How to use both CUDA GPUs?

Question

I'm using rpcminer-cuda to have a play around mining some bitcoins. I have 2 NVidia cards, and I understand this is not ideal for any profitable mining. I would however like to have them both working at the same time. Presently the output from rpcminer-cuda when it starts up is:

rpcminer-cuda.exe -url=myurl -user=myuser -password=password

Client will start 1 miner threads
2 CUDA GPU devices found
Setting CUDA device to first device found

How do I make it use both cards at the same time? Do I need to tell it to start two threads or is there some other commandline switch I need to use? I've tried adding -gpu=2 but it seems to be ignored.

Answer 1

The -gpu=X command line is telling it which of your CUDA devices it should use, not how many it should use. Like most things written by programmers, the index starts at zero. If you have two video cards, run it once at the command line with -gpu=0 and then a second time again with -gpu=1. Once you work out your ideal fine tuning on aggression, gpugrid and gputhreads (if you do at all), it's easiest if you make a batch file (unless you're using GUIMiner, in which case create a new CUDA miner for the second card, same extra flags on both other than gpu=0 or 1).

You can execute it twice without specifying the GPU and it will certainly run, but from the console output it appears it assigns both of them to the first device.

Answer 2

The simple solution is to copy and rename the command so you run 2 separate instances of the same command. The other option is seeing if adding -threads=2 ACTUALLY does let you use both cards. I have had issues where the threads were CPU threads and not cards and times it ' just worked ' on the same system. running the command twice had it run 2 instances one per card and seemingly random which card was first every once in a while.

I have a XP box for CUDA mining on dual ( not connected ) 8600 GT cards and used GUIMiner Script to find out what batch files were created for each ( as new CUDA miners for litecoin ). With some tweaking, they can do BTC or LTC mining. What little time I have used rpcminer ( and hair pulling ) it seemed to ' kinda work ' when I ran it twice to get both cards mining as separate cards.

As to the profitability of GPU mining... I still have a ' day job ' and am looking at investing in ASIC miners for BTC.