Thanks for your research Mike, i'm a bit puzzled. I have a custom random-array generator that i use to fill the available memory to it's max, about 1.5G on a 2G system, then passing it to a recursive json string generator. Not passing by reference did double my memory requirements, but i've changed about 7-10 functions so i dont know which one was the pig..
On Thu, Jan 28, 2010 at 12:41 PM, Ford, Mike <m.f...@leedsmet.ac.uk> wrote: >> -----Original Message----- >> From: Rene Veerman [mailto:rene7...@gmail.com] >> Sent: 27 January 2010 22:46 >> >> And if your script needs to pass large (> 5Mb) arrays around to >> functions, be sure to use passing-by-reference; failing to do so can >> double your memory requirements, >> possibly hitting the ini_set('memory_lmit', ??) > > Have you benchmarked this? PHP's copy-on-change philosophy means there > shouldn't be much difference in memory terms -- so unless you actually expect > to change the array's contents, you should pass by value. > > As proof, I constructed this little test: > > function test($arg, $base_mem) > { > echo "Additional inside func = ", > memory_get_usage()-$base_mem, "<br />\n"; > } > > $mem_start = memory_get_usage(); > > $array = array_fill(1,1000,"The quick brown fox jumps over the lazy > dog"); > $mem_array = memory_get_usage(); > > echo "After array creation = ", $mem_array-$mem_start, "<br />\n"; > > test($array, $mem_array); > > echo "Final = ", memory_get_usage()-$mem_start, "<br />\n"; > > Which produced: > > After array creation = 546104 > Additional inside func = 208 > Final = 546312 > > Changing the function definition to have &$arg instead of just $arg changed > this to: > > After array creation = 545984 > Additional inside func = 208 > Final = 546192 > > In other words, there was no difference in the memory used to call a function > with a by-reference argument vs a by-value argument, but the by-value version > occupied an additional 120 bytes overall (which is hardly going to break the > bank!). (By varying the numbers in the array_fill(), I found some minor > variations, but nothing significant.) > > Much more interesting is adding the following line to the beginning of the > function: > > $x = count($arg); > > In this case, the by-value figures were: > > After array creation = 546104 > Additional inside func = 280 > Final = 546384 > > ... but by-reference gave: > > After array creation = 545984 > Additional inside func = 64600 > Final = 610456 > > In other words, just using count() on the array increased memory usage for > the by-*reference* version by over 60 kbytes, whereas the by-value version > only went up by 72 bytes. > > Finally, I ran a quick benchmark of runtimes for a version of the function > that just returns a single element of the array, which doesn't incur the > ballooning memory demonstrated above. For 10,000 calls to the function (in a > for loop, and averaged over several runs), the respective times were: > > By value: 5.54sec > By reference: 5.64sec > > Or, the by-value version was very, very slightly quicker. > > All in all, there is no evidence that passing arrays to functions by value is > inherently more expensive than by reference, and in fact in some cases it can > be significantly cheaper. Which in turn says that you should only pass > arguments by reference when you actually expect to change them, which is > after all what the by-reference operator is there for in the first place! QED. > > Cheers! > > Mike > > -- > Mike Ford, > Electronic Information Developer, Libraries and Learning Innovation, > Leeds Metropolitan University, C507, Civic Quarter Campus, > Woodhouse Lane, LEEDS, LS1 3HE, United Kingdom > Email: m.f...@leedsmet.ac.uk > Tel: +44 113 812 4730 > > > > > To view the terms under which this email is distributed, please go to > http://disclaimer.leedsmet.ac.uk/email.htm > > -- > PHP General Mailing List (http://www.php.net/) > To unsubscribe, visit: http://www.php.net/unsub.php > > -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
