继续昨天的话题,昨天提到io:format对数据共享的间接影响,如果是下面两种情况恐怕更容易成为"坑", 呃,恰好我都遇到过;
如果是测试代码是下面这样,得到的结果会是怎样?猜!
s2()->
L=[1,2,3,4,5,6],
L2=[L,L,L,L],
erlang:display( {{erts_debug:size(L),erts_debug:flat_size(L)},{erts_debug:size(L2),erts_debug:flat_size(L2)}}
).
结果是
5> d:s2().
{{12,12},{56,56}}
这个结果出来之后,我足足用了5分钟用来怀疑人生,为什么和期望的结果不一样呢?是因为我现在用的最新版本(17.2)吗?是实现已经修改掉但是没有更新文档吗?出于好奇,我还是按照之前探索问题的套路,生成了一下to_core文件,真相大白:
's2'/0 =
%% Line 11
fun () ->
let <_cor5> =
%% Line 14
call 'erts_debug':'size'
([1|[2|[3|[4|[5|[6]]]]]])
in let <_cor4> =
%% Line 14
call 'erts_debug':'flat_size'
([1|[2|[3|[4|[5|[6]]]]]])
in let <_cor3> =
%% Line 14
call 'erts_debug':'size'
([[1|[2|[3|[4|[5|[6]]]]]]|[[1|[2|[3|[4|[5|[6]]]]]]|[[1|[2|[3|[4|[5|[6]]]]]]|[[1|[2|[3|[4|[5|[6]]]]]]]]]])
in let <_cor2> =
%% Line 14
call 'erts_debug':'flat_size'
([[1|[2|[3|[4|[5|[6]]]]]]|[[1|[2|[3|[4|[5|[6]]]]]]|[[1|[2|[3|[4|[5|[6]]]]]]|[[1|[2|[3|[4|[5|[6]]]]]]]]]])
in %% Line 14
call 'erlang':'display'
({{_cor5,_cor4},{_cor3,_cor2}})
修改一下代码:
s3(L)->
L2=[L,L,L,L],
{{erts_debug:size(L),erts_debug:flat_size(L)},{erts_debug:size(L2),erts_debug:flat_size(L2)}}
.
对应的s3的代码是
's3'/1 =
%% Line 18
fun (_cor0) ->
let <L2> =
%% Line 19
[_cor0|[_cor0|[_cor0|[_cor0|[]]]]]
in let <_cor5> =
%% Line 20
call 'erts_debug':'size'
(_cor0)
in let <_cor4> =
%% Line 20
call 'erts_debug':'flat_size'
(_cor0)
in let <_cor3> =
%% Line 20
call 'erts_debug':'size'
(L2)
in let <_cor2> =
%% Line 20
call 'erts_debug':'flat_size'
(L2)
in %% Line 20
{{_cor5,_cor4},{_cor3,_cor2}}
换句话,在编译阶段s2方法里面的常量数据就已经展开了,所以L2无论是size还是flat_size都是一样的.之所以要先把这个测试做了,就是避免后面的测试误入陷阱.
这个怎么破呢?除了上面传入参数的方法之外,还有一个路子:换成函数调用即可,如下:
s4()->
L=lists:seq(1,6),
L2=[L,L,L,L],
erlang:display( {{erts_debug:size(L),erts_debug:flat_size(L)},{erts_debug:size(L2),erts_debug:flat_size(L2)}}
).
对应的代码为:
's4'/0 =
%% Line 24
fun () ->
let <L> =
%% Line 25
call 'lists':'seq'
(1, 6)
in let <L2> =
%% Line 26
[L|[L|[L|[L|[]]]]]
in let <_cor5> =
%% Line 27
call 'erts_debug':'size'
(L)
in let <_cor4> =
%% Line 27
call 'erts_debug':'flat_size'
(L)
in let <_cor3> =
%% Line 27
call 'erts_debug':'size'
(L2)
in let <_cor2> =
%% Line 27
call 'erts_debug':'flat_size'
(L2)
in %% Line 27
call 'erlang':'display'
({{_cor5,_cor4},{_cor3,_cor2}})
不要小看这个问题,这样一个常量优化在极端情况下会有"大惊喜",论文里面给了这样一个例子:
show_compiler_crashes() -> L0 = [0], L1 = [L0, L0, L0, L0, L0, L0, L0, L0, L0, L0], L2 = [L1, L1, L1, L1, L1, L1, L1, L1, L1, L1], L3 = [L2, L2, L2, L2, L2, L2, L2, L2, L2, L2], L4 = [L3, L3, L3, L3, L3, L3, L3, L3, L3, L3], L5 = [L4, L4, L4, L4, L4, L4, L4, L4, L4, L4], L6 = [L5, L5, L5, L5, L5, L5, L5, L5, L5, L5], L7 = [L6, L6, L6, L6, L6, L6, L6, L6, L6, L6], L8 = [L7, L7, L7, L7, L7, L7, L7, L7, L7, L7], L9 = [L8, L8, L8, L8, L8, L8, L8, L8, L8, L8], L = [L9, L9, L9, L9, L9, L9, L9, L9, L9, L9], L.
$ erlc demo.erl
Crash dump was written to: erl_crash.dump
eheap_alloc: Cannot allocate 3716993744 bytes of
memory (of type "heap_frag").
Abort
好吧,勇于自黑,由于上面遇到这样让人恼火的问题,我决定在Shell中完成后续的测试,然后,我一脚踏进"新坑":
陷阱2 Shell ! Shell !
Eshell V6.0 (abort with ^G)
1> L=[1,2,3,4,5,6,7,8,9,10].
[1,2,3,4,5,6,7,8,9,10]
2> L2=[L,L,L,L,L,L].
[[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10]]
3> erts_debug:size(L2).
32
4> erts_debug:flat_size(L2).
132
5> io:format("~p",[L2]).
[[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10],
[1,2,3,4,5,6,7,8,9,10]]ok
6> erts_debug:size(L2).
32
7> erts_debug:flat_size(L2).
132
一开始启动shell的时候,Shell的Pid是<0.33.0>.然后我们在中间故意执行一个不存在的方法 fake:fake().这时查看一下,Shell已经重启,Pid变成<0.40.0>.注意再执行erts_debug:size(L2).结果已经变成了132了,换句话说,这里L2数据已经展开了.
Eshell V6.0 (abort with ^G) 1> self(). <0.33.0> 2> L=[1,2,3,4,5,6,7,8,9,10]. [1,2,3,4,5,6,7,8,9,10] 3> L2=[L,L,L,L,L,L]. [[1,2,3,4,5,6,7,8,9,10], [1,2,3,4,5,6,7,8,9,10], [1,2,3,4,5,6,7,8,9,10], [1,2,3,4,5,6,7,8,9,10], [1,2,3,4,5,6,7,8,9,10], [1,2,3,4,5,6,7,8,9,10]] 4> erts_debug:size(L2). 32 5> erts_debug:flat_size(L2). 132 6> fake:fake(). ** exception error: undefined function fake:fake/0 7> self(). <0.40.0> 8> erts_debug:size(L2). 132 9> erts_debug:flat_size(L2). 132 10>
那为什么会触发数据展开(expand ,flattening)呢? 看下面的代码,在Shell启动的时候,会把之前已经绑定的变量作为spawn_link参数以启动新的shell.
erl6.2\lib\stdlib-2.2\src
start_eval(Bs, RT, Ds) ->
Self = self(),
Eval = spawn_link(fun() -> evaluator(Self, Bs, RT, Ds) end),
put(evaluator, Eval),
Eval.
换句话说,Erlang中使用spawn创建进程,传入的参数(包括函数闭包),需要拷贝到新进程的heap,换句话说进程创建的时候需要考虑参数的大小.
OK,这个问题差不多了,休息.
