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.Ford@xxxxxxxxxxxxxx> wrote: >> -----Original Message----- >> From: Rene Veerman [mailto:rene7705@xxxxxxxxx] >> 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.ford@xxxxxxxxxxxxxx > 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
