Need help

Alan DeKok aland at
Mon Jan 15 21:16:46 CET 2007

Peter Nixon wrote:
> While there IS a difference in speed between writing to an on-disk spool and 
> your method (in memory queue) our method is "correct" and a dedicated disk 
> (or raid set) is more than fast enough to keep up with thousands of requests 
> per second. (I haven't benchmarked it recently but I suspect we are in 100K 
> requests per second territory here depending on disk spindle speed, 
> filesystem and cache configuration)

  To put it another way, if a system doesn't have to be correct, you can
make it run as fast as you want.

  Never underestimate that speed of a process that does "exit 0".  It's
not correct, but *man* is it fast!

> Basically a queue of more than a second (or the timeout configured on your 
> NAS) is worse than sending an Authentication reject to a couple of users as 
> the whole thing just snowballs! An Auth queue only helps in the case where 
> you have a huge peak of requests that cannot be serviced simultaneously but 
> CAN be serviced quicker than the configurable timeout of your NAS. If you 
> continually have a deep queue then you need to increase the speed of your 
> backend.

  The CVS head tries to be a little smarter.  The priority is:

a) replies from proxies (i.e. ongoing requests)
b) requests with State attribute (i.e. EAP)
c) everything else.

  This means that people get to finish their authentications.

> I would be interested to see you run a benchmark to show that your algorithm 
> for dealing with newest Auth request first is actually a performance gain in 
> a high load environment. (I suspect that it will make very little difference 
> compared with out caching system) If it does make a considerable difference, 
> then of course we would consider adding a similar feature to FreeRADIUS. 
> (ie. Please prove to us that what you have done is actually better!)

  Dealing the the newest request first is *bad*.  FreeRADIUS puts
requests into a queue, and discards requests from the queue if it
determines that they've timed out, or that the NAS has given up on them.

  Priority (or time-based) queues stabilize networks.  FILO queues do not.

  Alan DeKok.
--       - The web site of the book - The blog

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