FW: Performance issue ( maybe ) corrected

Simon Hobson dhcp1 at thehobsons.co.uk
Fri Sep 3 15:36:54 UTC 2010

Bjarne Blichfeldt wrote:

>During the problem I see something like (numbers added from memory):
>Cpu0  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
>Cpu1  :  0.3%us,  0.3%sy,  0.0%ni, 99.3%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
>è Cpu2  :  0.0%us,  0.3%sy,  0.0%ni, 1.3%id,  98%wa,  0.0%hi,  0.0%si,  0.0%st
>Cpu3  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
>Mem:   2075208k total,   714656k used,  1360552k free,   189160k buffers
>Swap:  4128760k total,      100k used,  4128660k free,   318604k cached


>The next I have done is to increase default lease time from 8 hours to 5 days.


>1352 subnets
>792 pools
>around 7500 active leases

That works out at one lease renewal every around 
1.5 to 2 seconds, not exactly a heavy load. 
However, there are two things that can cripple 
the server under heavy I/O :

Log files. Configure syslog to use async logs for 
whatever dhcp logs end up in. Otherwise, every 
discover, offer, request, and ack results in a 
wait while the log entry is synced out.

Leases file. Again, every offer or ack is 
preceded by writing to the leases file. If the 
machine is liable to heavy I/O from other 
processes then it may be worth putting the leases 
file on it's own storage so dhcpd doesn't find 
itself stuck in an I/O queue.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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