netdev - Re: Linux 2.6.27.5 / SFQ/HTB scheduling problems

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Date:	Fri, 14 Nov 2008 07:21:14 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Linux Networking Mailing List <netdev@...r.kernel.org>
Cc:	Sami Farin <safari-kernel@...ari.iki.fi>
Subject: Re: Linux 2.6.27.5 / SFQ/HTB scheduling problems

On 13-11-2008 15:29, Sami Farin wrote:
...
> Oh, I had old ethtool..
> These with v 6:
> 
> # ethtool -k eth0
> Offload parameters for eth0:
> rx-checksumming: off
> tx-checksumming: off
> scatter-gather: off
> tcp segmentation offload: off
> udp fragmentation offload: off
> generic segmentation offload: on
> 
> Wow.  I turned gso off and now it works just like before.
> No packets over size of mtu anymore, either.
> 
> State       Recv-Q Send-Q               Local Address:Port                 Peer Address:Port
> ESTAB      0      122334        80.223.84.180:57694        74.54.226.166:80     timer:(on,4.475ms,0) uid:518 ino:4546485 sk:2ea3ac80ffff8800
>          ts sackcubic wscale:8,3 rto:4499 rtt:2914.12/378.5 cwnd:134 ssthresh:30 send 522.4Kbps rcv_space:5728
> 
> --- 84.250.192.1 ping statistics ---
> 50 packets transmitted, 50 received, 0% packet loss, time 1877ms
> rtt min/avg/max/mdev = 22.463/38.354/99.126/13.738 ms, pipe 3, ipg/ewma 38.309/34.963 ms
> 
> ... 
>> Anyway these TSO/GSO/jumbo_frames are usually bad idea with packet
>> schedulers (or need more tweaking - e.g. sfq quantum).
> 
> Well this GSO on by default was an unexpected surprise for me, for sure.
> 
> Maybe a warning on htb (or something) module load
> "GSO enabled, everything might not work as expected, try 
> 'ethtool -K eth0 gso off' to turn GSO off"..? ;)

I agree, such a warning could be useful.

BTW, r2q == 1 in your config could harm latency too; if it's because
of quantum warnings, usually (if there are big rate differences
between classes) it's better to use htb class's "quantum" parameter
(tc class add htb help).

Jarek P.

PS: it seems your mailer setting prevents adding your email as "To:"
while doing "Reply All". Sometimes it's intended, but not the most
popular on this list (e.g. I'd prefer to be "To/Cc-ed", because I'm
not a subscriber at the moment).
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