Date:   Wed, 20 Feb 2019 23:18:20 -0500
To:     James Pearson <jcpearson@gmail.com>
Cc:     linux-nfs@vger.kernel.org
Subject: Re: nfsd thread limit and UDP ?
Message-ID: <20190221041820.GA4625@fieldses.org>
References: <CAK3fRr9EL-YOp3yUr5tMe5zuiERxHjkLDhAaWkxxEHjtfSwisQ@mail.gmail.com>
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From:   bfields@fieldses.org (J. Bruce Fields)
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On Wed, Feb 20, 2019 at 11:28:53AM +0000, James Pearson wrote:
> On a very busy NFSv3 server (running CentOS 6), we recently upped the
> nfsd thread count to 1024 - but this caused client mount requests over
> UDP to fail.
> 
> We configure all our clients to use TCP for NFS mounts, but the
> automounter (automountd) on MacOS (up to version MacOS 10.12) seeds a
> 'null call' to the NFS server over UDP before attempting the mount -
> but the server appears to ignore any UDP requests - and the automount
> fails

By the way, you might also just turn off UDP.  (Start run rpc.nfsd with
the -U option.)  Hopefully MacOS can handle that case.

--b.

> 
> I can also reproduce the issue on a Linux client via:
> 
>  mount -o udp,nfsvers=3 server:/export /mount/point
> 
> I've found, by trial and error, that the maximum number of nfsd
> threads that can be run on the server is 1017 before UDP mount
> requests fail
> 
> Running tcpdump on the server shows the UDP requests from the client,
> but the server never replies
> 
> It looks like more recent versions of MacOS will do its test 'null
> call' over TCP - so that is one 'solution' to this issue
> 
> However, I'm interested to know if we're hitting some hard limit, or
> if there are any settings we can tweak that could mitigate the
> problem?
> 
> Thanks
> 
> James Pearson