Question about Underclocking

[makdak]

Hello, i have an i7 920 on a Gigabute EX58-UD4 and i'm considering to underclock my cpu to save energy and to lengthen the lifespan of the chip. On a normal basis, the cpu is usually not doing much (cpu usage rarely exceeds 10%) so i was thinking maybe it is a good idea to underclock it (further than EIST does = 16x). I also think it's a good idea to keep Turbo mode ON so that when needed it would crank up the multiplier to 21x (=2,8GHz). This way i will have full power when needed and low energy consumption, temperatures etc. while my system is idle.
So, i would like you to tell me

-if what i describe above is possible
-will i have lower energy consumption/temperatures if i do it
-if there is any risk with my hardware (possible voltage issues/conflict with other hardware?).
-how should i do it (mess with the multiplier, the bus speed, the voltages?)

Thanks in advance!!

 

[randomizer]

Decreasing the voltage will have the biggest impact on temperatures. My i7 920 runs 15C cooler with the stock cooler using a 0.16V undervolt.

 

[makdak]

Woo great! Ok, and now a noob question.. The voltage i'm looking to undervolt is Vcore voltage, right? What's the lowest safe value for the system to be stable?

 

[randomizer]

Yep it's the vcore, but it's impossible to tell you how much to do it by. It's completely specific to each individual CPU. You can't possibly damage it by undervolting, however you can cause data corruption through instability. It's not common but it can happen, so backup any very important data onto an external drive first just in case.

I would start by undervolting by 0.1V. Run some stability tests like Prime95 Small FFTs for a couple of hours to ensure it's stable. Then decrease a little bit more and check again until you find the first signs of instability. Once you hit errors, start increasing the vcore by the smallest amount possible each time to find the lowest point where you maintain stability.

 

[makdak]

Thanks so much Randomizer! One last thing I dont touch the Multiplier and the Bus speed, only the Vcore right? What exactly is the result of this action though? I mean, will my clockspeed will still be 2,66GHz? And what happens when my system is under a heavy task? Will it's performance be lower than what it is now or the Turbo mode cancels the Underclocking temporarily so that it can perform to it's maximum anyway?

 

[randomizer]

Clock speed is not affected, which is why undervolting alone can cause instability. If you undervolt and underclock you can undervolt further and still be stable, however performance will suffer. Assuming complete stability, performance remains the same regardless of the vcore.

If you underclock, performance will drop, obviously. It may not drop much if you're just playing games though. Turbo mode adjusts the multiplier, so if you lower the BCLK (from which the other clock speeds are derived using multipliers), Turbo mode won't negate the effects of underclocking completely.

If you run 100MHz BCLK then your normal speed will be 100*20=2000MHz, and when Turbo kicks in it will be 100*21=2100MHz. This is still well below stock speeds. Of course, you wouldn't lower the BCLK, you'd lower the multiplier if you wanted to underclock so that the performance of other components is not affected. If you lowered the stock multiplier to 19x then you'd get 19*133=2527MHz and with Turbo it will be 20*133=2660MHz (your normal stock clocks). Turbo mode will always increase the multiplier by 1 unless the application is completely single-threaded, in which case it should (in theory) increase it by 2.

 

[makdak]

You couldnt possibly explain things better! Thanks a lot randomizer!