On Sat, 29 Nov 2008, Roland Dreier wrote:

quoted

The 8b10 encoding is already accounted for in the 2.5 Gbps figure;
the raw bit rate is 3.125 Gbps.

In addition, PCI Express transfers are broken up into packets, usually
with very small payloads (128 or 256 bytes are common).  So the packet
header overhead reduces throughput further, and then the transaction
layer adds further overhead.  Then transferring NIC control structures
over the link adds even more overhead.  So achieving 13.something Gb/sec
of real throughput on a PCIe link theoretically capable of 16 Gb/sec
seems pretty good to me.

For transfers with only 4 byte payloads, I get about 6.5 MB/sec over a PCIe
4x link.  For small transfers, the overhead is huge.