Re: perl process slower and slower when loop number increase

[Dallaylaen]

The total execution time is startup_time + count * iteration_time. For numbers like 10..10000 the loop time is negligible compared to reading the interpreter from disk, parsing script etc.

You can eliminate the startup time (assuming it's constant) by running several different big (10**6+) loops, then plotting time against count:

bash$ for i in `seq 10`; do echo $i; time perl -e 'for($i=0;$i<='$i'*1
+000000 ;$i++){}'; done 
1

real    0m0.055s
user    0m0.048s
sys    0m0.000s
2

real    0m0.051s
user    0m0.048s
sys    0m0.000s
3

real    0m0.074s
user    0m0.068s
sys    0m0.000s
4

real    0m0.099s
user    0m0.088s
sys    0m0.004s
5

real    0m0.124s
user    0m0.116s
sys    0m0.000s
6

real    0m0.146s
user    0m0.140s
sys    0m0.000s
7

real    0m0.166s
user    0m0.164s
sys    0m0.000s
8

real    0m0.197s
user    0m0.184s
sys    0m0.000s
9

real    0m0.217s
user    0m0.204s
sys    0m0.004s
10

real    0m0.235s
user    0m0.232s
sys    0m0.000s

bash$ for i in `seq 10`; do echo $i; time php -r 'for($i=0;$i<='$i'*10
+00000 ;$i++){}'; done 
1

real    0m0.034s
user    0m0.020s
sys    0m0.012s
2

real    0m0.036s
user    0m0.028s
sys    0m0.000s
3

real    0m0.035s
user    0m0.032s
sys    0m0.000s
4

real    0m0.040s
user    0m0.036s
sys    0m0.000s
5

real    0m0.048s
user    0m0.040s
sys    0m0.004s
6

real    0m0.055s
user    0m0.052s
sys    0m0.000s
7

real    0m0.065s
user    0m0.060s
sys    0m0.000s
8

real    0m0.071s
user    0m0.068s
sys    0m0.000s
9

real    0m0.080s
user    0m0.076s
sys    0m0.000s
10

real    0m0.087s
user    0m0.084s
sys    0m0.000s

What surprises me here is that PHP is much faster than Perl.

[hippo]

What surprises me here is that PHP is much faster than Perl.

It's no surprise to me. I always suspected PHP would be really fantastic at doing something utterly useless. ;-)

[karlgoethebier]

"...PHP is much faster than Perl"

I guess this must be a measurement error ;-)

Best regards, Karl

[Anonymous Monk]

not just php,

xxxx vs perl have same behaviour.

this loop test is maybe "useless" and "doing nothing" but why only perl slowing down for "do nothing" until "certain number",

and for "real useful app", try to parse 1M line of single file, perl already faster from other for read, match with regex, text processing, etc, but this "loop number" is the problem

the question is still: can that "certain number" be set ?

try:

time node -e 'for($i=0;$i<=1000000;$i++){}'

or use java, c, etc
for "doing nothing" they not slowing down, only perl does

[dave_the_m]

Bear in mind that C, Java etc are compiled languages. The compiler can take as long as it likes to optimise code, and is likely to detect an empty loop and optimise it away. Perl does its compilation as part of its execution, and can therefore only spend a limited amount of time applying optimisations. In particular, it won't optimise away a for(;;) loop with an empty body. So the timings you see for perl are a fixed compilation overhead, followed by N executions of a sequence of opcodes which collectively increment $i, test it against a limiting value, and jump back.

You haven't shown anything apart from perl taking a time linearly proportional the the number of times the loop is executed. It's not clear to me why you find that surprising.

Dave.

[Anonymous Monk]

>> Bear in mind that C, Java etc are compiled languages

AFAIK, C is compiled language, Java etc only "compile" to intermediate code, well, AFAIK (again) perl have optimizer that do like java, but no need create intermediate code file like java class

php and nodejs are not "compiled" language, but their loop process not slowing down

>> You haven't shown anything apart from perl taking a time linearly proportional the the number of times the loop is executed

in my vps box:

$ time perl -e ''

real    0m0.003s
user    0m0.000s
sys 0m0.000s

$ time perl -e 'for($i=0;$i<=0;$i++){}'

real    0m0.003s
user    0m0.000s
sys 0m0.000s

$ time perl -e 'for($i=0;$i<=10;$i++){}'

real    0m0.004s
user    0m0.004s
sys 0m0.000s
$ time perl -e 'for($i=0;$i<=100;$i++){}'

real    0m0.004s
user    0m0.000s
sys 0m0.000s
$ time perl -e 'for($i=0;$i<=1000;$i++){}'

real    0m0.003s
user    0m0.000s
sys 0m0.000s
$ time perl -e 'for($i=0;$i<=10000;$i++){}'

real    0m0.005s
user    0m0.004s
sys 0m0.000s
$ time perl -e 'for($i=0;$i<=100000;$i++){}'

real    0m0.015s
user    0m0.012s
sys 0m0.000s
$ time perl -e 'for($i=0;$i<=1000000;$i++){}'

real    0m0.094s
user    0m0.088s
sys 0m0.004s
$ time perl -e 'for($i=0;$i<=10000000;$i++){}'

real    0m0.731s
user    0m0.724s
sys 0m0.004s
$ time perl -e 'for($i=0;$i<=100000000;$i++){}'

real    0m7.253s
user    0m7.256s
sys 0m0.000s
$ time perl -e 'for($i=0;$i<=1000000000;$i++){}'

real    1m13.921s
user    1m13.828s
sys 0m0.008s

I still think there limit number between $i=100000 and $i=1000000;

but where to set it ?