netdev - Re: txqueuelen has wrong units; should be time

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Date:	Mon, 28 Feb 2011 13:23:41 +0200
From:	Jussi Kivilinna <jussi.kivilinna@...et.fi>
To:	Albert Cahalan <acahalan@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Mikael Abrahamsson <swmike@....pp.se>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: txqueuelen has wrong units; should be time

Quoting Albert Cahalan <acahalan@...il.com>:

> On Sun, Feb 27, 2011 at 5:55 AM, Jussi Kivilinna
> <jussi.kivilinna@...et.fi> wrote:
>
>> I made simple hack on sch_fifo with per packet time limits (attachment) this
>> weekend and have been doing limited testing on wireless link. I think
>> hardlimit is fine, it's simple and does somewhat same as what
>> packet(-hard)limited buffer does, drops packets when buffer is 'full'. My
>> hack checks for timed out packets on enqueue, might be wrong approach (on
>> other hand might allow some more burstiness).
>
> Thanks!
>
> I think the default is too high. 1 ms may even be a bit high.

Well, with 10ms buffer timeout latency goes to 10-20ms on 54Mbit wifi  
link (zd1211rw driver) from >500ms (ping rtt when iperf running same  
time). So for that it's good enough.

>
> I suppose there is a need to allow at least 2 packets despite any
> time limits, so that it remains possible to use a traditional modem
> even if a huge packet takes several seconds to send.
>

I made EWMA version of my fifo hack (attached). I added minimum 2  
packet queue limit and  probabilistic 1% ECN marking/dropping for  
timeout/2.

-Jussi


View attachment "sch_fifo_ewma.c" of type "text/x-csrc" (7810 bytes)