[zeromq-dev] Message batching in zmq

Francesco francesco.montorsi at gmail.com
Fri Aug 9 23:28:49 CEST 2019

Hi Luca,
[changing subject to make it easier to search this message in future]

Il giorno mer 7 ago 2019 alle ore 16:05 Luca Boccassi
<luca.boccassi at gmail.com> ha scritto:
> On Wed, 2019-08-07 at 14:50 +0200, Francesco wrote:
> > > Another improvement that can help is using the new zero-copy kernel
> > > TCP read/write APIs - I had started something a couple of years
> > > back, but again didn't have time to complete it.
> > >
> >
> > This looks very interesting as well.. do you have any pointer to
> > these new zero-copy kernel APIs?
> >
> > Thanks,
> > Francesco
> Kernel docs:
> https://www.kernel.org/doc/html/v5.2/networking/msg_zerocopy.html
> This is the initial experiment, very much incomplete:
> https://github.com/bluca/libzmq/commit/d021ea5f2c7526b388cb8f8005298e30b4cadd62

Thanks, this looks really interesting, however the kernel docs state
very clearly that "MSG_ZEROCOPY is generally only effective at writes
over around 10 KB."...
 this raises the question: can we better tune the ZMQ message batching

I see in
the following comment:
    //  Disable Nagle's algorithm. We are doing data batching on 0MQ level,
    //  so using Nagle wouldn't improve throughput in anyway, but it would
    //  hurt latency.

but did not find (didn't look too much though) the place where ZMQ
actually does its own data batching. Nor could I find any option in
zmq_setsockopt() to tune somehow that batching.

E.g. assuming I don't mind at all about message latency, can I improve
the throughput by forcing ZMQ somehow to only call the send() system
call when I have queued 10kB of data to the zmq socket?


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