linux-kernel - Re: how to tweak ARP timer on kernel 2.6.9

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Date:	Tue, 09 Mar 2010 23:10:14 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Ammon <tom.ammon@...h.edu>
Cc:	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: how to tweak ARP timer on kernel 2.6.9

Le mardi 09 mars 2010 à 12:32 -0700, Tom Ammon a écrit :
> Hi,
> 
> I've been trying to figure out how to adjust the ARP timeout on kernel 
> 2.6.9 and I found the following in /proc/sys/net/ipv4/neigh/ib0 (its an 
> IB interface I am interested in changing) with the following values. 
> This is on kernel 2.6.9-89ELsmp (RHEL4) :
> 

> When I test this, along with per-flow ECMP (using the iproute2 utils), I 
> see that the ARP cache is timing out about every 10 minutes (I observe 
> this by load balancing an iperf flow between two different gateway 
> machines and then graphing the interface traffic)
> 

10 minutes seems out of specs.

> On a newer kernel, 2.6.18-164.11.1.el5 (RHEL5), I see mostly the same 
> parms available, but a few new ones have been added. However, all of the 
> parms that are the same name between the two kernels are the same values:
> 

> Yet when I observe the same traffic flow with this machine, the ARP 
> cache times out about once per minute.
> 

This seems normal, old kernels were a bit lazy about arp.


> Is there another set of parameters somewhere that govern how often the 
> kernel times out the ARP cache? If so, where might I find that? Is there 
> any kernel documentation that talks about changing ARP timers on the 
> linux kernel?
> 

I would suggest :

man 7 arp

http://linux.die.net/man/7/arp



base_reachable_time
        Once a neighbour has been found, the entry is considered to be
        valid for at least a random value between base_reachable_time/2
        and 3*base_reachable_time/2. An entry's validity will be
        extended if it receives positive feedback from higher level
        protocols. Defaults to 30 seconds.

gc_interval
        How frequently the garbage collector for neighbour entries
        should attempt to run. Defaults to 30 seconds.

gc_stale_time
        Determines how often to check for stale neighbour entries. When
        a neighbour entry is considered stale it is resolved again
        before sending data to it. Defaults to 60 seconds.
        


and "ip -s neigh"


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