[erlang-questions] Lockfree now()

Wojtek Narczyński <>
Fri Jul 13 02:08:20 CEST 2012


On 2012-07-12 23:55, Erik Søe Sørensen wrote:
> 2012/7/12 Wojtek Narczyński < 
> <mailto:>>
>
>     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 :-)

I looked into the Erjang repo, and indeed now_unique_micros() looks 
good, so I honorably retreat.

> 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.

I suspect that there are some serious differences between platforms, 
because on Unix the POSIX API (clock_gettime and friends) is a clean one 
- you have a choice of different clocks with different characteristics. 
Erlang exposes only one clock, the most expensive one possible - 
strictly increasing [0, 1, 2, 3, ...], basically a clock and a counter 
in one. Using atomic operations instead of locks would definitely be 
faster. But still slower compared with an increasing clock [0, 1, 1, 2, 
...], and other clocks. Certainly, it would be nice to have that POSIX 
choices in erlang. I have been surprised once by erlang:now() 
performance on multiclore, but still I do not think this is big issue.

--Regards,
Wojtek
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