single outstanding DMA read operation. Most controllers targeted to
client with PCIe bus interface(e.g. BCM5761) may have this
limitation. All controllers for servers does not have this
limitation.
Collapsing mbuf chains to reduce number of memory reads before
transmitting was most effective way to workaround this. I got about
940Mbps from 850Mbps with mbuf collapsing on BCM5761. However it
takes a lot of CPU cycles to collapse mbuf chains so add tunable to
control the number of allowed TX buffers before collapsing. The
default value is 0 which effectively disables the forced collapsing.
For most cases 2 would yield best performance(about 930Mbps)
without much sacrificing CPU cycles.
Note the collapsing is only activated when the controller is on
PCIe bus and the frame does not need TSO operation. TSO does not
seem to suffer from the hardware limitation because the payload
size is much bigger than normal IP datagram.
Thanks to davidch@ who told me the limitation of client controllers
and actually gave possible workarounds to mitigate the limitation.
Reviewed by: davidch, marius
901b0b81fe