On Wed, Apr 08, 2020 at 11:01:52PM +0000, Jaromir Dolecek wrote:
> Module Name:  src
> Committed By: jdolecek
> Date:         Wed Apr  8 23:01:52 UTC 2020
> Modified Files:
>       src/share/man/man4: wm.4
> Log Message:
> add a warning in checksum offload that hardware TCP segmentation might be
> slow
> on I219 I observe about 35% transmit performance drop when tso4 enabled

This sounds familiar.  There was a bug affecting TCP segmentation
offload (I think) that we found at CoyotePoint.  ISTR
bus_dmamap_load_mbuf(9) failed with EFBIG because under some
circumstances the number of segments in the DMA map was too small
for the mbuf chain.  The driver would drop the whole mbuf chain
on the floor.  This showed up as terrible performance under some
circumstances---possibly when the TCP window grew long?  The solution
was to increase the number of DMA segments, *I think*.

I don't think CoyotePoint ever fed its change back to NetBSD,
unfortunately.  On the other hand, some other NetBSDer may have
independently fixed the bug.

Do any stats increase (vmstat -e, ifconfig -v wm0) when the poor
performance occurs? You may have to enable WM_DEBUG or something to see
all of the relevant stats.


David Young
dyo...@pobox.com    Urbana, IL    (217) 721-9981

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