[zeromq-dev] About inbound_poll_rate

[Luca Boccassi]

On Tue, 2017-10-24 at 15:43 +0200, Francesco wrote:
> Hi all,
> I'm running a process that creates a ZMQ_SUB socket and is subscribed
> to
> several publishers (over TCP transport).
> 
> Now I measured that this process saturates with CPU at 100% and
> slows-down
> the publisher (I'm using ZMQ_XPUB_NODROP=1) when subscribed to more
> than
> 600kPPS/1.6Gbps of traffic.
> 
> The "strange" thing is that when it's subscribed to much lower
> traffic
> (e.g,. in my case to around 4 kPPS / 140 Mbps) the CPU still stays at
> 100%.
> If I strace the process I find out that:
> 
> # strace -cp 48520
> 
> strace: Process 48520 attached
> ^Cstrace: Process 48520 detached
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  93.29    0.634666           2    327545           poll
>   6.70    0.045613           2     23735           read
>   0.01    0.000051           4        12           write
>   0.00    0.000001           1         1           restart_syscall
> ------ ----------- ----------- --------- --------- ----------------
> 100.00    0.680331                351293           total
> 
> the 93% of the time is spent inside poll(), which happens to be
> called with
> this stack trace:
> 
> #0  poll () at ../sysdeps/unix/syscall-template.S:84
> #1  0x00007f98da959a1a in zmq::signaler_t::wait(int) ()
> #2  0x00007f98da937f75 in zmq::mailbox_t::recv(zmq::command_t*, int)
> ()
> #3  0x00007f98da95a3a7 in zmq::socket_base_t::process_commands(int,
> bool)
> [clone .constprop.148] ()
> #4  0x00007f98da95c6ca in zmq::socket_base_t::recv(zmq::msg_t*, int)
> ()
> #5  0x00007f98da97e8c0 in zmq_msg_recv ()
> 
> Maybe I'm missing something but by looking at the code it looks to me
> that
> this is happening because of the config setting
> 'inbound_poll_rate=100';
> that is, every 100 packets received zmq_msg_recv() will do an extra
> poll.
> 
> Now my question is: is there any reason to have this
> inbound_poll_rate setting hardcoded and not configurable (e.g., via
> context
> option) ?

It might be as simple as nobody has needed it beforehand. I guess one
issue could be that by changing those values it's extremely easy to
shoot oneself in the foot.
But if you need it feel free to send a PR to implement the option.

-- 
Kind regards,
Luca Boccassi
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: This is a digitally signed message part
URL: <https://lists.zeromq.org/pipermail/zeromq-dev/attachments/20171024/59470337/attachment.sig>