Re: kirkwood mv643xx_eth wrong tcp checksums
Lennert Buytenhek wrote on 23.11.2009 15:50:
> The general idea here would be to do this in mv643xx_eth.c:
> - if (unlikely(tag_bytes & ~12)) {
> + if (unlikely(tag_bytes & ~12) || skb->len > MAGIC_VALUE) {
>
> If you set MAGIC_VALUE to 1514, regular MTU should still work, and
> use with hw checksum offload, and jumbo MTU should start to work,
> albeit with sw checksumming.
Instead of the latest 2.6.32rc8 kernel I have now compiled the 2.6.30
kernel that came with debian squeeze with your workaround and it works!
It is now possible to set any mtu between 1500 and 9000 without any
problems. I performed some tcp benchmarks (transmit from qnap arm device
to ubuntu x64 server) and here are the results:
nas:~# ifconfig eth0 mtu 1500
nas:~# iperf -N -c server.lan.froehlich.ws -t 60
------------------------------------------------------------
Client connecting to server.lan.froehlich.ws, TCP port 5001
TCP window size: 148 KByte (default)
------------------------------------------------------------
[ 3] local 172.24.0.70 port 41403 connected with 172.24.0.71 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-60.0 sec 3.26 GBytes 466 Mbits/sec
nas:~# ifconfig eth0 mtu 9000
nas:~# iperf -N -c server.lan.froehlich.ws -t 60
------------------------------------------------------------
Client connecting to server.lan.froehlich.ws, TCP port 5001
TCP window size: 73.9 KByte (default)
------------------------------------------------------------
[ 3] local 172.24.0.70 port 56614 connected with 172.24.0.71 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-60.0 sec 5.78 GBytes 827 Mbits/sec
As you can see tcp throughput has almost doubled when using jumbo
frames; even under the circumstances that software instead of hardware
does now calculate all the tcp checksums.
output of top, 9000 mtu
top - 20:11:14 up 1:05, 2 users, load average: 1.13, 0.36, 0.17
Tasks: 91 total, 1 running, 90 sleeping, 0 stopped, 0 zombie
Cpu(s): 1.7%us, 38.6%sy, 0.0%ni, 0.7%id, 0.0%wa, 2.0%hi, 57.1%si,
Mem: 515924k total, 209248k used, 306676k free, 57092k buffers
Swap: 1646620k total, 0k used, 1646620k free, 82104k cached
and with 1500 mtu
top - 20:12:36 up 1:07, 2 users, load average: 0.55, 0.33, 0.17
Tasks: 91 total, 1 running, 90 sleeping, 0 stopped, 0 zombie
Cpu(s): 4.0%us, 24.8%sy, 0.0%ni, 0.0%id, 0.0%wa, 5.0%hi, 66.2%si,
Mem: 515924k total, 223844k used, 292080k free, 57188k buffers
Swap: 1646620k total, 0k used, 1646620k free, 98436k cached
Seems as if the systems is under full load even when using hardware
checksumming. So the hardware checksum engine does not really have any
advantages compared to sw crc and 9k mtu.
Thanks a lot for your help. Maybe this patch could be merged into
debian's default kirkwood kernel before they freeze squeeze some day.
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