netdev - Re: TCP keepalive timer problem

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Message-ID: <20090827142927.GA17220@basil.fritz.box>
Date:	Thu, 27 Aug 2009 16:29:27 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Li_Xin2@....com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: TCP keepalive timer problem

On Thu, Aug 27, 2009 at 04:17:10PM +0200, Eric Dumazet wrote:
> Andi Kleen a écrit :
> > Eric Dumazet <eric.dumazet@...il.com> writes:
> >> Now, 7200 seconds might be inappropriate for special needs, and considering
> >> there is no way to change tcp_retries2 for a given socket (only choice being the global
> >> tcp_retries2 setting), I would vote for a change in our stack, to *relax* RFC,
> >> and get smaller keepalive timers if possible.
> > 
> > I think the better fix would be to just to only do that when
> > tcp_retries2 > keep alive time. So keep the existing behaviour
> > with default keep alive, but switch when the user defined
> > a very short keep alive.
> > 
> 
> tcp_retries2 is a number of retries, its difficult to derive a time from it.

That shouldn't be too hard. 

> 
> Also, it's not clear what behavior you are refering to.
> Imagine we can be smart and compute tcp_retries2_time (in jiffies) from tcp_retries2
> If keepalive_timer fires and we have packets in flight, what heuristic do you suggest ?

I didn't suggest to change something at firing time, just pattern
the code you removed with if (keepalive_time > retries2 time)

That's not perfect, but likely good enough.


> if (tp->packets_out || tcp_send_head(sk))
> 	if (tcp_retries2_time < keepalive_time_when(tp))
> 		goto resched;
> elapsed = tcp_time_stamp - tp->rcv_tstamp;
> ...
> 
> What would be the gain ?
> Arming timer exactly every keepalive_time_when(tp)
> instead of keepalive_time_when(tp) - (tcp_time_stamp - tp->rcv_tstamp) ?

The gain would be that you don't send unnecessary packets by default (following the RFC), but 
still give expected behaviour to users who explicitely set short keepalives.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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