<div dir="ltr">It is because you construct and execute the fun in the same timeslot. You want to benchmark something like this:<div><br></div><div>F = fun() -> ok end,</div><div>%% Take start time</div><div>[F() || _ <- List]</div>
<div>%% Take end time</div><div><br></div><div>Unfortunately fun does not come for free.</div><div><br></div><div>Lukas</div><div><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Jun 17, 2014 at 8:43 AM, fishball <span dir="ltr"><<a href="mailto:fishballian@qq.com" target="_blank">fishballian@qq.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>Hi</div><div>As I read the<font style="display:inline;font-size:14px;font-family:Verdana;color:rgb(0,0,0);font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)"> </font><font style="display:inline;font-size:14px;font-family:Verdana;color:rgb(0,0,0);font-weight:normal;background-color:rgba(0,0,0,0)"><i>Efficiency Guide</i></font><font style="display:inline;font-size:14px;font-family:Verdana;color:rgb(0,0,0);font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)">, I found something interesting:(see <a href="http://www.erlang.org/doc/efficiency_guide/functions.html#id67357" target="_blank">http://www.erlang.org/doc/efficiency_guide/functions.html#id67357</a>)</font></div>
<div><blockquote style="font-family:Verdana,Arial,Helvetica,sans-serif;font-size:medium;line-height:normal"><font style="display:inline;font-size:14px;font-family:Verdana;font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)" color="#0000ff">Here is an intentionally rough guide to the relative costs of different kinds of calls. It is based on benchmark figures run on Solaris/Sparc:</font><ul style="font-family:Verdana,Arial,Helvetica,sans-serif;font-size:medium;line-height:normal">
<li style="display:list-item;font-size:14px;font-family:Verdana;font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)"><font color="#0000ff">Calls to local or external functions (foo(), m:foo()) are the fastest kind of calls.</font></li>
</ul><ul style="font-family:Verdana,Arial,Helvetica,sans-serif;font-size:medium;line-height:normal"><li style="display:list-item;font-size:14px;font-family:Verdana;font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)">
<font color="#0000ff">Calling or applying a fun (Fun(), apply(Fun, [])) is about </font><font color="#ff0000">three times</font><font color="#0000ff"> as expensive as calling a local function.</font></li></ul><ul style="font-family:Verdana,Arial,Helvetica,sans-serif;font-size:medium;line-height:normal">
<li style="display:list-item;font-size:14px;font-family:Verdana;font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)"><font color="#0000ff">Applying an exported function (Mod:Name(), apply(Mod, Name, [])) is about twice as expensive as calling a fun, or about </font><font color="#ff0000">six times</font><font color="#0000ff"> as expensive as calling a local function.</font></li>
</ul></blockquote><p style="font-family:Verdana,Arial,Helvetica,sans-serif;font-size:medium;line-height:normal"></p><font style="display:inline;font-size:14px;font-family:Verdana;font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)"><div style="color:rgb(0,0,0)">
But I get another result when I run these code in my computer:</div><blockquote style="margin:0.8em 0px 0.8em 2em;padding:0px 0px 0px 0.7em;border-left-width:2px;border-left-style:solid;border-left-color:rgb(221,221,221)">
foo() -><br> ok.<br>test() -><br> List = lists:seq(1, 50000000),<br> statistics(wall_clock),<br> statistics(runtime),<br><font color="#0000ff"> %% [foo() || _ <- List], %%test result 358 ~ 358<br>
</font><font color="#0000ff"> %% [?MODULE:foo() || <span style="line-height:1.5;background-color:rgba(0,0,0,0)">_ <- List</span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">], </span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">%%test result 358 ~ 358<br>
</span></font><font color="#0000ff"><span style="line-height:1.5;background-color:rgba(0,0,0,0)"> %% [apply(?MODULE, foo, [] || </span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">_ <- List</span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">], </span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">%%test result 358 ~ 358<br>
</span></font><span style="line-height:1.5;background-color:rgba(0,0,0,0)"><font color="#0000ff"> </font> </span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">[apply(fun() -> ok end, []) || </span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">_ <- List</span><span style="line-height:1.5;background-color:rgba(0,0,0,0)">], <font color="#0000ff"> %%test result 2247 ~ 1716</font><br>
</span><span style="line-height:1.5;background-color:rgba(0,0,0,0)"><font color="#0000ff"> </font>{_, Time1} = statistics(</span><span style="line-height:1.5">wall_clock),<br></span><span style="line-height:1.5"> </span><span style="line-height:1.5">{_, Time2} =</span><span style="line-height:1.5"> </span><span style="line-height:1.5">statistics(runtime),<br>
</span><span style="line-height:1.5"> io:format("calc ~p ~~ ~p~n", [Time1, Time2]).</span></blockquote><div>Applying a fun is even slower than apply/3, Why?</div><div><span style="line-height:1.5">I also notice "</span><font style="display:inline;font-size:14px;font-family:Verdana;color:rgb(0,0,0);font-weight:normal;font-style:normal;background-color:rgba(0,0,0,0)">apply/3 must look up the code for the function to execute in a hash table. Therefore, it will always be slower than a direct call or a fun call".</font><span style="line-height:1.5"><br>
So I test in my project which include many modules and I get the same result.</span></div></font></div><br>_______________________________________________<br>
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<br></blockquote></div><br></div>