DHCP server answers with delay

John Wobus jw354 at cornell.edu
Fri Apr 4 20:21:21 UTC 2014

On Apr 3, 2014, at 6:47 AM, Simon Hobson wrote:
> Denis Taranushin <tda at suttk.ru> wrote:
>> We have trouble with DHCP. namely the DHCP server answers with delay.
>> Why the server answers in 4 secs after the first "Discover"?
>> Unfortunately I can't provide the config because it is enormous.
> Has it occurred to you that the two may be related ? Is this a new  
> problem, or has it always been like that ? If it's new, has anything  
> changed ? How much memory is the DHCP process taking ? Does the host  
> have enough ? Is it swapping ? What is disk I/O like ?
> How many clients are being served ? Is DNS updating in use ?
> Can you provide a shortened form of the config ? Eg, if you have a  
> gazzillion class definitions, just provide a couple and note how  
> many there are. There are some things you can do in the config which  
> are "expensive" in resource usage or time, but without seeing it, no- 
> one here can comment.

Yes, definitely look at dhcpd performance.  A large input udp buffer and
an inadequate disk write transaction rate can cause delays.

See if the disk is constantly busy at the time the issue
is happening, and compare tcpdump timing with DHCP log

Disk transaction rate is critical for a busy dhcp server.

John Wobus
Cornell University IT

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