[erlang-questions] Sending message at a specific and accurate time

[Joe Armstrong]

On Sun, Feb 21, 2016 at 10:56 PM, Jesper Louis Andersen
<jesper.louis.andersen@REDACTED> wrote:
> On a Archlinux machine, Core i7 Haswell:
>
> 6> z:test().
> [5001,5001,5001,5001,5001,5001,5001,5001,5001,5001]
>
> This is what I expect to see. I'm awoken on the right flank up to one
> millisecond after.
>
> Immediate thought: this has to do with CPU frequency scaling and battery
> saving features of OSX. Since nothing is done for 5 seconds, the system
> throttles down and goes to sleep. When it is awoken again, you are enjoying
> some latency.

It was the same with 100 ms delays

>
> Enable dtrace and hunt :)
>
>
> On Sun, Feb 21, 2016 at 10:40 PM, Joe Armstrong <erlang@REDACTED> wrote:
>>
>> mac os-x El Capitan 10.11.2
>> Macbook retina pro
>> 3.1 GHz core i7
>> 16 GB ram
>>
>> Can you run the above program locally and post back the results?
>>
>>
>>
>> /Joe
>>
>> On Sun, Feb 21, 2016 at 10:34 PM, Felix Gallo <felixgallo@REDACTED>
>> wrote:
>> > What os and hardware are you seeing these results on?
>> >
>> > On Feb 21, 2016 1:20 PM, "Joe Armstrong" <erlang@REDACTED> wrote:
>> >>
>> >> I tried a simpler program:
>> >>
>> >> test() -> test(10, []).
>> >>
>> >> test(0, L) ->
>> >>     L;
>> >> test(K, L) ->
>> >>     T1 = ms_time(),
>> >>     erlang:send_after(5000, self(), ping),
>> >>     receive
>> >>       ping ->
>> >>          T2 = ms_time(),
>> >>           test(K-1, [T2-T1|L])
>> >>     end.
>> >>
>> >> ms_time() ->
>> >>     erlang:system_time() div 1000000.
>> >>
>> >> Running this gives the following times
>> >>
>> >> [5001,5001,5006,5006,5002,5003,5006,5002,5002,5006]
>> >>
>> >> I'd expected 5000 or 5001
>> >>
>> >> This is in an unloaded OS with an unloaded erlang. 6ms seems very long
>> >> -
>> >> there are very few processes running and the system load is virtually
>> >> zero.
>> >>
>> >> I tried erl -snp disable and setting process_flag(priority, max) but
>> >> the
>> >> results
>> >> are pretty much the same.
>> >>
>> >> Waiting for shorter times like 100 ms makes no difference - still
>> >> events with a 6 ms delay.
>> >>
>> >> I want to use this for scheduling music events (controlling synths) and
>> >> these
>> >> delays are far more than I expected.
>> >>
>> >> /Joe
>> >>
>> >>
>> >>
>> >> On Sun, Feb 21, 2016 at 8:32 PM, Jesper Louis Andersen
>> >> <jesper.louis.andersen@REDACTED> wrote:
>> >> >
>> >> > On Sun, Feb 21, 2016 at 7:53 PM, Joe Armstrong <erlang@REDACTED>
>> >> > wrote:
>> >> >>
>> >> >> I'm getting about a 4 - 9 ms. inaccuracy in the time the messages is
>> >> >> sent
>> >> >> and
>> >> >> the time I want it to be sent - but I'd like it to be much more
>> >> >> accurate
>> >> >> (sub ms if possible)
>> >> >
>> >> >
>> >> > I would start by making measurements without the network component.
>> >> > When
>> >> > you
>> >> > would send the gen_udp message, you take a timestamp, and analyze the
>> >> > skew
>> >> > from the suggested skew. This lets you estimate the overhead of the
>> >> > rest
>> >> > of
>> >> > the system in isolation from your own code and the Erlang VM.
>> >> >
>> >> > Intuitively, 4 to 9 milliseconds is much higher than what I would
>> >> > expect.
>> >> > But note that if you sleep for, say, 40ms, you will be awoken on the
>> >> > 41ms
>> >> > flank at the earliest. This is because you are usually "inside" a
>> >> > millisecond when you make the call so you start by taking the ceiling
>> >> > of
>> >> > that milli-second before you.
>> >> >
>> >> > How much other work is your Erlang VM doing when you make these
>> >> > measurements? You are saying between 4 to 9 ms, which is variance
>> >> > suggesting
>> >> > the VM has lots of work to do at that moment. And of course such
>> >> > stuff
>> >> > will
>> >> > affect the running time. You can switch priority of your processes up
>> >> > to
>> >> > high, but this comes at the expense of other calculations if you
>> >> > can't
>> >> > finish your work quickly in the process with high priority.
>> >> >
>> >> >
>> >> > --
>> >> > J.
>> >> _______________________________________________
>> >> erlang-questions mailing list
>> >> erlang-questions@REDACTED
>> >> http://erlang.org/mailman/listinfo/erlang-questions
>
>
>
>
> --
> J.