I have a Z840 with upgraded PSU of over 1000W and thought it should be enough to support 2X 250W Titan X. I can boot up and run normal programs OK. Recently, I tried to run 2 computation extensive programs concurrently and the machine just restarts itself after a while. Note that if I run the programs seperately on either cards, it is fine. Based on my previous experience, it looks like power overload ( I had similar problem running 1X Titan X on a Z420 with 600W PSU). After some research, I had some very confusing findings:
(1) The Z840 whitepaper has some conflicting information about the power supply to graphical cards. Supprisingly, even with the upgraded PSU, HP only supplies 225W to each card. Seriously? Most of the top-of-the-line cards that requires 250w or 300w can NOT be used in the Z840? However, in the I/O part of the same whitepaper, HP says that they can support 600W total for the cards. Which one is true?
(2) In the HP website, they are selling the Z840 with the option of dual nvidia M6000 with the upgraded PSU. Each of the M6000 has max power of 250W, same as the Titan X. How did HP do that?
(3) I can only find 3X 6PIN PCI-e power cable in the Z840. Given that most graphical card needs 6PIN + 8PIN or 2X 8PIN, I had to buy third-party spliter/converter for the dual GPU. Maybe that's the root of the problem? Maybe there is one extra power cable that HP reserves for their own dual GPU configuration?
It has been a PITA especially when the Z840 were bought with the upgraded PSU for the potential dual GPU. In the future I will consider DELL 7910 with 675W GPU power.
Any suggestion or comment will be highly appreciated.