For PCI-E x8 links the nominal bandwidth is 2.0 GB/s per-direction (upstream or downstream) and 4.0 GB/s full-duplex (the calculation is shown below).


Nominal PCI-E BW per lane: 250 MB/s (M=10^6) with 8b10b encoding
(Wirespeed only, assumes *no* PCI-E protocol overhead)
8 lanes, 1 direction => 8*250 * 1 = 2000 MB/s (M=10^6)
8 lanes, 2 directions => 8*250*2 = 4000 MB/s (M=10^6)

However, to determine theoretical payload data bandwidth, we must account for PCI-E packet overheads

as well as link management such as flow control and acknowlegement traffic. As well, fabric and device

parameters of maximum packet size and maximum read request size affect achievable performance.


The following table shows theoretical PCI-E performance taking into accout a specific T5440 and Solaris configuration including 128B maximum payload size and 512B maximum read request size.


Theoretical DMA Performance for T5440 Default Configuration

Operation

T5440

Theoretical

Percentage of Theoretical

100% DMA Read

1520 MB/s

1580 MB/s

97%

100% DMA Write

1720 MB/s

1780 MB/s

97%

Bi-Directional

2940 MB/s

3160 MB/s

93%