OC at Last

Well, the problem was trying to use the onboard video.  Once I shut off the IGA and disabled multiple adapters, gpu-manager quit overwriting my xorg.conf and I was able to follow the rest of the usual steps to overclock the GPUs.  I started off with +100 MHz graphics clock and +500 MHz memory clock, and once I got the EVGA 1060 SC up to +1000 MHz memory, claymore started throwing errors.  Anything over +500 didn't seem to help the hashrate, although I needed to kick the power target on the #3 card up to 80W.  With 100/500 across the board, I'm getting about 21 MH/s rather than 19.5 MH/s, a total of 84 MH/s, up from 79 MH/s.  Not quite a 10% kicker, but it's welcome.

I was reading how ethminer has better optimization for Cuda, so I'm going to have to check out ethminer vs. claymore once I get it compiled.  WhatToMine says I should be getting 90 MH/s for four 1060s.  At least the system power consumption's lower than the 360W they quote; 345W for the whole box.

Linux Recovery

Well, this is just a note to myself rather than a bunch of weird links.  So...if you do "prime-select intel" on a hybrid graphics system and everything freezes when the boot sequence gets to X:

1. Hard reboot and wait for the BIOS screen to pass.
2. Hit Esc to bring up the Grub bootloader menu (holding down Shift didn't work on my system).
3. Type 'e' on the first entry to edit the boot commands.
4. On the line with "linux", append " 3" (space, 3).  This will restrict the runlevel to 3, which is multi-user mode.  Alternatively, one could append " single", but single-user mode is perhaps less useful because you can't start X from there, nor do you have network access.
5. Hit F10 to boot with those options.
6. Log in and do your thing.  In multi-user console mode, you can hit Ctrl-Alt-1 through 6 to get tty1-6.  Ctrl-Alt-7 is X, usually.  If you have something to try, like fix the graphics selection: "sudo prime-select nvidia ; systemctl stop lightdm ; systemctl start lightdm".

Of course, that may not completely fix things either, but getting access to the system is better than being locked out.