Load times for Large Subnets
David W. Hankins
David_Hankins at isc.org
Wed Oct 10 17:01:18 UTC 2007
On Tue, Oct 09, 2007 at 04:35:02PM -0700, Larry Apolonio wrote:
> Oct 9 16:32:11 dhcp-amd dhcpd: Lease IP hash: Contents/Size (%):
> 410524/100003 (410%). Min/max: 2/7
> Oct 9 16:32:11 dhcp-amd dhcpd: Lease UID hash: Contents/Size (%):
> 200000/100003 (199%). Min/max: 0/101
> Oct 9 16:32:11 dhcp-amd dhcpd: Lease HW hash: Contents/Size (%):
> 200000/100003 (199%). Min/max: 0/5
vs:
> Oct 9 15:40:25 dhcp-amd dhcpd: Lease IP hash: Contents/Size (%):
> 410524/20000003 (2%). Min/max: 0/1
> Oct 9 15:40:26 dhcp-amd dhcpd: Lease UID hash: Contents/Size (%):
> 200000/20000003 (0%). Min/max: 0/101
> Oct 9 15:40:26 dhcp-amd dhcpd: Lease HW hash: Contents/Size (%):
> 200000/20000003 (0%). Min/max: 0/1
It's curious that the UID hash turns out the same in both tables...and
so horribly wrong; max of 101 entries in one bucket!
It's hard to tell if it's going more wrong in the smaller table size.
I should have put in mean/std.dev as well.
I wouldn't have suspected such a big difference in performance for
such a (evidently) small change in hash distribution. I'd literally
expect to see more like '0/137<->0/1' than '0/7<->0/1' Possibly mean
and std.dev would give us a different idea of how much things have
shifted...I should have put those in the reporter but I was busy
looking at a different corner of the problem.
Could you mail your dhcpd.conf and dhcpd.leases to dhcp-bugs at isc.org
(or me gpg encrypted)? When we have time, it looks like we need to
look more closely at our "opaque string of bits" hash function. 0/5
is really quite bad at 200% load.
> What do those IP hash %ages mean?
That's how loaded the table is. Just doing the math for the
contents/size so I didn't have to do it in my head.
At 410%, I would have expected to see a min/max of 4/5 for perfect
distribution. 2/7 isn't so far away from that...nothing that would
explain the ludicrous shift in performance. The "0" in the uid/hw
hashes is pretty telling; some buckets aren't getting any leases at
all.
--
Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
Why settle for the lesser evil? https://secure.isc.org/store/t-shirt/
--
David W. Hankins "If you don't do it right the first time,
Software Engineer you'll just have to do it again."
Internet Systems Consortium, Inc. -- Jack T. Hankins
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