几次我读过unpack()比substr()更快,特别是随着子串数量的增加.然而,这个基准则反映出来.我的基准是否有缺陷,或者是从旧版本的Perl中解压缩()的性能优势？
use strict;
use warnings;
use Benchmark;
my ($data,$format_string,$n_substrings);
my %methods = (
unpack => sub { return unpack $format_string,$data },substr => sub { return map {substr $data,$_,1} 0 .. $n_substrings - 1 },);
for my $exp (1 .. 5){
$n_substrings = 10 ** $exp;
print $n_substrings,"\n";
$format_string = 'a1' x $n_substrings;
$data = 9 x $n_substrings;
Benchmark::cmpthese -2,\%methods;
}
输出(在Windows上)：
10
Rate unpack substr
unpack 131588/s -- -52%
substr 276802/s 110% --
100
Rate unpack substr
unpack 13660/s -- -57%
substr 31636/s 132% --
1000
Rate unpack substr
unpack 1027/s -- -68%
substr 3166/s 208% --
10000
Rate unpack substr
unpack 84.4/s -- -74%
substr 322/s 281% --
100000
Rate unpack substr
unpack 5.46/s -- -82%
substr 30.1/s 452% --
正如在一些答案中指出的那样,unpack()在Windows上表现不佳.这是一个solaris机器的输出 – 并不是那么决定性,但是substr()仍然赢得了足球比赛：
10
Rate unpack substr
unpack 202274/s -- -4%
substr 210818/s 4% --
100
Rate unpack substr
unpack 22015/s -- -9%
substr 24322/s 10% --
1000
Rate unpack substr
unpack 2259/s -- -9%
substr 2481/s 10% --
10000
Rate unpack substr
unpack 225/s -- -9%
substr 247/s 9% --
100000
Rate unpack substr
unpack 22.0/s -- -10%
substr 24.4/s 11% --
解决方法
事实上,你的基准是有缺陷的,一个真正有趣的方式,但是归结的是,你真正比较的是相对效率,解压缩与地图可以抛出一个列表,因为基准:: cmpthese()正在执行void context中的函数.
你的substr出来的原因是pp_ctl.c pp_mapwhile()中的这行代码：
if (items && gimme != G_VOID) {
即perl的地图奇怪地跳过一堆工作(即为地图的结果分配存储),如果它知道它在void context中被调用的话！
(我对窗口的预感和上面的其他看法都是基于windows的perl内存分配是非常糟糕的,所以跳过分配是一个更大的节省 – 只是一个预感,虽然,我没有一个Windows框可以玩但是实际的解压缩实现是直接的C代码,并且不应该从Windows到其他的不同.)
我有三个不同的解决方案来解决这个问题,并产生一个更公平的比较：
>将列表分配给数组
>循环在函数内的列表中,并且不返回
>返回对列表的引用(隐藏void上下文)
以下是我的％方法版本,所有这三个版本：
my %methods = (
unpack_assign => sub { my @foo = unpack $format_string,$data; return },unpack_loop => sub { for my $foo (unpack $format_string,$data) { } },unpack_return_ref => sub { return [ unpack $format_string,$data ] },unpack_return_array => sub { return unpack $format_string,substr_assign => sub { my @foo = map {substr $data,1} 0 .. ($n_substrings - 1) },substr_loop => sub { for my $foo ( map {substr $data,1} 0 .. ($n_substrings - 1)) { } },substr_return_ref => sub { return [ map {substr $data,1} 0 .. ($n_substrings - 1) ] },substr_return_array => sub { return map { substr $data,);
我的结果：
$perl -v
This is perl,v5.10.0 built for x86_64-linux-gnu-thread-multi
$perl foo.pl
10
Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array
substr_assign 101915/s -- -20% -21% -28% -51% -51% -65% -69%
substr_return_ref 127224/s 25% -- -1% -10% -39% -39% -57% -62%
substr_loop 128484/s 26% 1% -- -9% -38% -39% -56% -61%
unpack_assign 141499/s 39% 11% 10% -- -32% -32% -52% -57%
unpack_return_ref 207144/s 103% 63% 61% 46% -- -1% -29% -37%
unpack_loop 209520/s 106% 65% 63% 48% 1% -- -28% -37%
unpack_return_array 292713/s 187% 130% 128% 107% 41% 40% -- -12%
substr_return_array 330827/s 225% 160% 157% 134% 60% 58% 13% --
100
Rate substr_assign substr_loop substr_return_ref unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array
substr_assign 11818/s -- -25% -25% -26% -53% -55% -63% -70%
substr_loop 15677/s 33% -- -0% -2% -38% -40% -51% -60%
substr_return_ref 15752/s 33% 0% -- -2% -37% -40% -51% -60%
unpack_assign 16061/s 36% 2% 2% -- -36% -39% -50% -59%
unpack_return_ref 25121/s 113% 60% 59% 56% -- -4% -22% -35%
unpack_loop 26188/s 122% 67% 66% 63% 4% -- -19% -33%
unpack_return_array 32310/s 173% 106% 105% 101% 29% 23% -- -17%
substr_return_array 38910/s 229% 148% 147% 142% 55% 49% 20% --
1000
Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array
substr_assign 1309/s -- -23% -25% -28% -52% -54% -62% -67%
substr_return_ref 1709/s 31% -- -3% -6% -38% -41% -51% -57%
substr_loop 1756/s 34% 3% -- -3% -36% -39% -49% -56%
unpack_assign 1815/s 39% 6% 3% -- -34% -37% -48% -55%
unpack_return_ref 2738/s 109% 60% 56% 51% -- -5% -21% -32%
unpack_loop 2873/s 120% 68% 64% 58% 5% -- -17% -28%
unpack_return_array 3470/s 165% 103% 98% 91% 27% 21% -- -14%
substr_return_array 4015/s 207% 135% 129% 121% 47% 40% 16% --
10000
Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array
substr_assign 131/s -- -23% -27% -28% -52% -55% -63% -67%
substr_return_ref 171/s 30% -- -5% -6% -38% -42% -52% -57%
substr_loop 179/s 37% 5% -- -1% -35% -39% -50% -55%
unpack_assign 181/s 38% 6% 1% -- -34% -38% -49% -55%
unpack_return_ref 274/s 109% 60% 53% 51% -- -6% -23% -32%
unpack_loop 293/s 123% 71% 63% 62% 7% -- -18% -27%
unpack_return_array 356/s 171% 108% 98% 96% 30% 21% -- -11%
substr_return_array 400/s 205% 134% 123% 121% 46% 37% 13% --
100000
Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array
substr_assign 13.0/s -- -22% -26% -29% -51% -55% -63% -67%
substr_return_ref 16.7/s 29% -- -5% -8% -37% -43% -52% -58%
substr_loop 17.6/s 36% 5% -- -3% -33% -40% -50% -56%
unpack_assign 18.2/s 40% 9% 3% -- -31% -37% -48% -54%
unpack_return_ref 26.4/s 103% 58% 50% 45% -- -9% -25% -34%
unpack_loop 29.1/s 124% 74% 65% 60% 10% -- -17% -27%
unpack_return_array 35.1/s 170% 110% 99% 93% 33% 20% -- -12%
substr_return_array 39.7/s 206% 137% 125% 118% 50% 36% 13% --
所以回到原来的问题：“unpack()比substr()更快？答案：总是对于这种类型的应用程序 – 除非你不关心返回值;)