Bug #34540 [Com]: dramatic count and strlen slow down on variable reference

Edit report at https://bugs.php.net/bug.php?id=34540&edit=1 ID: 34540 Comment by: sergio dot nalin at gmail dot com Reported by: php at vicaya dot com Summary: dramatic count and strlen slow down on variable reference Status: Not a bug Type: Bug Package: Performance problem Operating System: Linux 2.6.12 PHP Version: 4.4.0 Block user comment: N Private report: N New Comment: May I ask why does PHP need to make a copy of the variable? Shouldn't copy-on-write suffice to handle this? strlen(), count(), etc aren't going to write anything to the variable and therefore should not cause a copy (at least for performance's sake). Previous Comments: ------------------------------------------------------------------------ [2005-09-18 22:54:21] php at vicaya dot com Fair enough, as strlen and count is documented to accept a mixed var not a reference. But the builtin functions like count and strlen should be able to handle reference efficiently. Why it'd be so hard to do so? There're a lot of cases you want to use count and strlen on a reference to variable. eg. you want to modify some huge array or string but you want to use count and strlen first to find out the length... Maybe it's not a language bug per se, But a limitation/problem of the interface of the builtin functions like strlen and count. ------------------------------------------------------------------------ [2005-09-18 20:13:33] derick@php.net Thank you for taking the time to write to us, but this is not a bug. Please double-check the documentation available at http://www.php.net/manual/ and the instructions on how to report a bug at http://bugs.php.net/how-to-report.php In case you pass it by reference to the function, and then use the same variable as parameter to a function that doesn\'t accept by reference, then PHP needs to make a copy of the array. In case you do not pass by reference, in this case PHP doesn\'t have to copy the array because it\'s internal refcounting mechanism is smart enough. If you are just passing by reference to ensure PHP doesn\'t create a copy, then you\'re doing things wrong. You should only pass by reference when you are actually modifying the array inside the function. ------------------------------------------------------------------------ [2005-09-18 08:18:26] php at vicaya dot com Description: ------------ There is a dramatic (talking about orders of magnitude here) slow down when use count or strlen on referenced variables (including pass by reference and using the global keyword). strace shows excessive brk and mremap activities in such cases. If you use the foreach loop (commented out in the attached code) to count the elements, the version using reference is faster. See attached code and benchmark for details. # pass by value and count() $ /usr/bin/time php passva.php 2.43user 0.30system 0:02.77elapsed 98%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+47193minor)pagefaults 0swaps # pass by reference and count() /usr/bin/time php passra.php 70.32user 14.52system 1:27.73elapsed 96%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+2982532minor)pagefaults 0swaps # pass by value and foreach loop $ /usr/bin/time php passva.php 127.99user 1.72system 2:13.97elapsed 96%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+276203minor)pagefaults 0swaps # pass by reference and foreach loop $ /usr/bin/time php passra.php 47.73user 0.37system 0:49.36elapsed 97%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+47194minor)pagefaults 0swaps Reproduce code: --------------- <? # passva.php for ($i = 0; $i < 1000000; ++$i) { $a[] = $i; } for($i = 0 ; $i < 100 ; ++$i ) doit($a); #function doit($arg) { $s = 0; foreach($arg as $i) { ++$s; } } function doit($arg) { count($arg); } ?> <? # passra.php for ($i = 0; $i < 1000000; ++$i) { $a[] = $i; } for($i = 0 ; $i < 100 ; ++$i ) doit($a); #function doit(&$arg) { $s = 0; foreach($arg as $i) { ++$s; } } function doit(&$arg) { count($arg); } ?> ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=34540&edit=1