/lp0870.pdf https://lenovopress.com/lp0870.pdf "Powersave" means saving power, it doesn't mean saving energy. Your battery holds a fixed amount of energy, not a fixed amount of power. Energy is what matters.
  - BeeOnRope 5y ago
    > "Powersave" means saving power, it doesn't mean saving energy. Powersave means saving energy (for a given amount of work), despite the name. Any other behavior is a bug. It usually saves energy by running at a lower frequency, where the frequency/watt and work/joule are better (those aren't the same: there is a region where the first is better but the latter is worse, and you want to avoid that!). "Race to sleep" isn't binary: it's not a choice between the slowest supported speed and the fastest. Rather, you can select any supported speed. The fastest speeds (usually north of 3 GHz) will almost never provide the best work/joule, regardless of whether you sleep earlier.
christkv 5y ago
I think it’s more that a lower TDP might mean the compile takes 50% longer but I’m still multitasking and I might get 2h more out of a battery charge than when the cpu is spiking to max TDP

riedel 5y ago

Would be great to have more real world benchmarks (in a climate chamber). To me any difference should show empirically. Does anyone have a good suggestion to "debug" performance/battery/thermal issue as a matter of trade-offs. I feel that laptop performance is so subjective and very hard to track. Particularly thermal throttling on mobile Ryzen CPU makes my new Lenovo sometimes close to unusable. Particularly trying to do work while pandemic video conferencing seems to be more difficult than necessary...

marcan_42 5y ago
You don't need a climate chamber; you can just measure energy consumption. Most modern battery management ICs should have a joule counter (or equivalent).

Tenoke 5y ago

For what is worth, whatever adjustments are made on Linux (or at least Ubuntu) seem terrible compared to on Windows. If nothing else I can usually hear my fans working overtime even under minimal load on Linux compared to on Windows so I can see why people would look for more advanced tools.