> -----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
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