Saturday, September 12, 2009

Setting CPU Fan Speed Linux

I recently assembled a new desktop system. I want it to as reasonably quiet as possible. The motheboard's cpu fan header is 4 pin. What these new four pin headers do is allow for a variable pwm to be applied to them, which varies the cpu fan's speed.

One issue is that the stock fan, while not that loud, was leaving my temps a bit too warm. It is a lower powered AMD 245 "regor", but my case is a SUGO S02 from Silverstone - very cramped. I upgraded to an aftermarket Zalman 7000cu fan. This thing is a giant copper beast, with a bigger fan, but still compact enough to fit in my case. I was hoping I could shave off a few degrees and quiet it up a bit.

The one issue is that this new fan has only a 3 pin header, supposedly no pwm. It came with a manual adjustment knob that can let me vary the voltage by hand, but that meant another thing sticking out of the back of the case, or filling up the insides.

So at first I used a tool called fancontrol. It is installed in Arch Linux by default, but I'm sure its in all the big distribution's repositories. Fancontrol is a bash script that runs as a daemon, monitoring a temp you specify, and at speeds you want. You have to go through some time playing with its configuration tool, "pwmconfig".

To my surprise, it manages to control the fan speed of my 3-pin fan. It worked well, eventually I got things pretty quiet. As my load increases, and thus CPU temperatures, the fan speed does as well. There is one issue with this.

For a given load, the CPU will heat up, along with fan increases, until things stabilize at a certain level of heat/rpm/temperature. If you are lucky, it stabilizes at a particular value. But usually it stabilizes at a cycle. The temp hits 37 degrees, fan 1250rpm for me, and this lets the cpu warm to 38 degrees, where the fan is then kicked up to 1350rpm, cooling the cpu back down to 37. This can be an irritating cycle. At either speed, the fan is relatively quiet, but to hear the speedup and slowdown every 30 seconds is distracting.

So let's just set it manually. Googling for how to do this was not immediately apparent, so I wanted to clarify. Linux detects and has support for hardware monitoring devices, which use the very low level i2c bus. It exports an interface at /sys/class/hwmon. For me, /sys/class/hwmon/hwmon0/device is the usable device interface, which has numerous values within.

"pwm1" is the fan control value that we can play with. It stores a value from 0-255, indicating the voltage driving the fan. A value of 0 means the fan wont spin, a value of 255 means it spins at full speed. However, a value of up to 28 for me has the fan not moving either, so each fan is probably different.

'cat fan1_input' will return the rpm of the first fan, the cpu fan for me.
'cat pwm1' returns the value 0 - 255 that the fan is being told to spin at.
I use lm_sensors to see cpu temp and fan speed, but this data is all controlled by this hwmon device.

To alter the fan speed, first 'echo 1 > pwm1_enable', this will let the 'pwm1' value control it. Then experiment. I have found that 'echo 150 > pwm1' sets the fan high enough to where it cools down to the low 30's, but is not too loud. 'echo 65 > pwm1' is silent, although the cpu temp rises to almost 40C.

The nice thing about this is that I set a single temp/rpm/noise volume, no irritating cycles. Another nice thing about just setting this manually is that my fan has certain regions that have additional vibration. 1250rpm and things get a slight buzz. 1200rpm and even 1300rpm are quieter. I like the finer manual control.

As for now, my rc.local has both "echo 1 > /sys/class/hwmon/hwmon0/device/pwm1_enable" and "echo 65 > /sys/class/hwmon/hwmon0/device/pwm1". I generally have a similar load on the system, and don't do anything that will tax it too much for too long, so I am comfortable with this setup.

I'm still curious as to what the fourth pin on a 4 pin cpu fan was for. Obviously motherboards can modulate the 12v line in the 3 pin, which is the same in the 4. I can only guess that this is a legacy issue. Certain fans may not work well with changing the 12v line, and certain motherboards may lack a hardware monitoring device. But basically, if you see a good cpu fan and it is only 3 pin, you can still have full automated or manual control.

1 comment:

  1. Fans speeds can be controlled in 2 ways :

    1. Constantly apply a voltage, but vary that voltage according to how fast you want the fan to turn.
    2. Apply voltage pulses at full voltage, but vary the on/off ratio of the pulses according to how fast you want the fan to turn. This is called Pulse-Width Modulation ( PWM ) and the on/off ration is called the duty cycle.

    The 3-pin header contains a pin for "common" ( the reference "0" voltage of the computer ), a "read" voltage pin to measure the speed of the fan rotation, and a "write" voltage pin where the fan power ( anything between 0 and 12v DC ) is applied.

    The 4-pin header adds a pin for PWM, where the voltage cycles between "low" and "high" ( a square wave ). The voltage on the "write" pin is applied only when the PWM pin is "high".

    Thus, a 3-pin fan connecter can only vary fan speed via direct voltage control, whilst the 4-pin fan connector can vary fan speed by direct voltage control or by varying the square wave on the PWM pin.

    PWM is more accurate and more efficient than voltage control.