linux-kernel - Re: txqueuelen has wrong units; should be time

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Date:	Sun, 27 Feb 2011 21:07:53 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jussi Kivilinna <jussi.kivilinna@...et.fi>
Cc:	Albert Cahalan <acahalan@...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

Le dimanche 27 février 2011 à 12:55 +0200, Jussi Kivilinna a écrit :
> 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).
> 


Qdisc should return to caller a good indication packet is queued or
dropped at enqueue() time... not later (aka : never)

Accepting a packet at t0, and dropping it later at t0+limit without
giving any indication to caller is a problem.

This is why I suggested using an EWMA plus a probabilist drop or
congestion indication (NET_XMIT_CN) to caller at enqueue() time.

The absolute time limit you are trying to implement should be checked at
dequeue time, to cope with enqueue bursts or pauses on wire.



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