Re: monitors received out of order

[Tim Mooney <mooney@aps.anl.gov>]

Nick,

As I understand the responses, each record has a per-subscription queue 
(actually a set
of queues, but they behave as a single queue for time-ordering purposes).

I don't recall ever being told that monitors are guaranteed to be received in 
order.  In fact,
I think I started with the assumption that this was not guaranteed, and got 
lulled over time
into a false sense of security because vxWorks is so nimble.

This is not a one-in-a-million problem.  On solaris, I'm seeing the raw symptom 
about 7%
of the time, from software that I would guess is not exceptionally vulnerable to 
it (e.g.,
switches threads more than once between offending db_post_events calls).  I haven't
measured systematically, but so far the largest time "reversal" I've seen is 
around 280
microseconds.

For clarity and completeness, I probably should add that this software is not in the
regime suggested in Jeff's reply (intermediate events discarded; producer outpaces
consumer).  That's not possible in this case, because the events are what drive the
software from one state to the next; their number and sequence is known, and all of
them appear in the diagnostic output.

About your snapshot: in a roughly analogous application, I'm doing a get_callback
on all the PVs in the list after I know that they all must contain up-to-date 
values, to
sweep up any time crumbs.  I haven't seen any need to do this within a single IOC,
but I have noticed arrays from different IOCs coming in late.

Tim

On 11/2/2010 4:26 AM, nick.rees@diamond.ac.uk wrote:
> Hi All,
>
> I read Tim's initial post with interest because I am sure that I was told at an early EPICS training course (probably in 1992 or 93) that monitors posted to a given client within a given lockset were guaranteed to be received in order (and, conversely, ca_puts from a client were delivered in order). I have since used this for a lot of systems that need to collect a coherent block of information. I have not known it to fail, but I haven't looked for the 1 in a million problem, just the 1 in 100 or so. Now this old wives tale is clearly being challenged.
>
> I find Jeff's response ambiguous, and maybe that's the problem. Jeff says there is a per client event queue. Tim seems to imply it is per-subscription. Ralf seems to imply the truth is somewhere in between, and since it is more specific it has the ring of truth about it.
>
> Was my memory ever true? I have a suspicion that by the tone of his message Tim may have been labouring under the same misapprehension. It may have changed, for example, when we lifted the fixed (read 16k) CA limit. I would agree with Tim in that in my experience it is true most of the time - especially on vxWorks, and since I have mostly used it to get synchronised hard real-time data from a server to a client, vxWorks is where I have used it.
>
> I am not sure what to do about it now, but I definitely have a frequent requirement to get a coherent set of data from a server. Up till now, I have monitored everything and then taken a snapshot whenever I got a monitor from the last item in a processing chain (which I have ensured always updates whenever the chain processes). This has seemed to work with VxWorks servers. I would like to still be able to achieve the same effect. How else can I do this?
>
> Cheers,
>
> Nick Rees
> Principal Software Engineer           Phone: +44 (0)1235-778430
> Diamond Light Source                  Fax:   +44 (0)1235-446713
>
>
> -----Original Message-----
> From: tech-talk-bounces@aps.anl.gov [mailto:tech-talk-bounces@aps.anl.gov] On Behalf Of Jeff Hill
> Sent: 01 November 2010 21:42
> To: 'Tim Mooney'; 'EPICS tech-talk'
> Subject: RE: monitors received out of order
>
> Hi Tim,
>
> The Channel Access publish and subscribe system is a classic
> consumer/producer situation. The producer is of course record
> processing and the consumer is the Channel Access client side
> application. In basic queuing theory, it is known that no
> matter how big the finite buffer capacity between the producer
> and the consumer, all of it will be consumed if the production
> rate is even a marginal amount faster than the consumption rate.
> Furthermore, to prevent one client from disrupting subscription
> updates sent to another client, each client attaching to an IOC
> is serviced out of a per-client dedicated event queue connecting
> the higher priority database processing threads with the lower
> priority per-client server threads. So to avoid consuming all
> available memory this event queue is finite length, and if full
> capacity is reached the last event on the queue for a particular
> subscription is replaced; this has the net impact of causing an
> intermediate subscription update to be discarded.
>
> So one can easily see, given the above design with intermediate
> events being discarded when the producer out-paces the consumer,
> that events will sometimes be delivered out of order when comparing
> different channel subscriptions, but that events will never be
> delivered out of order within a single channel subscription.
>
> HTH
>
> Jeff
> ______________________________________________________
> Jeffrey O. Hill           Email        johill@lanl.gov
> LANL MS H820              Voice        505 665 1831
> Los Alamos NM 87545 USA   FAX          505 665 5107
>
> Message content: TSPA
>
> With sufficient thrust, pigs fly just fine. However, this is
> not necessarily a good idea. It is hard to be sure where they
> are going to land, and it could be dangerous sitting under them
> as they fly overhead. -- RFC 1925
>
>
> > -----Original Message-----
> > From: tech-talk-bounces@aps.anl.gov [mailto:tech-talk-bounces@aps.anl.gov]
> > On Behalf Of Tim Mooney
> > Sent: Monday, November 01, 2010 3:10 PM
> > To: EPICS tech-talk
> > Subject: monitors received out of order
> >
> > Dear folks,
> >
> > I have two records, and a separate task monitoring a field that both
> > records
> > post, all in the same IOC.  Most of the time, my task receives monitors in
> > the
> > order in which they were posted.  But sometimes, the task receives a
> > monitor
> > from record B before it receives a previously posted monitor from record
> > A.
> > (I know for sure which record is posting first, because record A posts
> > before
> > causing record B to process.  Also, I've modified the record to set its
> > time
> > stamp immediately before posting, and I get another time stamp on entry to
> > the monitor routine.  The time ordering of those stamps does not agree.)
> > I've seen this on solaris and Linux, but not on vxWorks.
> >
> > I have code that misbehaves when this happens, so I started digging around
> > and have convinced myself that I should not be relying on the time
> > ordering of
> > monitors received from different records (even when those records are
> > running
> > in the same task).  I now think I can rely on posts from a single record
> > arriving in
> > time order, but not posts from different records.  I think this because
> > events from
> > different record go into different queues, and there doesn't seem to be
> > any code
> > in the vicinity that seems worried about time ordering across event
> > queues.
> >
> > Am I right about this?
> >
> > --
> > Tim Mooney (mooney@aps.anl.gov) (630)252-5417
> > Software Services Group, Advanced Photon Source, Argonne National Lab.

--
Tim Mooney (mooney@aps.anl.gov) (630)252-5417
Software Services Group, Advanced Photon Source, Argonne National Lab.