Two Is Better Than One
Now that I have a single video card configured and running and stable the next step is to purchase and install a second.
My previous power supply was rated at 430 Watts, which was not going to be enough to power an additional video card. So I upgraded to this: Corsair HX 850 It cost $150 from Amazon.com and had a variety of connectors.
My motherboard only has 3 PCIe slots to use, and I knew I didn't want to plug two video cards in next to each other. That didn't leave very many choices. This is what the inside of my case ended up looking like.
Placing the cards right next to each other definitely (and obviously?) caused some heat issues. I tried this out briefly just to see what would happen. As predicted, the top card (with it's fan pointing at the back of the other card) overheated often.
Configuring Multiple Cards
Purchasing a second video card that was the same as the first made configuration and setup go pretty fast. I had the card plugged in and running solo with a single night's effort.
The cgminer Way
Setting up a single cgminer with multiple GPUs is possible. You can use the configuration file supported by cgminer and add a comma followed by a second parameter for the second card to each of the settings. For example:
"intensity": "18,18", "worksize": "256,256",
The Unix Way
The other way is to simply run two separate instances of cgminer. This is the option that I went with. The main thing to remember is to specify a different value for the -d/--device option.
Here are the abbrivated command lines that I use for each of my cgminer instances.
cgminer --device 0 --intensity 18 --thread-concurrency 7168 --remove-disabled --scrypt cgminer --device 1 --intensity 18 --thread-concurrency 7168 --remove-disabled --scrypt
Tweaking and Tuning
Updating the cgminer git repo can be done with the following commands. First change to the directory that contains the cgminer source code and run:
git pull make
This should result in a new version of the cgminer application.
This works most of the time. I just recently tried this and found an error with the ./configure script for compat/libusb-1.0. I'm not sure I needed to, but I ended up installing this library via apt-get and ignored the error from configure. I was able to build without errors.
To uninstall the AMD drivers you can use the following command.
sudo apt-get remove --purge fglrx*
Then follow the install instructions from my previous post.
Tuning the cgminer application is much more difficult. I used a two phase approach:
- Used google to find cgminer config strings for my video card
- Use them and modify each value slightly up and down until I found something optimal
It's a very repetitive process and can vary widely between chipsets and video card vendors.
Here are the final numbers that I settled on for 331 Kh/s average from both of my 5870s.
cgminer --device 0 --auto-fan --temp-target 78 --gpu-engine 710 --gpu-memclock 1250 --intensity 18 --thread-concurrency 7168 --remove-disabled --scrypt cgminer --device 1 --auto-fan --temp-target 80 --gpu-engine 710 --gpu-memclock 1250 --intensity 18 --thread-concurrency 7168 --remove-disabled --scrypt
Here are some final numbers from my Litecoin experiments.
- I spent a total of $350 for two video cards and a new power supply
- After 2 months of mining, I've made almost 25 LTC.
- I'm collecting 2 Litecoins approximately every three days.
- Both video cards running at 331 Kh/s now consume 440 Watts of power
As is, it looks like I'm making a slight profit of about $0.66 per week not including the money invested in hardware. I guess this is a long-term investment.