[lwip-users] Low Iperf performance of lwip 1.4.1 on STM32 and FreeRTOS

[Claudius Zingerli]

Hello all,

I'm working on a project using lwIP 1.4.1, FreeRTOS 7.4.2 on an
STM32F407 MCU.
I have several UDP/TCP/Multicast services running well, but when I tried 
to measure TCP bandwidth with Iperf as well as with dd|nc, I get very 
low results.
Iperf basically just sends a lot of data and lwIP drops it (using 
netconn_recv();netbuf_delete() or netconn_recv_tcp_pbuf();pbuf_free();)

An analysis with Wireshark shows the following:
(TCP_MSS=TCP_WND=1460)
- SYN,SYNACK,ACK,PSH,PSH (as usual)
- ZeroWindow (client stuck), WindowUpdate (some ms later)
- PSH, ZeroWindow, WindowUpdate,...

As I understand it, this is how TCP works. Quite low bandwidth (a few
hundred kBps) with these settings, but it works.
When I try to increase TCP_WND to p.e. 5kB, the following problems arise:
- Dup ACKs (from lwIP)
- lots of Retransmissions (from Linux)
The bandwidth is in the Bps to kBps range (at most). I spent hours, but 
have no clue where to look next. Any ideas what could be the 
reason?(Iperf Linux to Linux results in the full line speed)

One interesting thing is: I get about 0.5% packet drop if I do a ping -f 
(100 Pings per second, packets seem to never arrive at the Eth 
interrupt). MCU load is always quite low (I have a low prio blink task 
that still gets its CPU time as well as )
Things I already fixed: (my design bases on ST's ethernet code)
- Check any stacks/NULL/malloc fails
- Check if pbuf fits into Tx buffer
- Check if there is enough pbuf_mem to fits Rx packet
- In packet reception I try to drain the input queue (by checking
DMARxDescToGet->Status & ETH_DMARxDesc_OWN )
- ETH_DMASR_RBUS cleared in low_level_input()

I just ran out of ideas how to fix the problem. Is this about tuning 
lwipopts.h? Attached, my current version.

Best regards

Claudius

lwipopts.h
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