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From Fraser Adams <fraser.ad...@blueyonder.co.uk>
Subject Re: Optimising Proton Messenger data transfers & msgr-send/msg-recv oddities
Date Sat, 01 Nov 2014 09:07:04 GMT
Thanks for the responses Rafael.

Re the pn_messenger_send stuff, I guess I should have been clearer; what 
I was really meaning was that I'd seen pn_messenger_put actually 
transfer data without requiring the send. I realise that 
pn_messenger_put doesn't block the question was more about at which 
point is it likely that *transmission* would cause blocking (and thus 
the message not be sent by the put), my hunch was that with small 
messages the transmission probably won't block.

If I'm honest part of my problem is that most of the experiments I'm 
doing with Messenger are in non-blocking mode, so I can see the 
relationship between put and send somewhat more in blocking mode but 
what goes on in non-blocking mode is a bit of a mystery - sorry I should 
have been clearer.


Re the credit stuff, many thanks for the comprehensive explanation, one 
remaining question then - am I correct in thinking that in Messenger it 
is pn_messenger_recv that would be the API call that would be used to 
set the credit? From your explanation of the credit based flow control 
that seems the most likely place, but the documentation for that makes 
no reference to credit, nor that there's a relationship with the sender. 
If my assumption is actually correct it might be useful if you could add 
your explanation of credits below to the API docs for pn_messenger_recv.

Many thanks again for your response,
Frase


On 31/10/14 21:21, Rafael Schloming wrote:
> Hi,
>
> I'll do my best to answer what I can... comments inline...
>
> On Fri, Oct 31, 2014 at 8:10 AM, Fraser Adams <fraser.adams@blueyonder.co.uk
>> wrote:
>> Hey all,
>> OK I'll 'fess up I have to admit that although I've been tinkering with
>> Messenger for a while now I don't *really* understand some of the terms
>> that get used such as credit, disposition, settlement. I think that I was
>> OK with qpid::messaging's setCapacity stuff and how to use that to optimise
>> prefetch and also using qpid::messaging link controls in the Address string
>> to set/disable reliability but the settings on Messenger remain a mystery
>> to me.
>>
>> What really brought that home in my mind was when I started playing with
>> the msgr-send and msgr-recv applications in <proton>/tests/tools/apps to
>> try to figure what sort of throughput I might get with Messenger.
>>
> First off let me just say that I'm not that familiar with msgr-send and
> msgr-recv. I think Ken wrote those, and as I understand it they are command
> line sending and receiving programs that happen to be implemented via
> messenger, however they aren't intended to be command line interfaces to
> messenger, so the terminology and parameters, may not line up 100% with the
> messenger api-doc.
>
>
>> A while back Gordon was involved in a performance conversation and
>> mentioned testing with the settings below
>> ./msgr-recv -c 1000000
>> ./msgr-send -c 1000000 -b 64
>>
>> So sending/receiving a million 64 octet messages and seeing what the
>> performance is - so far so good.
>>
>> But I then tinkered around and hacked in some code to display the count
>> for the sent and received messages and then did:
>>
>> ./msgr-recv -c 100
>> ./msgr-send -c 100 -b 1000000
>>
>> in other words sending & receiving 100 1MB messages - I actually used the
>> large message size as much to slow things down as anything, but what I
>> observed was that the 100 messages were all being sent before msgr-recv
>> started to display any received count numbers.
>>
>> When I looked at the usage I noticed the -p option "Send batches of #
>> messages" and sure enough if I do
>> ./msgr-send -c 100 -p 10 -b 1000000
>>
>> I see msgr-recv catch up every 10 messages.
>>
>>
>> What I *think* is going on is that when the count in the internal
>> Messenger queue (pn_messenger_outgoing(messenger)) exceeds the batch size
>> it calls pn_messenger_send(messenger, -1);
>>
> Yes, from my brief perusal of the code I would concur.
>
>
>> But that makes me unclear in my mind what the differences between
>> pn_messenger_put and pn_messenger_send actually are, I've certainly seen
>> pn_messenger_put actually send messages. I realise that there's a comment
>> "The message may also be sent if transmission would not cause blocking" but
>> I'm not clear at exactly which point blocking would occur, I'm guessing
>> that I'm noticing this because of my large messages? The problem of course
>> is that if I use tiny little messages I can't actually see if any batching
>> actually occurs or whether in the small message case pn_messenger_put
>> merrily whizzes out the small messages without really needing
>> pn_messenger_send to give them a helping kick.
>>
> I'm a little confused by this question. The pn_messenger_put operation
> hands a new message over to the messenger and as stated in the API doc is
> guaranteed not to block. The pn_messenger_send operation on the other hand
> does not take a new message, and exists solely for the purpose of blocking
> until previously "put" messages are actually sent. The pn_messenger_send
> operation is the equivalent of
> pn_mesenger_work_until_you_send_N_messages(N).
>
>
>> Does that make sense? It'd be useful for someone who knows this stuff to
>> explain how the Messenger store works and how the various API calls relate
>> to credit, disposition & settlement (I'm pretty sure the latter relates to
>> the tracker/window/accept/settle stuff but not so sure about the first
>> two). I'd also quite like to know how this stuff relates to the
>> capacity/reliability stuff on qpid::messaging.
>>
> Credit refers to credit based flow control. In a credit based flow control
> scheme, the receiver maintains a "credit balance". The credit balance is
> just a number that indicates how many messages the receiver is capable of
> receiving at any given point in time. The receiver periodically informs the
> sender of this number, and the sender guarantees never to send unless this
> number is positive. The sender will also decrement its copy of the credit
> balance whenever it sends a message. This guarantees that the sender will
> never send more messages than the receiver has requested.
>
> So the term credit can mean a bunch of different things in different
> contexts, but when used as a "unit", e.g. 10 credits, a credit pretty much
> translates into permission to send one message. So when the receiver issues
> 10 credits, it is issuing permission to send up to 10 messages.
>
> Now depending on what exact policy you use to issue credits, you can
> implement a lot of different semantics, e.g. you can send 1 credit to fetch
> exactly one message and be done with it, or you can renew the sender's
> credit whenever it falls below a certain threshold, and there are many more
> options. The concept of capacity is just a specific policy for managing
> credits. This policy assumes the receiver has a fixed buffer for holding
> messages, and it issues credit to reflect the number of empty slots in that
> buffer.
>
> Settlement of a delivery refers to an endpoint being done with and
> forgetting everything about a given delivery. Disposition refers to the
> state of the delivery at the time of settlement, e.g. was it accepted vs
> rejected. Sometimes the disposition is null because the state at the time
> of settlement is unknown, e.g. the sender can choose to "pre-settle" the
> message as it is sent, i.e. forget about it as soon as it hits the wire. In
> this case the state when the receiver settles it will never be known. Just
> like credit is a lower level/more general concept than capacity and can be
> used to implement a greater variety of semantics, settlement is a lower
> level/more general concept than reliability and can be used to implement a
> variety of different QoS levels.
>
>
>> Also I *think* that there is a problem with the python version of
>> msgr-send.py I'd expect that to run more slowly than the C version, but
>> when I did:
>>
>> ./msgr-recv.py -c 100
>> ./msgr-send.py -c 100 -b 1000000
>>
>> it returned more or less immediately and when I increase the -c value I
>> appear to be seeing the same throughput irrespective of the value of the -b
>> value. I've not really looked too deeply at the code but I wonder if that
>> rings any bells for anyone?
>>
> Doesn't ring a bell for me. I'd file a JIRA and maybe poke Ken.
>
>
>> Sorry if these things are obvious to the people who know, but I figured I
>> probably wasn't the only one who didn't actually know this stuff and as
>> I've got no shame I thought I'd raise my head above the parapet and expose
>> my ignorance to the world :-)
>>
> I hope this helps a bit, please follow up and let me know if you have
> further questions.
>
> --Rafael
>


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