[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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