any place to look at for PCI-express performance issues ?

John Baldwin jhb at
Thu Jun 9 13:22:46 UTC 2011

On Wednesday, June 08, 2011 11:59:52 pm Luigi Rizzo wrote:
> hi,
> during my tests with netmap with 10Gbit cards (82599, dual port),
> i notice that a motherboard with an AMD 880G chipset
> is performing significantly worse than an intel based one.
> In both cases the NIC is mounted on a 16x PCIe slot,
> and in both cases the driver reports the use 5Gb/4x per port.
> On the intel i reach easily 14.88Mpps, on the AMD the card tops
> at 1.8Mpps, and is not CPU limited (changing dev.cpu.0.freq does not change
> the throughput).
> Disabling flow control does not help (and in any case
> the other end of the link is the same), and since
> I am using the same picobsd image (based on FreeBSD/i386
> head w/ my netmap code) i suspect that the difference in
> performance has to do with the PCIe controller.
> My netmap code
> does nothing special on the bus.
> Now, the question is, is there any place in FreeBSD sources that
> might be related to PCIe performance, e.g. initialising specific
> features in one or another northbridge, etc ?

No, in general we rely on the firmware (BIOS, etc.) to configure those bits.

You might take a gander at comparing 'pciconf -lc' output as that displays a 
few of the PCI-e settings such as the actual number of lanes used, etc.

John Baldwin

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