In other news, the risers I ordered nearly a month ago finally arrived earlier this week. After fixing an unrelated power-supply issue, I rearranged my rig to use the risers, powered it up...and saw only about half of the hashrate I had previously been getting, with nvidia-smi indicating per-card power consumption fluctuating all over the place. BIOS settings are as recommended. I've updated the BIOS and redid the settings. I've tried slowing down the bus. I've tried plugging the risers into 1x slots instead of 16x. PCIe spread spectrum is still enabled; would disabling it likely help or hurt? Beyond that, the only thing I can think to try is to install Windows and see if it'll work any better, but then I'd have to port my mining switchers to it (if that'e even possible...are the command-line overclocking tools used by nvOC supported on Windows?). I got so fed up with it that I just put everything aside late yesterday evening and have been mining sweet bugger-all since.
In other, other news, as an alternative to flaky USB PCIe risers that never seem to work right, I stumbled across this:
http://shop.dmp.com.tw/INT/products/23Depending on how the module's configured, it might need a jumper removed to change the PCIe 1x connector at the end from a target to a host, but once that's done, I'm thinking a simple adapter board might be possible that would hold a GPU in one slot, this card in another, and some power-supply circuitry as appropriate. Add a MicroSD card and an Ethernet jack, and it'd potentially turn a GPU into a standalone miner. The module's also available by itself for inclusion in your own designs, which might be more appropriate in the long run...but just to see if the idea would work? If 128MB RAM isn't likely to be enough, the 1GB version is $15 more. It should run Linux without issue, and as long as nVidia and AMD avoid certain unsupported instructions (someone said CMOV isn't supported), their drivers ought to work on it. It's not particularly speedy, but it ought to be fast enough to keep a GPU fed with mining data.
). Also, CryptoSwitcher used full-node coin daemons (bitcoind, litecoind, etc.) as an independent source of mining stats (and I might've had pools paying out to local wallets); having a bunch of those running chews up lots of RAM and disk I/O.