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I'm researching dual QSFP 40GBit network cards for a work project and have a few questions regarding PCIe 3.0's theoretical max bandwidth.
I'm currently looking at a dual QSFP PCIe 3.0 x8 card on CDW (Mellanox MCX314A-BCBT), but I don't think PCIe x8 would provide enough bandwidth for both 40Gbit links at 100% utilization.
Wikipedia states that PCIe 3.0 has a theoretical max bandwidth of 985MB/s per lane. Thus, by my calculations, PCIe 3.0 x8 would yield a max bandwidth of 7880MB/s. If this is true, the card would saturate the PCIe connection if both links are at 100% utilization.
Here are my specific questions:
What is the max bandwidth that a dual QSFP 40Gbit network card can output?
What is the max bandwidth that a PCIe 8x can handle before saturation?
Is there an easy way to calculate this?
3There's actually a little more to it than you realise. You need fast memory and fast CPU as this also becomes a bottleneck. Particularly the memory as memory copy operations become the bottleneck at these speeds. That is why RDMA has become so important and is now a standard feature of 10Gbit+ NICs. – Matt H – 2013-04-23T22:42:27.013
In our app we're using FDR infiniband and unfortunately spec'd the CPU toward the low end. This limited the memory speed and we hadn't anticipated the knock on effect that this would have on IPoIB performance. – Matt H – 2013-04-23T22:44:14.723
1Yeah, that was the next thing on my plate... We'll definitely have to upgrade the memory from dual to triple channel ddr3-1333, but we might have to go to 1600. – SPARCpenguin – 2013-04-23T22:49:41.953
2BTW Welcome to Super User! – James Mertz – 2013-04-23T22:54:52.087