Re: [PHP] Return Values Copied? Copied if By Reference?
On Thu, July 27, 2006 1:05 pm, Adam Zey wrote: Then how come when I do a foreach on an array (without modifying anything within the foreach), it still makes a copy of the array that consumes memory? I think it's dangerous to generalize that it's always best to let PHP make copies of things. In the foreach situation, the preferred solution when memory is a problem is to either use a reference, or have foreach iterate over the keys of the array. I think this is because you can sometimes use to modify the contents of the iteratee (is that a word?) so PHP does a copy there blindly, whether you have or not, because you might have there sometimes. I think Dmitry or Antony or ??? is looking at changing that so the copy is only done when is present. At least, that's how I understand (or not) the upshot of a thread on Internals. It's entirely possible, even likely, that I'm completely misunderstanding both of these threads. :-) -- Like Music? http://l-i-e.com/artists.htm -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Return Values Copied? Copied if By Reference?
On Thu, 2006-07-27 at 01:35, Larry Garfield wrote: On Wednesday 26 July 2006 21:41, Robert Cummings wrote: I'm working on some code that would be called to generate a cell in a possibly large table and therefore a small difference in performance may have a significant impact. PHP uses copy-on-write and so copies are essentially shared until such time as you modify one of them. If you don't need references then copies are faster than references. By the same token, then, if I have a function that generates a large string and returns it, is there any benefit to return-by-reference? Nope. You should only use references if you really need them. Attempting to improve efficiency by using references instead of copies when you aren't actually in need of a reference will result in less efficiency since the engine spends more time creating the reference than the copy. Cheers, Rob. -- .. | InterJinn Application Framework - http://www.interjinn.com | :: | An application and templating framework for PHP. Boasting | | a powerful, scalable system for accessing system services | | such as forms, properties, sessions, and caches. InterJinn | | also provides an extremely flexible architecture for | | creating re-usable components quickly and easily. | `' -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Return Values Copied? Copied if By Reference?
Robert Cummings wrote: On Thu, 2006-07-27 at 01:35, Larry Garfield wrote: On Wednesday 26 July 2006 21:41, Robert Cummings wrote: I'm working on some code that would be called to generate a cell in a possibly large table and therefore a small difference in performance may have a significant impact. PHP uses copy-on-write and so copies are essentially shared until such time as you modify one of them. If you don't need references then copies are faster than references. By the same token, then, if I have a function that generates a large string and returns it, is there any benefit to return-by-reference? Nope. You should only use references if you really need them. Attempting to improve efficiency by using references instead of copies when you aren't actually in need of a reference will result in less efficiency since the engine spends more time creating the reference than the copy. Cheers, Rob. Then how come when I do a foreach on an array (without modifying anything within the foreach), it still makes a copy of the array that consumes memory? I think it's dangerous to generalize that it's always best to let PHP make copies of things. In the foreach situation, the preferred solution when memory is a problem is to either use a reference, or have foreach iterate over the keys of the array. Regards, Adam. -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
RE: [PHP] Return Values Copied? Copied if By Reference?
Robert Cummings wrote:
Then how come when I do a foreach on an array (without modifying
anything within the foreach), it still makes a copy of the array that
consumes memory? I think it's dangerous to generalize that it's always
best to let PHP make copies of things. In the foreach situation, the
preferred solution when memory is a problem is to either use a
reference, or have foreach iterate over the keys of the array.
Regards, Adam.
-
PHP doesn't seem to make a real copy of data until one of the copies is
modified, making it necessary to create a new set of data. So, it is pretty
smart about that.
Here is a small CLI script that seems to support this:
?php
$a_array = array();
for($i = 0; $i 1; $i++) {
$a_array[] = time(); // Just an arbitrary piece of data
}
echo 'Memory Usage: ' . memory_get_usage() . \n;
echo Making a copy of the array.\n;
$a_copy = $a_array;
echo 'Memory Usage: ' . memory_get_usage() . \n;
echo Modifying the copy:\n;
$a_copy[] = time();
echo 'Memory Usage: ' . memory_get_usage() . \n;
?
On my machine, this displays:
Memory Usage: 640280
Making a copy of the array.
Memory Usage: 640440
Modifying the copy:
Memory Usage: 1106056
-K. Bear
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Re: [PHP] Return Values Copied? Copied if By Reference?
KermodeBear wrote:
Robert Cummings wrote:
Then how come when I do a foreach on an array (without modifying
anything within the foreach), it still makes a copy of the array that
consumes memory? I think it's dangerous to generalize that it's always
best to let PHP make copies of things. In the foreach situation, the
preferred solution when memory is a problem is to either use a
reference, or have foreach iterate over the keys of the array.
Regards, Adam.
-
PHP doesn't seem to make a real copy of data until one of the copies is
modified, making it necessary to create a new set of data. So, it is pretty
smart about that.
Here is a small CLI script that seems to support this:
?php
$a_array = array();
for($i = 0; $i 1; $i++) {
$a_array[] = time(); // Just an arbitrary piece of data
}
echo 'Memory Usage: ' . memory_get_usage() . \n;
echo Making a copy of the array.\n;
$a_copy = $a_array;
echo 'Memory Usage: ' . memory_get_usage() . \n;
echo Modifying the copy:\n;
$a_copy[] = time();
echo 'Memory Usage: ' . memory_get_usage() . \n;
?
On my machine, this displays:
Memory Usage: 640280
Making a copy of the array.
Memory Usage: 640440
Modifying the copy:
Memory Usage: 1106056
-K. Bear
I note that your example doesn't use a foreach loop, which is what my
post was about.
Some testing shows my original premise was wrong, but it's still
possible to get PHP to copy the array when it doesn't need to. In this
case, using a reference can save memory even when a reference isn't
needed. If you want to modify the original array in the foreach, then it
is better to step over the array yourself and save memory, or pass the
foreach a reference to the array to stop it from doing a copy.
I'm just trying to say, I don't think it's always a good idea to rely on
the language deciding when to copy. Sometimes it's a good idea to use a
reference to stop the copy if memory is a concern.
Regards, Adam Zey.
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Re: [PHP] Return Values Copied? Copied if By Reference?
On Thu, 2006-07-27 at 14:48, Adam Zey wrote:
KermodeBear wrote:
Robert Cummings wrote:
Then how come when I do a foreach on an array (without modifying
anything within the foreach), it still makes a copy of the array that
consumes memory? I think it's dangerous to generalize that it's always
best to let PHP make copies of things. In the foreach situation, the
preferred solution when memory is a problem is to either use a
reference, or have foreach iterate over the keys of the array.
Regards, Adam.
-
PHP doesn't seem to make a real copy of data until one of the copies is
modified, making it necessary to create a new set of data. So, it is pretty
smart about that.
Here is a small CLI script that seems to support this:
?php
$a_array = array();
for($i = 0; $i 1; $i++) {
$a_array[] = time(); // Just an arbitrary piece of data
}
echo 'Memory Usage: ' . memory_get_usage() . \n;
echo Making a copy of the array.\n;
$a_copy = $a_array;
echo 'Memory Usage: ' . memory_get_usage() . \n;
echo Modifying the copy:\n;
$a_copy[] = time();
echo 'Memory Usage: ' . memory_get_usage() . \n;
?
On my machine, this displays:
Memory Usage: 640280
Making a copy of the array.
Memory Usage: 640440
Modifying the copy:
Memory Usage: 1106056
-K. Bear
I note that your example doesn't use a foreach loop, which is what my
post was about.
Some testing shows my original premise was wrong, but it's still
possible to get PHP to copy the array when it doesn't need to. In this
case, using a reference can save memory even when a reference isn't
needed. If you want to modify the original array in the foreach, then it
is better to step over the array yourself and save memory, or pass the
foreach a reference to the array to stop it from doing a copy.
I'm just trying to say, I don't think it's always a good idea to rely on
the language deciding when to copy. Sometimes it's a good idea to use a
reference to stop the copy if memory is a concern.
PHP creates a real copy when one of the original arrays changes. This is
the principle of copy-on-write. If you are getting a copy then one of
the arrays changed, so now you really have to choose between a copy and
reference knowing that a reference will affect all arrays to which you
made a reference or a copy which will only affect the copy to which the
change was made. So this returns to my original assertion that you
should only use references when a reference is necessary.
Cheers,
Rob.
--
..
| InterJinn Application Framework - http://www.interjinn.com |
::
| An application and templating framework for PHP. Boasting |
| a powerful, scalable system for accessing system services |
| such as forms, properties, sessions, and caches. InterJinn |
| also provides an extremely flexible architecture for |
| creating re-usable components quickly and easily. |
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Re: [PHP] Return Values Copied? Copied if By Reference?
On Wed, 2006-07-26 at 22:29, Michael B Allen wrote:
Is a function return value copied? If the value is an integer I suppose
it is but what about a string or an array? If you pass by reference is
the return value still copied?
For example, is this:
function foo($arr) {
$arr[] = bar;
}
faster than this?
function foo($arr) {
$arr[] = bar;
return $arr; // is this copied?
}
I'm working on some code that would be called to generate a cell in a
possibly large table and therefore a small difference in performance
may have a significant impact.
PHP uses copy-on-write and so copies are essentially shared until such
time as you modify one of them. If you don't need references then copies
are faster than references.
Cheers,
Rob.
--
..
| InterJinn Application Framework - http://www.interjinn.com |
::
| An application and templating framework for PHP. Boasting |
| a powerful, scalable system for accessing system services |
| such as forms, properties, sessions, and caches. InterJinn |
| also provides an extremely flexible architecture for |
| creating re-usable components quickly and easily. |
`'
--
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Re: [PHP] Return Values Copied? Copied if By Reference?
On Wednesday 26 July 2006 21:41, Robert Cummings wrote: I'm working on some code that would be called to generate a cell in a possibly large table and therefore a small difference in performance may have a significant impact. PHP uses copy-on-write and so copies are essentially shared until such time as you modify one of them. If you don't need references then copies are faster than references. By the same token, then, if I have a function that generates a large string and returns it, is there any benefit to return-by-reference? -- Larry Garfield AIM: LOLG42 [EMAIL PROTECTED] ICQ: 6817012 If nature has made any one thing less susceptible than all others of exclusive property, it is the action of the thinking power called an idea, which an individual may exclusively possess as long as he keeps it to himself; but the moment it is divulged, it forces itself into the possession of every one, and the receiver cannot dispossess himself of it. -- Thomas Jefferson -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
