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From: liam@cs.qmw.ac.uk (William Roberts;)
Newsgroups: comp.protocols.nfs
Subject: Re: how many nfsd's should I run?
Message-ID: <2936@redstar.cs.qmw.ac.uk>
Date: 25 Feb 91 19:34:40 GMT
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In <4218@skye.ed.ac.uk> richard@aiai.ed.ac.uk (Richard Tobin) writes:

>In article <1991Feb22.012532.26075@murdoch.acc.Virginia.EDU> 
gl8f@astsun7.astro.Virginia.EDU (Greg Lindahl) writes:
>>If you have too many processes competing for the limited slots in the
>>hardware context cache, your machine will roll over and die.

>Given that nfsd runs in kernel mode inside nfssvc(), is this statement
>about contexts correct?  If so, why is the default number of nfsds for
>Sun 3s 8?

1) Hardware contexts are a feature of the MMU/Instruction cache, and so Greg's 
comment is specific to Sun4 machines.

2) The nfssvc() system call never returns, but the process slot of the caller 
is used as a cheap way to implement multi-threading in the kernel. When an 
nfsd runs out of work, it does a sleep() waiting for requests to come in on 
the socket bound to UDP port 2049. When it gets handles a request involving 
disk I/O, it sleeps waiting for the result. All nice straightforward kernel 
stuff provided that you have a process table entry to work with, but 
impossible if you don't. Using a kernel like Mach or Chorus it would be done 
with true multi-threading of the code, so lightweight process switches would 
be the only overhead; under most UNIX systems it takes the full context switch 
and hence can thrash the hardware contexts (if applicable).


Historical note: prior to SunOS 3.2, there was no wakeupone() routine and so 
ALL sleeping nfsd processes would be made runnable when a request came in. One 
would get to handle the request, and the others would needlessly run, find 
nothing to do and go back to sleep. Under this scheme it was possible to find 
that the file server got slower at night, and needless to say it was 
impossible to choose a "good number" since there were penalties for having too 
many for the current load. I once tried 30 nfsds on a WhiteChapel MG1 server: 
it stopped.

Nowadays only one nfsd will wake up for an incoming request, hence the change 
in "good number" from "4" to "4 or more". One plausible way to determine how 
many nfsds you need is to ask for lots, then see how many use up CPU time (NB. 
this depends on sleep queues being managed unfairly, so it might not work 
these days).
--

William Roberts                 ARPA: liam@cs.qmw.ac.uk
Queen Mary & Westfield College  UUCP: liam@qmw-cs.UUCP
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