测试环境
Linux jason-lxw 3.2.0-39-generic #62-Ubuntu SMP Thu Feb 28 00:28:53 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
4G内存
Erlang R14B04 (erts-5.8.5) [source] [64-bit] [smp:4:4] [rq:4] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.8.5 (abort with ^G)
1>
测试数据：
add()->
III="langxw",
MMM= 10,
OFFFFF = 100,
"display uuuuuuuuu, ffstate ," ++
" uuusername, uubio, "++
" uuavatar_url," ++
" uusex todo langgfriendship f , langgusers u where fffolllleriii=uuuuuuuuu " ++
" aaa | fffollllediii = '", III, "')" ++
" aaa Lype='1'" ++
" rrrr by ffctttte_datt ssss lllll " ++ integer_to_list(MMM) ++" FFeeee " ++ integer_to_list(OFFFFF) ++";".
concat()->
III="langxw",
MMM= 10,
OFFFFF = 100,
Sql = lists:concat(["display uuuuuuuuu, ffstate ,"
" uuusername, uubio, ",
" uuavatar_url,",
" uusex todo langgfriendship f , langgusers u where fffolllleriii=uuuuuuuuu ",
" aaa | fffollllediii = '", III, "')",
" aaa Type='1'",
" rrrr by ffctttte_datt ssss lllll ", MMM," FFeeee ", OFFFFF,";"
]).
lists:concat操作结果
49> test_lists_concat:start(1,10000). ：参数：第一个参数是单进程，第二个参数是 add/concat执行次数
===============================================================
process #1 send 10000 request spent 62 ms
total spent=62 ms,avg spent=62 ms
=====================================================================
true
50> test_lists_concat:start(1,100000).
===============================================================
process #1 send 100000 request spent 554 ms
total spent=554 ms,avg spent=554 ms
=====================================================================
true
51> test_lists_concat:start(1,100 0000).
===============================================================
process #1 send 1000000 request spent 5699 ms
total spent=5699 ms,avg spent=5699 ms
=====================================================================
true
++操作结果
54> test_lists_concat:start(1,10000).
===============================================================
process #1 send 10000 request spent 2 ms
total spent=2 ms,avg spent=2 ms
=====================================================================
true
55> test_lists_concat:start(1,100000).
===============================================================
process #1 send 100000 request spent 27 ms
total spent=27 ms,avg spent=27 ms
=====================================================================
true
56> test_lists_concat:start(1,100 0000).
===============================================================
process #1 send 1000000 request spent 206 ms
total spent=206 ms,avg spent=206 ms
=====================================================================
true
57> test_lists_concat:start(1,10000000).
===============================================================
process #1 send 10000000 request spent 5257 ms
total spent=5257 ms,avg spent=5257 ms
=====================================================================
结论：（仅代表当前测试总结学习）乍看lists:concat源码，以为lists:concat执行效率会比++操作快，但实际测试结果，lists:concat远不如++操作。这可能因为lists:concat里面的递归（非尾递归）造成的。从上面测试数据可以看出lists:concat 比++操作要慢几十倍。
-module(test_lists_concat).
-export([test/2]).
-export([start/2,dotest/3]).
start(Processes,Loops)->
ets:new(ib,[named_table,set]),
[spawn(?MODULE,dotest,[self(),X,Loops])||X<-lists:seq(1,Processes)],
recv(Processes,Loops),
ets:delete(ib).
recv(Processes,Loops)->
receive
{fin,ProcessNum,Elapse}->
ets:insert(ib,{ProcessNum,Elapse}),
case ets:info(ib,size) of
Processes->
%%got all the resp
print_result(Processes,Loops);
_->
recv(Processes,Loops)
end;
Err->
io:format("recv unknowen msg~n"),
recv(Processes,Loops)
end.
print_result(Processes,Loops)->
io:format("===============================================================~n"),
TotalElapse=
lists:foldr(
fun({ProcessNum,Elapse},Acc)->
io:format("process #~p send ~p request spent ~p ms~n",[ProcessNum,Loops,Elapse]),
Acc + Elapse
end,
0,
ets:tab2list(ib)
),
AvgElapse=TotalElapse div Processes,
io:format("total spent=~p ms,avg spent=~p ms~n",[TotalElapse,AvgElapse]),
io:format("=====================================================================~n").
dotest(From,ProcessNum,Loops)->
Start=timestamp_in_millinsec(),
[lists_concat()||X<-lists:seq(1,Loops)],
Elapse=timestamp_in_millinsec()-Start,
From!{fin,ProcessNum,Elapse}.
timestamp_in_millinsec()->
{MegaSec,Sec,MicoSec}=erlang:now(),
MegaSec * 1000000000+Sec*1000+MicoSec div 1000.
lists_concat() ->
%%lists:concat(["langxw", "langxw", "langxw"]),
%%""++"langxw"++"langxw",
%%concat(),
add(),
ok.
add()->
III="langxw",
MMM= 10,
OFFFFF = 100,
"display uuuuuuuuu, ffstate ," ++
" uuusername, uubio, "++
" uuavatar_url," ++
" uusex todo langgfriendship f , langgusers u where fffolllleriii=uuuuuuuuu " ++
" aaa | fffollllediii = '", III, "')" ++
" aaa Lype='1'" ++
" rrrr by ffctttte_datt ssss lllll " ++ integer_to_list(MMM) ++" FFeeee " ++ integer_to_list(OFFFFF) ++";".
concat()->
III="langxw",
MMM= 10,
OFFFFF = 100,
Sql = lists:concat(["display uuuuuuuuu, ffstate ,"
" uuusername, uubio, ",
" uuavatar_url,",
" uusex todo langgfriendship f , langgusers u where fffolllleriii=uuuuuuuuu ",
" aaa | fffollllediii = '", III, "')",
" aaa Type='1'",
" rrrr by ffctttte_datt ssss lllll ", MMM," FFeeee ", OFFFFF,";"
]).
test(Pro, Num)->
o.
补充：
51> test_lists_concat:start(1,100 0000).
===============================================================
process #1 send 1000000 request spent 5699 ms
total spent=5699 ms,avg spent=5699 ms
56> test_lists_concat:start(1,100 0000).
===============================================================
process #1 send 1000000 request spent 206 ms
@石头哥哥 这两次结果比较 ++ 比lists:concat快了大约27倍。
@erlang-菜鸟
个人理解（没测试）假设有两个list分别是A和B
1、如果A的长度远远大约B的长度时。A++B操作 会比 B++A慢
2、如果1成立，同样假设有A1，A2，A3，A4...An
如果A1长度>A2长短>A3长度>A4长度>...>An长度，
A1++A2++A3++A4++...++An 会比
{A4++..++An :记为R
A3++R :记为R
A2++R :记为R
A1++R
}慢。
基于这样的理解，简单的误认为lists:concat和++的快慢显示是错误的。还有就是lists:concat里面调用了非尾递归。感兴趣的同学继续测试一下。
找时间测试研究一下++操作顺序