The original send problem turned out that it would send one packet,
wait for the ack, then send the next. This was because the
TCP_SND_BUF was only 2048 in the unixsim contrib folder, which I used
as a basis.
The values used in the examples are often very conservative defaults,
and not tuned for performance. lwIP is in general targeted at low
memory systems rather than high performance ones, which is why the
defaults lean in that direction. Some time ago someone posted a
list of
sensible values to use for performance. These may have made it
onto the
lwIP wiki, but I'm not sure. Searching the mailing list archives
might
help if not.
I'll check. Thanks.
My test program linux-to-linux gets 117.8MB/s (940Gbps). It gets
113.2MB/s with TCP window scaling off, but this is off topic.
Linux_send-lwip_recv gets 923.3MB/s (738Gbps). Linux_send-lwip_recv
gets 92.3MB/s (738Gbps). lwip_both gets 92.3MB/s (738Gbps).
I think you've confused Gbps and Mbps, and got a decimal place or
two in
the wrong place in those figures, but I think I can make sense of
them.
It looks like you're getting pretty good performance from lwIP. I
wouldn't expect it to beat the native Linux stack.
Sorry if there is any confusion here. For clarification, I mean MB is
megabytes or 1 million bytes. Gb is Gigabits or 1 billion bits or 125MB.
It seems that on the recv side, LWIP sends multiple requests for TCP
window_size updates.
I think we have an outstanding bug about window updates. Might be
worth
checking to see if that matches your observed behaviour.
The window size drops significantly and
fluctuates between 30K and 60K. Also, there seems to be a lot of PSH
flags when lwip sends. The packets are truncated, but the data
payload is just '0x7a7a7a7 ....'
I'll try and find some time to take a look at your packet captures.
Finally, somehow linux can send packets bigger than the 1500 MTU. How
is this possible? Is it fragmenting the packets? The IP flags say
'don't fragment'.
Most likely Linux is doing segmentation offload to your NIC. i.e.
passing a larger than MTU socket buffer to the NIC, which then
fragments
it into MTU sized chunks. This is a pretty standard feature in modern
NICs to reduce the overheads on the network stack.
I guess I could do the same?
Thanks.
Rishi
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