Root SetCPU

[d.knight99]

SetCPU claims to be able to increase your battery life by limiting the speed your processor can run at. This seems like sound reasoning at first, but then I remember roughly what a CPU is and how it works...

So let's say you've set a profile so that when the screen is off it will be limited to half the clock speed; around 500mHz. Lets then say your email wants to get updated, so your processor chugs away that that. If it was set to full capacity it may take say, 1 second to complete the task then go back to it's idol state. With SetCPU limiting it, it would take twice as long and would still use exactly the same number of cycles as before.

In fact, when you think about it, it doesn't actually matter what speed your phones processor is running at all the time, what matters is how much work you're asking it to do. Surely the best and only way to increase battery life is to simply have your processor complete less tasks, therefore using less clock cycles? Sending a text message uses the same amount of clock cycles no matter what your max setting is, so as a result uses the same amount of power.

If anything, having SetCPU running all the time just gives your phone more work to do and therefore decreases battery life?

Or am I missing something?

 

[SUroot]

Well yea you're right about the email example except you made one huge assumption in that it uses all of the CPU's ability to download am email, when I'm truth it takes very little.

I find that it does increase battery and with the correct settings, can drastically increase it.

Just try it for yourself for a while

 

[d.knight99]

Yeah that's true, but the assumption about the actual clock speed doesn't matter, the point is that tasks require a finite number of cycles to complete, whether it's 1000, 10, or 10,000. So the CPU will use the same amount of power completing a task in 10 seconds as it would in 20 seconds at half the speed?

At least that's how I understand it anyway. Unless there is something written into the OS so that when it doesn't have any headroom left it starts throwing out processes that are of a lower priority and therefore actually does less work as a result?

May I ask what these settings are to obtain the drastic increases you say are possible?

 

[SUroot]

Look at it this way,

Your CPU handles 998,000 cycles per second.

Imagine receiving an email takes 1,000 cycles to receive (a number pulled out of the air). So unless you drop the CPU to 500Hz (which is not possible - minimum is 245,000), then it wont matter. If the CPU did drop to to 500 Cycles per second, yes, it would take 2 seconds to receive an email.

Fact is you can't drop the cpu low enough to make any single task take twice as long.

 

[d.knight99]

Ah but receiving the email at full speed would not have taken 1 second so that argument doesn't make sense.

at 998,000 cycles per second, 1000 cycles would take exactly 1.00200401 microseconds. At say half speed, around 499,000 hz it would take 2.00400802 microseconds, exactly double the time. Yes, slowing that 1000 cycles down to take 2 whole seconds would require a clock speed of 500hz, but that would be thousands of times slower, not twice as slow.

So the point still stands, you're executing EXACTLY the same number of clock cycles (no matter how long it takes to complete) every time you do something with your phone. But then again, people have apparently reported that they get more battery out of their devices, but how? I'm just trying to get my head around how this program is working and what's physically happening.

 

[SUroot]

Email the dev

 

[Tunga]

You're right, except you are making the false assumption that the CPU only uses power when it's doing something useful. Even when idle it is using power because a processor cannot literally do nothing so long as it has a clockspeed, it has to do something on every clock step. In fact the power usage of a specific CPU is determine pretty much entirely by the clock speed and not by how much load it is currently carrying.

In other words, it doesn't matter if your e-mail takes 1ms or 1sec or 1hour to download since the CPU won't use more power than if it was idle anyway.

The important thing is that it is running/idling at ~250MHz rather than ~1GHz which means it's using a lot less power in doing so.

(Warning: this post is a somewhat of a huge simplification given that modern CPUs are ludicrously complicated but the principle is reasonably sound, enough to demonstrate the logic behind SetCPU.)

 

[d.knight99]

I see, so the idea is that although it's idling, the clock cycles are (for the most part) just being wasted. A few are obviously necessary to keep the system running, but not all 250MHz worth. So when you ask it to do something in the background, it can simply use those wasted cycles to do the work instead of upscaling the speed?

Yeah that makes sense!

 

[rx93]

This app sucks big time!!!!!!

I installed it this afternoon and it slowed down my Desire like hell!!!! Didn't see any battery life improvement though.

Already uninstalled it and asked for refund.

 

[SUroot]

This app sucks big time!!!!!!

I installed it this afternoon and it slowed down my Desire like hell!!!! Didn't see any battery life improvement though.

Already uninstalled it and asked for refund.

What settings did you use? If you didn't do device selection when you installed it, the main page sets your clock speed at the lowest all the time, which would indeed slow down your phone.

 

[rx93]

Hi danfrance,
Do I have to set device in SETCUP manually? I thought everything would be set up automatically.

Anyway, I uninstalled it since it would help on battery life.

 

[SUroot]

Yeah when you first load Set CPU, you have to go into device selection and select automatically detect Settings, otherwise, it sticks at 245Mhz.

To be honest, since you returned teh app in time to get a refund, you didn't really give the app enough chance to see if it does make a difference or not.

This is a really big problem with the 24 hour refund period as it just isn't long enough.

 

[Tunga]

I see, so the idea is that although it's idling, the clock cycles are (for the most part) just being wasted. A few are obviously necessary to keep the system running, but not all 250MHz worth. So when you ask it to do something in the background, it can simply use those wasted cycles to do the work instead of upscaling the speed?

Yeah that makes sense!

Correct .