I built a mining rig with 3 Radeon 6850 GPUs. When I run one miner (either Phoenix or m0mchil's poclbm) it reports a hash rate of a little over 200 Mhash/sec. When I fire up a second miner pointed at a different GPU, the hash rate of both miners is about half what the one running alone would get. When I fire up a third pointed at the last GPU, all three miners report an even lower hash rate, such that the total is around the 200 Mhash/sec that a solo miner would get.
Initially, I thought that the miners were ignoring the
DEVICE=# argument and all using the same GPU. I posted a question seeking help in figuring out why they would do that. @Lodewijk's answer there clued me in to the fact that the miners are using separate GPUs and that there's a bottleneck elsewhere in the system.
So, my question is, what's the likely bottleneck?
I used the recommendations at the bitcoin.it wiki entry on Mining Rigs to pick out my hardware. I'm running on a Ubuntu 11.10 system, with an msi 890FXA-GD70 motherboard, 2GB Kingston 1333MHz DDR3 Non-ECC RAM, 80GB WD IDE Drive, and a 2.8Ghz AMD Semperon 145 processor. It's on a cable Internet connection with tons of bandwidth and I'm connecting to the Arsbitcoin mining pool.