Testing 10Gb switches using iPerf … pt2

I have a HP dual 10Gb NIC (model number) and also a cisco SFP+ DAC 5m, both from eBay.

CN1100e in a dell optiplex 7010 MT, in an x16 wired x4 slot, PCIe Gen2.

So they are actually CNA’s, useful to know. these are in the FCoE mode.

output from lspci
output of: sudo lspci -vv | grep -P “[0-9a-f]{2}:[0-9a-f]{2}.[0-9a-f]|LnkSta:”

I tested these with a switch but as I only had one 10Gb nic so I went ahead and brought 2 more. Once they arrived I put them in the old xw4600 based workstation and gave them a go with iPerf. The one in the top x16 slot got 8.84 gb/s and in the x8 slot below that got 4.33 gb/s.

So why is this? Well firstly lets look at the hardware – the x8 slot is PCIe Gen1…and wired only x4. And then x16 slots are Gen2. So the theory says that the x16 slot should be able to run a 10Gbit nic just fine, in x8 mode.

The 8x slot is actually only x4 electrically, and only Gen1, which means a max of 8Gb/s.

system topology from the manual
PCIe versionYearTransfersx1 speedx4 speedx8 speedx16 speed

Here’s the output showing the PCI link status’. Top card is running x8 at 5GT/s so Gen2, and bottom card is running x4 at 2.5GT/s so Gen1.

output of: sudo lspci -vv | grep -P “[0-9a-f]{2}:[0-9a-f]{2}.[0-9a-f]|LnkSta:”

You might notice there are in the iSCSI mode rather than FCoE. The 10Gb nics are actually only Gen 2 anyway. (PCIe x8 Gen2.) See https://support.hpe.com/hpesc/public/docDisplay?docLocale=en_US&docId=c03572683

Great article https://www.infoworld.com/article/2627603/fibre-channel-vs–iscsi–the-war-continues.html

I can also tell you that in an iPerf loopback the Dell 7010 gets 19.8Gb/s, and the xw4600 gets 14.8Gb/s.

So i’m not 100% sure where the bottleneck is. In subsequent testing I managed to get a 9.22Gb/s rate which I think is probably the limit of transfer given the TCP overheads – i’ll try with Jumbo packets.

With an MTU of 9000 I managed to get bursts of 9.90Gb/s. While running htop on both the dell and the xw4600 I noticed that the puney dual core E6550 at 2.33GHz in the xw4600 was pegged at 100%, While the i3-2120 at 3.3GHz in the dell didn’t go above 60%. I’m pretty sure the bottleneck is in the CPU/PCIe workings, after all using a dual core from 2007 probably wasn’t designed to handle 10Gb networking.