timer:tc/3, erlang:now/0 documentation

[István Lõrincz]

Hi!

I think the documentation of timer:tc/3 is misleading, because it states, that the function returns the real execution time of the specified MFA. In reality it returns a duration based on virtual time returned by erlang:now/0.

On the other hand, the documentation of erlang:now/0 is misleading too somewhat, because it states that it returns the POSIX time, without mentioning that it's true only in case you don't make repetitive calls to erlang:now/0 in a shorter time than 1 microsec.

To illustrate this I wrote the following code (tested on Windows Vista Enterprise SP1, Erlang OTP R13B04, Sun SPARC Solaris 5.8, Erlang OTP R11B):

----------------------------------------------------------------

-module(now).

-compile(export_all).

now_test(0)->

ok;

now_test(Counter) ->

now(),%%generating unique time-stamps

now_test(Counter-1).

test(Amount)->

NowStartTime = calendar:now_to_datetime(now()),

SysStartTime = calendar:local_time(),

{TCRunTime, _Result} = timer:tc( now, now_test, [Amount]),

SysRunTime = calendar:time_difference(SysStartTime,

calendar:local_time()),

NowTime = calendar:now_to_datetime(now()),

SysTime = calendar:local_time(),

io:format("calendar:local_time(): ~p~n",[SysStartTime]),

io:format("calendar:now_to_datetime(now()): ~p~n",[NowStartTime]),

io:format("Run time by timer:tc( now, now_test, [Amount]): ~p~n", [TCRunTime]),

io:format("Run time by calendar:local_time(): ~p~n",[SysRunTime]),

io:format("calendar:local_time(): ~p~n",[SysTime]),

io:format("calendar:now_to_datetime(now()): ~p~n",[NowTime]).

 

----------------------------------------------------------------

332> erlang:system_info(otp_release).

"R13B04"

333> now:test(10000000).

calendar:local_time(): {{2010,6,17},{12,5,23}}

calendar:now_to_datetime(now()): {{2010,6,17},{10,5,23}} Run time by timer:tc( now, now_test, [Amount]): 10000001 Run time by calendar:local_time(): {0,{0,0,2}}

calendar:local_time(): {{2010,6,17},{12,5,25}}

calendar:now_to_datetime(now()): {{2010,6,17},{10,5,33}}

334>

----------------------------------------------------------------

2> erlang:system_info(otp_release).

"R11B"

3> now:test(10000000).

calendar:local_time(): {{2010,6,17},{12,42,45}}

calendar:now_to_datetime(now()): {{2010,6,17},{10,42,45}} Run time by timer:tc( now, now_test, [Amount]): 10000004 Run time by calendar:local_time(): {0,{0,0,7}}

calendar:local_time(): {{2010,6,17},{12,42,52}}

calendar:now_to_datetime(now()): {{2010,6,17},{10,42,55}} ok

4>

Regards,

István