6 ms·

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,

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
        I am not a hardware engineer, so I don't have actual stats on me, unfortunately. But the reasoning behind it makes sense: https://mjg59.livejournal.com/88608.html https://mjg59.livejournal.com/88608.html. Given that everyone is doing it, I am sure that there is data that shows that; I just don't have it :P
        hnaccount_rng 5y ago
        Well you can take the battery size (and screen times you get from those). iPhone batteries are tiny compared to android. Like seriously tiny that my 10Wh Powerbank can charge it 5 times with some to spare (I only really get 4 due to losses but still). So yes comparing iPhones to anything else highly suggests that the tuning Apple did pays off. That being said: I’m not aware of a useful comparison. I’m not even sure how you would design something like that, ie what would you like to keep constant?
  - slavik81 5y ago
    This strategy is typically known as "race-to-sleep" or "race-to-idle". Just figured it was worth mentioning, as there's lots of additional information that can be found if you know what to search for.
  - tjoff 5y ago
    With the modern web there is no "rush back to idle". Browsers will burn your CPU until you turn off your browser, which in most cases is only when you turn your computer off.
    - saagarjha 5y ago
      Assuming you are using a reasonable browser with a reasonable set of websites open, a browser should not really burn unbounded CPU in the background. Tabs that do are generally throttled, allowing the scheduler to burst their work and then have the processor go back to sleep.
      - tjoff 5y ago
        Are we assuming no tracking and ads as well? But yeah I agree, it should not. But it does.
  - Neil44 5y ago
    I see what you’re saying but I believe there’s an issue at the silicon level where the gates literally use more power to do the same work at higher frequencies.
    - jhgb 5y ago
      It's a well-known feature of CMOS gates that they consume almost all of their power at the moments of switching. AFAIK only recently has static leakage become substantial, necessitating power gating.
      - garaetjjte 5y ago
        Higher frequencies require higher voltage though, and this will increase power consumption.
  - rtomanek 5y ago
    To add context: this is especially true for tasks that are likely to end in the device sleeping again. Two examples: user checking phone after hearing a notification sound or opening the laptop lid to check train departure time in the browser (then closing it again). The faster the CPU deals with the task once the device is fully awake the shorter the time other power hungry parts of the system (like screen) need to be awake. That's why slow phones, where the user needs to wait for their browser to finish loading the page will never be battery effective.
- deleted 5y ago
  [deleted]
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

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
        I am not a hardware engineer, so I don't have actual stats on me, unfortunately. But the reasoning behind it makes sense: https://mjg59.livejournal.com/88608.html https://mjg59.livejournal.com/88608.html. Given that everyone is doing it, I am sure that there is data that shows that; I just don't have it :P
      - hnaccount_rng 5y ago
        Well you can take the battery size (and screen times you get from those). iPhone batteries are tiny compared to android. Like seriously tiny that my 10Wh Powerbank can charge it 5 times with some to spare (I only really get 4 due to losses but still). So yes comparing iPhones to anything else highly suggests that the tuning Apple did pays off. That being said: I’m not aware of a useful comparison. I’m not even sure how you would design something like that, ie what would you like to keep constant?
- slavik81 5y ago
  This strategy is typically known as "race-to-sleep" or "race-to-idle". Just figured it was worth mentioning, as there's lots of additional information that can be found if you know what to search for.
- tjoff 5y ago
  With the modern web there is no "rush back to idle". Browsers will burn your CPU until you turn off your browser, which in most cases is only when you turn your computer off.
  - saagarjha 5y ago
    Assuming you are using a reasonable browser with a reasonable set of websites open, a browser should not really burn unbounded CPU in the background. Tabs that do are generally throttled, allowing the scheduler to burst their work and then have the processor go back to sleep.
    - tjoff 5y ago
      Are we assuming no tracking and ads as well? But yeah I agree, it should not. But it does.
- Neil44 5y ago
  I see what you’re saying but I believe there’s an issue at the silicon level where the gates literally use more power to do the same work at higher frequencies.
  - jhgb 5y ago
    It's a well-known feature of CMOS gates that they consume almost all of their power at the moments of switching. AFAIK only recently has static leakage become substantial, necessitating power gating.
    - garaetjjte 5y ago
      Higher frequencies require higher voltage though, and this will increase power consumption.
- rtomanek 5y ago
  To add context: this is especially true for tasks that are likely to end in the device sleeping again. Two examples: user checking phone after hearing a notification sound or opening the laptop lid to check train departure time in the browser (then closing it again). The faster the CPU deals with the task once the device is fully awake the shorter the time other power hungry parts of the system (like screen) need to be awake. That's why slow phones, where the user needs to wait for their browser to finish loading the page will never be battery effective.
deleted 5y ago
[deleted]

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