netdev - Re: ARP table question

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Date:	Mon, 17 Nov 2008 17:39:45 -0800
From:	Rick Jones <rick.jones2@...com>
To:	Ben Greear <greearb@...delatech.com>
CC:	netdev@...r.kernel.org, Patrick McHardy <kaber@...sh.net>
Subject: Re: ARP table question

Ben Greear wrote:
> Rick Jones wrote:
> 
>>> +static unsigned long neigh_rand_retry(struct neighbour* neigh) {
>>> +    if (neigh->parms->retrans_rand_backoff) {
>>> +        return net_random() % neigh->parms->retrans_rand_backoff;
>>> +    }
>>> +    return 0;
>>> +}
>>> +
>>>  /* Called when a timer expires for a neighbour entry. */
>>
>>
>> I thought that mod was something we tried to avoid?  Could you instead 
>> use something that isn't random but perhaps varies among all the 
>> requests?  Say some of the low-order bits of the IP being resolved?
> 
> 
> This is only called when we are going to retransmit an ARP, which shouldn't
> be in any sort of hot path, so I figured MOD was fine.
> 
> The net_random is a very cheap method (last I checked), as well.
> 
> So, I think that part is OK as it is, but I'm open to
> persuasion :)

Perhaps I'm confused, or simply channeling Emily Litella again, but if 
you only do this on the 1st through Nth retransmissions (ie after the 
first retransmission timer has popped) don't you still have a thundering 
herd problem on the first transmission and the first retransmission of 
ARP requests?

rick jones
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