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

[Joe Armstrong]

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.
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