[zeromq-dev] Timing issues

[Lindley French]

Maybe I'm missing something, but what purpose is there in disabling Nagle's
algorithm, only to then re-implement the same concept one layer higher?


On Thu, Jan 16, 2014 at 9:15 AM, Charles Remes <lists at chuckremes.com> wrote:

> Nagle’s algo is already disabled in the codebase (you can confirm that
> with a quick grep). I think what Bruno is referring to is that zeromq
> batches small messages into larger ones before sending. This improves
> throughput at the cost of latency as expected.
>
> Check out the “performance” section of the FAQ for an explanation:
> http://zeromq.org/area:faq
>
>
> On Jan 16, 2014, at 7:04 AM, Lindley French <lindleyf at gmail.com> wrote:
>
> Ah, that would explain it, yes. It would be great to have a way of
> disabling Nagle's algorithm (TCP_NODELAY sockopt).
>
>
> On Thu, Jan 16, 2014 at 4:24 AM, Bruno D. Rodrigues <
> bruno.rodrigues at litux.org> wrote:
>
>> Without looking at the code I assume ØMQ is not trying to send each
>> individual message as a TCP PDU but instead, as the name implies, queues
>> messages so it can batch them together and get the performance.
>>
>> This then means the wire will be filled up when some internal buffer
>> fills, or after a timeout, which looks like 100ms.
>>
>> On the other hand I can’t see any setsockopt to configure this possible
>> timeout value.
>>
>> Any feedback from someone else before I have time to  look at the code?
>>
>> On Jan 15, 2014, at 16:20, Lindley French <lindleyf at gmail.com> wrote:
>>
>> > I have a test case in which I'm communicating between two threads using
>> zmq sockets. The fact that the sockets are in the same process is an
>> artifact of the test, not the real use-case, so I have a TCP connection
>> between them.
>> >
>> > What I'm observing is that a lot of the time, it takes ~100
>> milliseconds between delivery of a message to the sending socket and
>> arrival of that message on the receiving socket. Other times (less
>> frequently) it is a matter of microseconds. I imagine this must be due to
>> some kernel or thread scheduling weirdness, but I can't rule out that it
>> might be due to something in 0MQ.
>> >
>> > If I follow the TCP socket write with one or more UDP writes using
>> Boost.Asio, the 100 millisecond delay invariably occurs for the ZMQ TCP
>> message but the UDP messages arrive almost instantly (before the TCP
>> message).
>> >
>> > My design requires that the TCP message arrive before *most* of the UDP
>> messages. It's fine if some come through first----UDP is faster after all,
>> that's why I'm using it----but this big of a delay is more than I counted
>> on, and it's concerning. I don't know if it would apply across a real
>> network or if it's an artifact of testing in a single process.
>> >
>> > Any insights?
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