6x MI50's on PCIE vs 4x MI50's on PEX8749 and 2x on PCIE
I am really excited to share this one. On the X99-E-WS motherboard.. while old and PCIE 3.0 - I think it's still pretty capable for what I'm trying to do (1TB VRAM across 3 machines). The board has seven physical PCIe x16 slots shared through the onboard PLX PEX8747 PCIe 3.0 switches and with a 40 lane CPU + all seven slots populated, the board supports an x16/x8/x8/x8/x8/x8/x8.
What I tested was putting a PEX8749 card on the x16 slots so that 4x MI50's ran on the switch thus freeing up 3 PCIE slots for additional cards.
Online data is scarce for folks running the PEX8749 card and Claude/ChatGPT gave me conflicting answers on wether this would increase tg/pp speeds or decrease tg/pp speeds so I figured I'd just test the before and after.
Hardware:
Asus X99-E-WS (Modded BIOS to support a large number GPU's )
Intel(R) Xeon(R) CPU E5-2680 v4 @ 2.40GHz
128GB DDR4 RAM
SSD
Here are the results:
| Test | 6× MI50 all direct | 4× PLX + 2× direct | Difference | Change |
|---|---|---|---|---|
pp512 | 139.27 | 138.24 | −1.03 | −0.74% |
tg128 | 24.87 | 25.55 | +0.68 | +2.73% |
pp512+tg128 | 71.12 | 72.47 | +1.35 | +1.90% |
pp4096+tg128 | 120.95 | 120.96 | +0.01 | +0.01% |
pp16384+tg128 | 117.69 | 117.27 | −0.42 | −0.36% |
pp32768+tg128 | 103.56 | 103.64 | +0.08 | +0.08% |
pp65536+tg128 | 81.99 | 82.67 | +0.68 | +0.83% |
I ran llama bench multiple times and surprisingly tg was always just a smidge better .8% ~ 2.8% with the PP speed loss at less than 1%.
Posted Jul 11, 2026, 2:23 PM UTC