Which slots for dual GPUs in z820?

[echoout]

Hey there, I have a z820 with dual E5-2687Ws and 1125W power supply. I just bought a second EVGA GTX 780Ti and was trying to get it to play friendly with PCI-e arrangement. On Page 94 of the manual, it states that 2 high performance GPU computer cards should reside in slots 4 and 6 (if I'm reading this correctly) to each have a PCIe-x16 slot assigned to each CPU. The same appears to be true for standard PCIe graphics cards.

With that in mind, I already had my existing 780Ti slot 6 so I simply added the new (indentical) one to slot 4. Turned it on and got 4 beeps (I think) which seems to mean power supply overload. I thought this was strange since my z820 shipped from Promax with dual GTX 680s. The 680s required 2 6-pin power cables each, while the 780Tis require 1 6-pin and 1 8-pin each. I suspected my adapters or the power load increase in the new cards had something to do with it but I played around with the arrangement before I gave up.

It turns out the 2 GPUs in just about any other arrangement seem to work just fine as far as booting up, displaying and under benchmarking. I had success with them in slots 2 and 6 (same CPU) and currently in slots 2 and 4 (seperate CPUs), neither which are recommended.

FYI, using one GPU for display and the second for its CUDA computation and it seems to work well.

Am I missing something in this arrangement? Will I damage anything? Any ideas why just the Slots 4/6 arrangement gives me the PSU boot error?

Thanks, sorry so long-winded.

[Dan_WGBU]

Echoout, you are right, the manual is confusing on how to plug in expansion cards.  The table (line 3) does say to install the the first PCIe graphics card in Slot 2.  This is the configuration for about 90% of the Z820 systems that HP ships (I confirmed this).  The table's Load order 1 and 2 probably refer to GPU compute cards (like Tesla cards), rather than graphics cards that are used as GPU compute (like you are doing). 

Putting your primary 780T8 in Slot 2 and the second one used for GPU compute in Slot 4 will work.  As you said, this puts the cards on separate CPUs, which should give the best performance.  Using Slots 2 and 6 will work, but will put both cards on the first CPU, which might not give the best performance. 

I do not know why you have problems installing the cards in Slots 6 and 4, but using Slots 2 and 4 is a good configuration.

[echoout]

Awesome! Thanks for looking into that. Consumer GPUs are getting so crazy that it's hard to know what is what anymore.

Yeah, the PSU Overload warning is just strange. 

Thanks, Dan!