Over-running gc_thresh3

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Sep 3 22:18:35 UTC 2009

Peter Laws wrote:
>Had to plumb up a new interface on the DHCP server (running 
>dhcp-3.0.1-10.2_EL3 - I know, old and bad on several counts!) and 
>it's now directly exposed to a network with /19 hungry clients.
>So, we're overrunning the above-named kernel threshold, which is 
>defaulted to 1024 (high-water mark for ARP table entries for those 
>of you who, like me at this time yesterday, had never heard of that 

Now you mention it, I hit this problem at home a while ago when I 
first fired up Azureus (bit torrent client) - the router (Linux box) 
threw it's hand up at all the remote addresses it was being asked to 
keep track of. I think I bumped them up fairly high - I currently 
have 2048 and 4096 for gc_thresh2 and gc_thresh3.

>Any suggestions on what this should be bumped to?  It's exposed to a 
>/19, so 8k+ potential clients (we're running around 7k leases) so 
>1024 isn't enough and we get this stuff:
>Sep  3 13:21:24 $HOST dhcpd: send_packet: No buffer space available
>... And ...
>Sep  3 13:21:21 love kernel: Neighbour table overflow.
>... when the water gets high enough.
>I don't think it needs to go to 8192, because I've been watching the 
>ARP cache and it only hits 1024 every few hours.  Would 2048 be 
>I'm assuming I can just echo the new value to the kernel, but will 
>it make anything hiccup or just up the thresholds (there are 
>actually three variables that need updating)?

IIRC I was able to just change this on the fly. You would also want 
to change gc_thresh2, and gc_thresh1 apparently isn't used. I had a 
quick look but didn't find a maximum, but I can't see that it would 
harm anything by going all the way to 8192 - it'll just use a bit 
more memory. You could try a lower value and just keep an eue out for 
Neighbour table overflow messages.

While looking it up, I did come across this which makes for a good 
read if you like details like that :

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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