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

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Message-ID: <20110227125540.40754c5y78j9u2m8@hayate.sektori.org>
Date:	Sun, 27 Feb 2011 12:55:40 +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 2:54 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> Le dimanche 27 février 2011 à 08:02 +0100, Mikael Abrahamsson a écrit :
>>> On Sun, 27 Feb 2011, Albert Cahalan wrote:
>>>
>>> > Nanoseconds seems fine; it's unlikely you'd ever want
>>> > more than 4.2 seconds (32-bit unsigned) of queue.
> ...
>> Problem is some machines have slow High Resolution timing services.
>>
>> _If_ we have a time limit, it will probably use the low resolution (aka
>> jiffies), unless high resolution services are cheap.
>
> As long as that is totally internal to the kernel and never
> getting exposed by some API for setting the amount, sure.
>
>> I was thinking not having an absolute hard limit, but an EWMA based one.
>
> The whole point is to prevent stale packets, especially to prevent
> them from messing with TCP, so I really don't think so. I suppose
> you do get this to some extent via early drop.

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).

-Jussi
View attachment "sch_fifo_to.c" of type "text/x-csrc" (6139 bytes)