[zeromq-dev] Implementing efficient recv timeouts [was Re: zmq::poll timeout returns immediately on MacOS/X 10.5]

Matt Weinstein matt_weinstein at yahoo.com
Fri Jun 11 15:14:46 CEST 2010


Folks,

I'm going to recycle the old question, which could be entitled   
"implementing efficient recv() timeouts".

My basic topology is the client-server setup shown in the sample

[client] [ZMQ_REQ] -[inproc]-- [ZMQ_QUEUE] --- [ZMQ_XREQ] ~network~  
[ZMQ_XREP] -- [ZMQ_QUEUE] --- [inproc] -- [ZMQ_REP] -- [server thread]

(don't fault me if I mislabel a few things, I'm on a train ;) )

Anyway, after thinking about this overnight, one possible approach  
would be to create a ZMQ_TIMEOUT device, which would be inserted in  
place of the ZMQ_QUEUE:

[client] [ZMQ_REQ] -[inproc]-- [ZMQ_TIMEOUT_QUEUE] --

(It can subsume the function of ZMQ_QUEUE, otherwise I would push it  
onto the stack).

This device would have some intelligence about the flow of REQs and  
REPs through it, and every N ms would synthesize an empty (or error)  
response back to the REQ that was waiting by constructing a synthetic  
X packet.  Later, when the REP came through it would ignore it.

One problem that results is that the receiving socket would be in the  
"ready" state after receiving the response, but the rest of the  
network would not reflect that.  If another request were sent with the  
same identity it could trounce state elsewhere.

One way to handle this might be to append out-of-band status to the  
identity packet in the XREQ/XREP protocol, which would be tell the  
receiving socket that e.g. this is an error message, rather than a  
message that causes a state change.
In general, it would be nice if you could add such trailing bytes to  
the identity to allow for general out of band data as part of an XREQ/ 
XREP, which could be useful for QoS etc.  It might require inserting a  
delimiter at the end of the identity if the length is not globally  
defined.

Another approach might be to have the ZMQ_QUEUE remap the requests to  
a new identity, but that's just painful.

Otherwise, I think you would have to build the device into zmq where  
it could directly mangle the pipes and sockets :-)

Also, I'm presuming identities are not recycled, meaning I can mark an  
identity as "down permanently" once a socket closes.  That way I can  
track all of the pending replies without worrying about the  
application "walking around me" to another socket and reusing the  
identity.

I've just started looking at the zmq kernel in depth, does anyone have  
any thoughts about how practical this approach is?

Thanks,
Matt

On Jun 11, 2010, at 8:15 AM, Martin Sustrik wrote:

> Marcin Gozdalik wrote:
>
>> Avg. us/call Elapsed Name
>> 0.064833 0.648334 clock_gettime(2/CLOCK_REALTIME)
>> 0.009635 0.096351 clock_gettime(2/CLOCK_REALTIME_COARSE)
>
> That's a pretty good performance! I wouldn't hesitate to use it in
> zmq_poll. The impact on non-Linux systems is still unknown though.
>
> Martin
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