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From: lm@slovax.Berkeley.EDU (Larry McVoy)
Newsgroups: comp.protocols.nfs,comp.arch
Subject: Re: how many nfsd's should I run?
Message-ID: <476@appserv.Eng.Sun.COM>
Date: 1 Mar 91 21:37:30 GMT
References: <28975@cs.yale.edu> <PCG.91Feb28195347@odin.cs.aber.ac.uk>
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In article <PCG.91Feb28195347@odin.cs.aber.ac.uk>, 
pcg@cs.aber.ac.uk (Piercarlo Grandi) writes:
|> richard> Given that nfsd runs in kernel mode inside nfssvc(), is this
|> richard> statement about contexts correct?
|> 
|> Yes and no, depending on who is your vendor, and which OS revision and
|> machine model you have. For Sun there is some history that may be worth
|> mentioning. Under SunOS 3 the nfsds were in effect kernel processes, so
|> that they could access the buffer cache, held in the kernel address
|> space, without copies. Since all nfsds run in the kernel page table
|> there was no problem.
|> 
|> Under SunOS 4 the buffer cache went away, so each nfsd was given its own
|> address space (memory mapped IO), while still being technically a kernel
|> process. This meant that MMU slot thrashing was virtually guaranteed, as
|> the nfs daemons are activated more or less FIFO and the MMU has a LIFO
|> replacement policy. As soon as the number of nfsds is greater or equal
|> to the number of MMU slots problems happen.

More misinformation spoken in an authoritative manner.  Ah, well, I've done 
the same I guess.  From sys/nfs/nfs_server.c:

nfs_svc()
....
        /* Now, release client memory; we never return back to user */
        relvm(u.u_procp);

From the SCCS history (note the date):

	D 2.83 87/12/15 18:34:42 kepecs 88 87
	remove virtual memory in async_daemon and nfs_svc as
	it's not needed. Remove pre-2.0 code to set rdir in nfs_svc.
	make sure these guys exit if error, since no vm to return to.

In other words, this problem went away 3 years ago, never to return.

|> I have seen the same server running the same load under SunOS 3 the day
|> before with 10-20% system time and 100-200 context switches per second,
|> and with SunOS 4 the day after with 80-90% system time and 800-900
|> context switches per second. An MMU slot swap on a Sun 3 will take about
|> a millisecond, which fits.

You may well have seen this.  Jumping to the conclusing that it is caused
by NFS is false, at least the reasons that you list are not true.

|> richard> If so, why is the default number of nfsds for Sun 3s 8?
|> 
|> Sun bogosity :-).

Piercarlo ignorance :-)

|> The latter argument is somewhat doubtful as there is contradictory
|> evidence about the relative merits of FCFS and of elevator style sorting
|> as used by the Unix kernel.

Ahem.  References, please?  I've looked at this one in detail, this should be interesting.

---
Larry McVoy, Sun Microsystems     (415) 336-7627       ...!sun!lm or lm@sun.com