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From Rafael Schloming <...@alum.mit.edu>
Subject Re: messenger credit concern
Date Thu, 28 Feb 2013 20:11:57 GMT
On Tue, Feb 26, 2013 at 12:18 PM, Ken Giusti <kgiusti@redhat.com> wrote:

> Hi Mick
>
> ----- Original Message -----
> >
> > One hole that I feel like I'm seeing in the messenger
> > interface concerns credit.
> >
> > I have a way of using credit to set a max number of
> > messages that a recv call should return in one gulp,
> > or a way of doing ... something that I'm still figuring
> > out ... by setting credit=-1.
> >
> > What I don't have is any way of getting guidance about
> > what effect my credit allocation is having.
> >
> > A messenger app might have some flexibility in how
> > much time it spends servicing incoming messages vs.
> > time it spends doing its own processing, and it might
> > be able to allocate more time to servicing incoming
> > messages if it knows more about what's happening.
> >
> > Alternatively, it might want to set the credit allocated
> > per recv call based on the number of current incoming
> > links.  ( And assume that the credit will be distributed
> > round-robin across all incoming links. )
> >
> > Would it be practical / desirable / catastrophic
> > to expose current backlog or number of incoming links,
> > or both, at the messenger level ?
> >
> > Or would that violate part of the messenger API philosophy?
> > ( And if so, what is that philosophy?  I want to be able
> > to explain it. )
> >
>
> I feel your pain - the more I use the "recv(n)" interface, the more I
> think it's an awkward approach to the credit/flow issue.
>

I don't think it's aiming to solve a credt/flow issue at all. That's the
job of the messenger implementation. It's simply providing a way for the
application to ask for n messages.


>
> From what I understand, Messenger's model is based on two queues - one for
> incoming messages and one for outgoing messages.
>
> For flow control, the recv(n) method indicates how many messages are
> allowed on the incoming queue (max n).  But recv(n) is also used as the
> "pend for incoming messages" call.  So every time my app needs to fetch
> some messages, it also needs to compute flow control.  It's that dual use
> that I don't like.
>

As I mentioned above, I don't think recv should be thought of as a flow
control thing. It does provide input into what messenger does for flow
control, but it's really just about a way for the app to fetch messages,
and so far I've been considering three scenarios:

  (1) an app wants to receive and process messages indefinitely, in which
case pn_messenger_recv(-1) now does that job pretty nicely
  (2) an app wants to make a simple request/response, in which case it
wants to receive exactly one message back and getting anymore would be a
bug.
  (3) a generalized form of option (2) where an app makes N requests and
processes each one as they arrive back. In this case you have to do
pn_messenger_recv(N - what you've already processed).

I think these are probably the 3 most common scenarios, and I can see how
using a pattern like (3) to cater to scenario (1) would be awkward, however
I think it's less awkward when used in scenario (3).

That said I'm open to trying to simplify the API here, but I fundamentally
don't think of this as a wire level flow control API. I get the impression
from the comments in this thread that there is an idea that the app
developer somehow has more knowledge or is in a better position to
distribute credit than the messenger implementation, whereas I think the
opposite is true. In my experience, the nature/shape of incoming message
traffic is not a variable that is well known at development time. Perhaps
you can define the extreme bounds of what loads you want to be able to
handle, but at runtime there are many unpredictable factors:

  - your service can go from idle to extreme bursts with no warning
  - round trip times can fluctuate based general network activity
  - message processing times might vary due to other activity on
    your machine or degradation in services you depend on
  - your load might be unevenly distributed across different
links/connections
  - a buggy or malicious app might be (unintentionally) DoSing you

All of these factors and more go into determining the optimal and/or fair
credit allocation at any given point in time, and that means a robust flow
control algorithm really needs to be dynamic in nature. Not only that, but
a robust flow control algorithm is a huge part of the value that messaging
infrastructure provides, and should really be a fundamentally separate
concern from how apps logically process messages.


> Since Messenger models a queue of incoming messages, I'd rather see flow
> control configured as thresholds on that queue, and recv() not take any
> arguments at all.
>
> Something like this:
>
>  Messenger m;
>  ...
>  m.set_flow_stop( 10000 )
>  m.set_flow_resume( 9000 )
>  ...
>  for (;;) {
>     m.recv()
>     while (m.incoming())
>     ....
>
> IMHO, this is a lot "cleaner" than the current approach.  Of course, some
> may find my sample names too cryptic :)
>

I think limits the API to only the first scenario I described above. At
least it's not clear to me how you'd fetch exactly N messages.


>
> From an implementation point of view, the "flow stop" threshold is really
> just a suggestion for how much credit should be distributed across the
> links.  We could distribute more, as we would need to if the number of
> links is greater than the flow stop threshold.  Or less, assume a point of
> diminishing returns.
>
> Once the flow stop threshold is hit, credit would be drained from all
> links.  No further credit would be granted until the number of "queued"
> messages drops below "flow resume".
>
> This is the same model we use for queue flow control in the C++ broker.
>

 This is starting to mix two things: (1) how the application fetches
messages from the messenger, and (2) how to tune the messengers internal
flow control algorithm in the specific case that the application wants to
receive messages indefinitely. I think (2) is premature given that we
haven't really done any performance work yet. Ideally I'd say we don't want
to have to tune it, rather just give it some bounds to work within, e.g.
limit to no more than X megabytes or no more than Y messages.

In any case I think we need to be clear on the application scenarios we're
trying to support. I've given (3) common ones above. Are there cases that
you think are missing, and do you have a better way to cater to the 3 I've
mentioned?

--Rafael

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