[erlang-questions] I'm having a SNAFU moment: timer:sleep/1

[Roberto Ostinelli]

Indeed. From very basic try-outs:

1>  test:sleep_periodical(6, 1000).
Finished in 6022381 microseconds.
ok
2>  test:sleep_periodical(12, 500).
Finished in 6042683 microseconds.
ok
3>  test:sleep_periodical(24, 250).
Finished in 6062096 microseconds.
ok
4>  test:sleep_periodical(48, 125).
Finished in 6133169 microseconds.
ok
5> test:sleep_periodical(96, 62).
Finished in 6320419 microseconds.
ok
6> test:sleep_periodical(192, 31).
Finished in 6492321 microseconds.
ok
7> test:sleep_periodical(384, 16).
Finished in 7327693 microseconds.
ok

So it somewhat is ok until we get in the 125ms range.



On Mon, Jan 12, 2015 at 12:12 PM, Sergej Jurečko <sergej.jurecko@REDACTED>
wrote:

> timer:sleep is not even remotely as accurate for those low numbers. It can
> not go lower than 20ms or so from what i remember.
>
> I have had this issue once when I needed low interval for timer. I used an
> external c app which printed to stdout every 5ms. I dont think there is any
> way in erlang to get a timer that low.
>
> Sergej
> On Jan 12, 2015 11:46 AM, "Roberto Ostinelli" <roberto@REDACTED> wrote:
>
>> Additionally:
>>
>> 6> test:sleep_periodical(1000, 1).
>> Finished in 1999097 microsecond
>>
>> So:
>>
>>    - Looping 1,000 times and sleeping 1 ms each time takes 2.00 seconds
>>    (instead of 1).
>>
>> Looks like it's ms related, not total count.
>>
>> Best,
>> r.
>>
>>
>>
>> On Mon, Jan 12, 2015 at 11:40 AM, Roberto Ostinelli <roberto@REDACTED>
>> wrote:
>>
>>> Dear list,
>>> I've probably missed out something since I'm not understanding what is
>>> going here.
>>>
>>> I'm basically just trying to send messages periodically, though I found
>>> out that I wasn't seeing the behavior that I was expecting. It looks like
>>> the timer has some erratic behavior.
>>>
>>> I've stripped out the code to the bare minimum and it looks like this:
>>>
>>> -module(test).
>>> -export([sleep_periodical/2]).
>>> -export([sleep_loop/2]).
>>>
>>> sleep_periodical(Num, IntervalMs) ->
>>> {Time, _Value} = timer:tc(?MODULE, sleep_loop, [Num, IntervalMs]),
>>> io:format("Finished in ~p microseconds.~n", [Time]).
>>>
>>> sleep_loop(Num, IntervalMs) ->
>>> SleepFun = fun(_) -> timer:sleep(IntervalMs) end,
>>> lists:foreach(SleepFun, lists:seq(1, Num)).
>>>
>>>
>>> When I run this code, I see the following:
>>>
>>> Erlang/OTP 17 [erts-6.3] [source] [64-bit] [smp:8:8] [async-threads:10]
>>> [hipe] [kernel-poll:false] [dtrace]
>>>
>>> Eshell V6.3  (abort with ^G)
>>> 1> c(test).
>>> {ok,test}
>>> 2> test:sleep_periodical(1000, 10).
>>> Finished in 12257397 microseconds.
>>> ok
>>> 3> test:sleep_periodical(2000, 10).
>>> Finished in 24518070 microseconds.
>>> ok
>>> 4> test:sleep_periodical(10000, 1).
>>> Finished in 20000280 microseconds.
>>> ok
>>> 5> test:sleep_periodical(20000, 1).
>>> Finished in 40000685 microseconds.
>>> ok
>>>
>>> So:
>>>
>>>    - Looping 1,000 times and sleeping 10 ms each time takes 12.26
>>>    seconds (instead of 10).
>>>    - Looping 2,000 times and sleeping 10 ms each time takes 24.52
>>>    seconds (instead of 20).
>>>    - Looping 10,000 times and sleeping 1 ms each time takes 20.00
>>>    seconds (instead of 10).
>>>    - Looping 20,000 times and sleeping 1 ms each time takes 40.00
>>>    seconds (instead of 20).
>>>
>>> Up to 10,000 times it takes 12,16% (1/8) more time, from 10,000 times it
>>> takes 100% (double).
>>>
>>> Can some kind soul explain what I'm doing wrong here and how to achieve
>>> the desired results?
>>>
>>> I'm on OSX, Erlang 17.4.
>>>
>>> Help! ^^_
>>> r.
>>>
>>>
>>
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>>
>>
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