Bug #352

A factor of 4 disimprovement, and mere 60Mbit performance on ipv6 is not terrible but fixing it will require oprofiling and deeper investigation.

I’ve seen a similar problem on ipv6 tunnels.

ipv6 runs at less than half the speed as ipv4.

CPU: MIPS 24K, speed 678 MHz (estimated)
Counted INSTRUCTIONS events (1-0 Instructions completed) with a unit
mask of 0x00 (No unit mask) count 100000
samples  %        app name                 symbol name
——————————————————————————-
6703     22.7367  vmlinux                  do_ade
 6703     100.000  vmlinux                  do_ade [self]
——————————————————————————-
3754     12.7336  ip6_tables               /ip6_tables
 3754     100.000  ip6_tables               /ip6_tables [self]
——————————————————————————-
2834      9.6130  ipv6                     /ipv6
 2834     100.000  ipv6                     /ipv6 [self]
——————————————————————————-
2240      7.5981  vmlinux                  __copy_user
 2240     100.000  vmlinux                  __copy_user [self]
——————————————————————————-
2164      7.3403  vmlinux                  csum_partial
 2164     100.000  vmlinux                  csum_partial [self]

m@cruithne:~\$ iperf -V -c fd25:b7ff:b098:8001::1
————————————————————
Client connecting to fd25:b7ff:b098:8001::1, TCP port 5001
TCP window size: 64.6 KByte (default)
————————————————————
[  3] local fd43:e2b3:341c:5071::1 port 35233 connected with
fd25:b7ff:b098:8001::1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec   185 MBytes   155 Mbits/sec

And on ipv6, we get a lot of them

———- Forwarded message ———-
From: Felix Fietkau nbd@openwrt.org
Date: Sat, Feb 18, 2012 at 8:41 AM
Subject: Re: oprofile magic? (ipv6 terrible)
To: Dave Taht dave.taht@gmail.com

On 2012-02-18 5:35 PM, Dave Taht wrote:
> I was curious as to what oprofiling magic tricks you used
> to track down the unaligned exception traps. I haven’t
> touched oprofile since august…
>
> ipv6 runs at about half the speed as ipv4.
I’m not using oprofile to track down such traps, there’s a much easier
way: echo 2 > /sys/kernel/debug/mips/unaligned_action
After that, the kernel will throw a stack trace for every single
unaligned exception. You can then run gdb to track down the line of code
that triggered the exception, assuming you’ve enabled debug symbols for
the kernel.
Just enter l *+
When it’s trapping based on access to a struct, just mark the struct as
__packed in a patch.

• Felix

I’m going to dismiss the slight lowering of ipv4 as statistical noise.

d@io:~\$ iperf -t 60 -c 149.20.63.27
————————————————————
Client connecting to 149.20.63.27, TCP port 5001
TCP window size: 64.0 KByte (default)
————————————————————
[ 3] local 149.20.63.20 port 35441 connected with 149.20.63.27 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-60.0 sec 1.83 GBytes 262 Mbits/sec

d@io:~\$ iperf -V -t 60 -c 2001:4f8:3:203::217
————————————————————
Client connecting to 2001:4f8:3:203::217, TCP port 5001
TCP window size: 64.0 KByte (default)
————————————————————
[ 3] local 2001:4f8:3:203::14 port 44866 connected with 2001:4f8:3:203::217 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-60.0 sec 1.29 GBytes 185 Mbits/sec

This bug was too epic in scope. Please do not delete any patches, they are useful to have around as blind alleys.