[erlang-questions] wierd:statistic(runtime) and statistic(wall_clock)

Oscar Hellström <>
Sat May 9 20:57:29 CEST 2009


Hi Junli,

Please keep in mind that Erlang will use all your processor cores. The 
wall_clock time is the time spend since your program started, as if you 
were checking with a clock on your wall. runtime is the time spent on 
the CPU. If there is more than one CPU, runtime can be more than 
wall_clock. This is the same for any multi threaded C program (see 
attached, compile with -lpthreads and run with time ./executable).

statistics/1 will give you statistics for the VM though, and not for one 
erlang process. You can probably use either timer:tc/3 or now/0 together 
with timer:now_diff/2 to find out how much wall time time a process has 
used, but I don't know how you would get the run time, since the run 
time could be less if you have several processes running in the system.

junli gu wrote:
> we have two ways to get time :statistic(*runtime*) and 
> statistic(wall_clock).I have not find any exact desciption about the two 
> methods.
> Some one have guessed:"Since erlang is not
> guaranteed to have 100% of your CPU time, *runtime* would be the actual 
> amount
> of processing time used by erlang. "wall_clock" time is simply recording 
> the
> actual time the process started and the actual time the process ended and
> giving you the difference in "real time" ."
> 
> So we can came to the conclusion that *runtime* is less than 
>  wall_clock.I did some experiments to verify this,the result is really 
> wired that I can't understand:  *runtime* is probably both more or less 
> than  wall_clock! Waht's happenging? Would some one explain this?
> process spawn average time =*runtime* <wall_clock> us
> process spawn average time =1.5 <3.2> us
> process spawn average time =3.44 <2.5> us
> 
> just wondering how to get the exact time that my process needed……
> 
> -- 
> ************************************************
> Gu Junli--谷俊丽
> PHD Candidate of Tsinghua University
> Beijing 100084,China
> Tel: 86-10-62795139
> 
> ************************************************
> 
> 
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Best regards
-- 
Oscar Hellström, 
Phone:  +44 (0)798 45 44 773
Mobile: +44 (0)207 65 50 337
Web:    http://www.erlang-consulting.com
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