GPU Fan Control
If I'm understanding the documentation correctly the --gpu-temp-retain option should result in hashcat controlling the GPU fans. However, when testing with --gpu-temp-retain=1 there is no affect on the fans when I'd expect them to be set at 100%.

Some system details:

Ubuntu Server 16.04
(2) GeForce GTX 1080
(2) GeForce GTX 1070
OpenCL Version : OpenCL C 1.2 
Driver Version : 381.09

Example run:

$ hashcat64.bin --gpu-temp-retain=1 -m 400 -a 0 -o text.dic test.hash words.txt

HWMon.Dev.#1.....: Temp: 68c Fan: 44% Util: 99% Core: 106MHz Mem:4513MHz Bus:16
HWMon.Dev.#2.....: Temp: 64c Fan: 41% Util: 77% Core:  14MHz Mem:3802MHz Bus:16
HWMon.Dev.#3.....: Temp: 70c Fan: 46% Util: 99% Core:  93MHz Mem:4513MHz Bus:16
HWMon.Dev.#4.....: Temp: 67c Fan: 43% Util: 99% Core: 106MHz Mem:3802MHz Bus:16

You're right, if you set --gpu-temp-retain=1 it should set the fans to 100% and it works in my case. Maybe you do no have setup the X11 correctly so that an external program can set fans, or you lack the permission to do it. Ideally run it from an xterm or so inside an X session on that machine. If it works, you just need to create the required access to do the same from SSH or whatever you're using.