Greetings, I'm using PCIe on the IMX8M Mini and testing PCIe performance with a NVMe constrained to 1 lane. The NVMe in question is a Samsung SSD980 500GB which claims 3500MB/s read speed (with a gen3 x4 link). My understanding of PCIe performance would give the following theoretical max bandwidth based on clock and encoding: pcie gen1 x1 : 2500MT/s*1lane*80% (8B/10B encoding) = 2000Mbps = 250MB/s pcie gen2 x1 : 5000MT/s*1lane*80% (8B/10B encoding) = 4000Mbps = 500MB/s pcie gen3 x1 : 8000MT/s*1lane*98.75% (128B/130B encoding) = 7900Mbps = 987.5MB/s pcie gen3 x4 : 8000MT/s*4lane*98.75% (128B/130B encoding) = 31600Mbps = 3950MB/s My assumption is an NVMe would have very little data overhead and thus be a simple way to test PCIe bus performance. Testing this NVMe with 'dd if=/dev/nvme0n1 of=/dev/null bs=1M count=500 iflag=nocache' on various systems gives me the following: - x86 gen3 x4: 2700MB/s (vs theoretical max of ~4GB/s) - x86 gen3 x1: 840MB/s - x86 gen2 x1: 390MB/s - cn8030 gen3 x1: 352MB/s (Cavium OcteonTX) - cn8030 gen2 x1: 193MB/s (Cavium OcteonTX) - imx8mm gen2 x1: 266MB/s The various x86 tests were not all done on the same PC or the same kernel or kernel config... I used what I had around with whatever Linux OS was on them just to get a feel for performance and in all cases but the x4 case lanes 2/3/4 were masked off with kapton tape to force a 1-lane link. Why do you think the IMX8MM running at gen2 x1 would have such a lower than expected performance (266MB/s vs the 390MB/s an x86 gen2 x1 could get)? What would a more appropriate way of testing PCIe performance be? Best regards, Tim
