FW: Performance issue ( maybe ) corrected

Bjarne Blichfeldt bjb at jndata.dk
Fri Sep 3 16:26:53 UTC 2010


> -----Original Message-----
> From: Simon Hobson
> Sent: 3. september 2010 17:37
> To: Users of ISC DHCP
> Subject: Re: FW: Performance issue ( maybe ) corrected
> 
> 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.
I agree, and I have a hard time believing the machines are overloaded, there must be something else, but the traces...

> 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.

Excellent suggestion, I have done that now.

> 
> 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.
Good suggestion, but unfortunately not immediately possible.

Thank you very much for your input.

regards,
Bjarne




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