linux-kernel - Re: [PATCH] NFSv4: Use exponential backoff delay for NFS4

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Message-ID: <20130425181932.GA5049@fieldses.org>
Date:	Thu, 25 Apr 2013 14:19:32 -0400
From:	"bfields@...ldses.org" <bfields@...ldses.org>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	David Wysochanski <dwysocha@...hat.com>,
	Dave Chiluk <chiluk@...onical.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] NFSv4: Use exponential backoff delay for NFS4_ERRDELAY

On Thu, Apr 25, 2013 at 02:10:36PM +0000, Myklebust, Trond wrote:
> On Thu, 2013-04-25 at 09:49 -0400, bfields@...ldses.org wrote:
> > On Thu, Apr 25, 2013 at 01:30:58PM +0000, Myklebust, Trond wrote:
> > > On Thu, 2013-04-25 at 09:29 -0400, bfields@...ldses.org wrote:
> > > 
> > > > My position is that we simply have no idea what order of magnitude even
> > > > delay should be.  And that in such a situation exponential backoff such
> > > > as implemented in the synchronous case seems the reasonable default as
> > > > it guarantees at worst doubling the delay while still bounding the
> > > > long-term average frequency of retries.
> > > 
> > > So we start with a 15 second delay, and then go to 60 seconds?
> > 
> > I agree that a server should normally be doing the wait on its own if
> > the wait would be on the order of an rpc round trip.
> > 
> > So I'd be inclined to start with a delay that was an order of magnitude
> > or two more than a round trip.
> > 
> > And I'd expect NFS isn't common on networks with 1-second latencies.
> > 
> > So the 1/10 second we're using in the synchronous case sounds closer to
> > the right ballpark to me.
> 
> OK, then. Now all I need is actual motivation for changing the existing
> code other than handwaving arguments about "polling is better than flat
> waits".
> What actual use cases are impacting us now, other than the AIX design
> decision to force CLOSE to retry at least once before succeeding?

Nah, I've got nothing, and I agree that the AIX problem is there bug.

Just for fun I looked at re-checked the Linux server cases.  As far as I
can tell they are:

	- delegations: returned immediately on detection of any
	  conflict.  The current behavior in the sync case looks
	  reasonable to me.
	- allocation failures: not really sure it's the best error, but
	  it seems to be all the protocol offers.  We probably don't
	  care much what the client does in this case.
	- some rare cases that would probably indicate bugs (e.g.,
	  attempting to destroy a client while other rpc's from that
	  client are running.)  Again we don't care what the client does
	  here.
	- the 4.1 slot-inuse case.

We also by default map four errors (ETIMEDOUT, EAGAIN, EWOULDBLOCK,
ENOMEM) to delay.  I thought I remembered one of those being used by
some HFS system, but can't actually find an example now.  A quick grep
doesn't show anything interesting.

--b.
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