[zeromq-dev] inproc PUB/SUB problem, how to profile

Pieter Hintjens ph at imatix.com
Mon Mar 30 12:59:29 CEST 2015

Some points:

- slow subscriber will cause data to be dropped, not delayed
- Linux kernel will often drop UDP packets, even in a trivial example
- to understand the behavior of a distributed system, measure at each interface


On Mon, Mar 30, 2015 at 12:45 PM, Jan Müller <217534 at gmail.com> wrote:
> Hi,
> I changed a set of tcp pub/sub connected processes into a single application
> with inproc sockets. Now it seems to me this application is somewhat slower
> than the multiple process implementation.
> The application looks something like this: (runs on Linux)
> 1. get the data from the network through UDP (around 10kbits at 20kHz)
> 2. "unpack the data" and publish (PUB) it over inproc
> 3. multiple threads subscribe and do various operations (I/O, filtering
> etc.)
> Now it seems to me, when the system is under load (other applications are
> running) everything gets a bit unresponsive. And the kernel drops some of
> the incoming UDP packets (even though I set the kernel receive buffer quite
> high, to 40MB). I'm pretty sure, nothing in the zmq part gets dropped
> (HWM==0).
> It looks like more and more memory gets used. Although the zmq application
> uses only few tens of MBs, when I check it with e.g. "top".
> I'm not sure where the bottleneck is (e.g. a slow subscriber).
> What is the behavior of inproc PUB/SUB when there is a slow subscriber?
> Will one slow subscriber delay everyone else or will the publisher happily
> continue with filling up the shared memory?
> Is there more shared memory allocated once the subscriber does not catch up
> with reading it?
> In short: can the "suicidal snail" pattern be applied to _inproc_ PUB/SUB as
> well?
> Can I have a set HWM on the PUB side and HWM==0 on the SUB side (where I
> need to write the data to disk with sometimes long respond times)?
> Any ideas, how I could performance test this application? AFAIK I cannot
> inspect the "fill-level" of the sockets. How can I find out who is the slow
> subscriber?
> Thanks a lot!
> Best,
> Jan
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