[erlang-questions] Lockfree now()

[Erik Søe Sørensen]

2012/7/12 Wojtek Narczyński <wojtek@REDACTED>

> On 07/12/2012 08:23 PM, Erik Søe Sørensen wrote:
>
>> Because of atomic operations.
>> I may not have been explicit enough there: latest_now is a
>> java.util.concurrent.**AtomicLong, and the two updating accesses to it
>> are atomic.
>>                 long prev;
>>                 while ((prev = latest_now.get()) < micros) {
>>                         if (latest_now.compareAndSet(**prev,micros)) {
>>                                 return micros;
>>                         }
>>                 }
>>                 return latest_now.incrementAndGet();
>>
>> This - and only because of this - means that no lock is necessary.
>>
> Oh, I think I understand now.
>
> You want to have one coarse-grained clock - protected by a lock, and
> another one fine-grained - updated atomically.
>
> Do I understand correctly now?
>

Indeed :-)
Roughly speaking, Erlang calculates now() from a monotonic, hi-res time
source plus a lower-res absolute-time source.
The now-time is the monotonic time plus a certain offset, which may change
over time in a controlled fashion, and which is adjusted only when the
monotonic source has made a certain amount of progress.
This is more or less what already happens, at

https://github.com/erlang/otp/blob/master/erts/emulator/beam/erl_time_sup.c
line 172. The variant at line 262 and forth is somewhat more complex to
follow; I'm afraid it doesn't follow the pattern as things are.
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