- PHPng is significantly faster and uses significantly less memory
- objects need more memory than arrays (+ 5-250 percent)
- if array values are numeric, don't save them as strings!
- saving 1M integers takes 33M of memory with PHPng (increase by factor 8)
- saving 1M integers as strings takes 79M of memory with PHPng (increase by factor 20)
- using SplFixedArray can reduce memory usage by 20-100 percent
- avoid big Arrays and Objects in PHP whenever possible
(don't use file('big_file') or explode("\n", file_get_contents('big_file')), etc.) - use streams whenever possible (fopen, fsockopen, etc.)
- use generators when available with PHP 5.5 (RFC)
- comparing 32bit to 64bit systems, memory consumption increases by 100-230 percent
If you need to save memory temporarily in your script, you can stringify your array (increasing cpu usage and runtime):
function test(){
$a = array();
for ($i=0; $i<1000000; $i++) $a[] = $i;
$a = implode(',', $a); // or $a = json_encode($a);
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
}
test();
// 7.5 M / 0.64s (php), 10.4 M / 0.55s (hhvm), 7.25 M / 0.20s (phpng)
Here is a small script to show how much memory PHP needs to handle big Arrays:
the total runtime of the script is 15.5s (php), 8.4s (hhvm), 4.6s (phpng)
(tests made with PHP 5.5.9, HHVM 3.2.0-2014-06-06, PHP 5.7.0-dev-2014-06-05, 2.8GHz single-core openvz)
ini_set('memory_limit', '1024M');
function test(){
$a = '';
for ($i=0; $i<1000000; $i++) $a .= '1';
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 1.50 (php), 3.5 (hhvm), 1.50 (phpng)
$a = array();
for ($i=0; $i<1000000; $i++) $a[$i] = true;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 138.25 (php), 30.4 (hhvm), 32.75 (phpng)
$a = new stdclass;
for ($i=0; $i<1000000; $i++) $a->$i = true;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 146.25 (php), 80.92 (hhvm), 82.50 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<1000000; $i++) $a[$i] = true;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 54.25 (php), 30.56 (hhvm), 16.50 (phpng)
$a = array();
for ($i=0; $i<1000000; $i++) $a[$i] = $i;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 138.50 (php), 30.56 (hhvm), 33.25 (phpng)
$a = new stdclass;
for ($i=0; $i<1000000; $i++) $a->$i = $i;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 146 (php), 80.97 (hhvm), 82.50 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<1000000; $i++) $a[$i] = $i;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 54.25 (php), 30.58 (hhvm), 16.75 (phpng)
$a = array();
for ($i=0; $i<1000000; $i++) $a[$i] = (string)$i;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 230 (php), 61.10 (hhvm), 78.50 (phpng)
$a = new stdclass();
for ($i=0; $i<1000000; $i++) $a->$i = (string)$i;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 237.75 (php), 111.50 (hhvm), 128.25 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<1000000; $i++) $a[$i] = (string)$i;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 145.5 (php), 61.11 (hhvm), 61.75 (phpng)
$a = array();
for ($i=0; $i<1000000; $i++) $a[(string)$i] = (string)$i;
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 230 (php), 61.12 (hhvm), 78.50 (phpng)
$a = array();
for ($i=0; $i<1000000; $i++) $a[$i] = "";
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 138.5 (php), 30.62 (hhvm), 33.75 (phpng)
$a = new stdclass;
for ($i=0; $i<1000000; $i++) $a->$i = "";
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 146 (php), 81.02 (hhvm), 82.50 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<1000000; $i++) $a[$i] = "";
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 54 (php), 30.63 (hhvm), 17.00 (phpng)
$a = array();
for ($i=0; $i<1000000; $i++) $a[$i] = "hello";
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 138.5 (php), 30.64 (hhvm), 33.75 (phpng)
$a = new stdclass;
for ($i=0; $i<1000000; $i++) $a->$i = "hello";
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 146 (php), 81.04 (hhvm), 82.50 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<1000000; $i++) $a[$i] = "hello";
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 54 (php), 30.65 (hhvm), 17.25 (phpng)
$a = array();
for ($i=0; $i<1000000; $i++) $a[$i] = array();
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 222.5 (php), 30.65 (hhvm), 34.00 (phpng)
$a = new stdclass;
for ($i=0; $i<1000000; $i++) $a->$i = array();
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 230 (php), 81.05 (hhvm), 82.50 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<1000000; $i++) $a[$i] = array();
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 138 (php), 30.66 (hhvm), 17.50 (phpng)
$a = array();
for ($i=0; $i<500000; $i++) $a[$i] = array("");
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 214.75 (php), 18.67 (hhvm), 18.25 (phpng)
$a = new stdclass;
for ($i=0; $i<500000; $i++) $a->$i = array("");
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 218.25 (php), 41.81 (hhvm), 41.75 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<500000; $i++) $a[$i] = array("");
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 176.00 (php), 30.68 (hhvm), 17.50 (phpng)
$a = array();
for ($i=0; $i<500000; $i++) $a[$i] = array("hello");
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 214.50 (php), 18.68 (hhvm), 18.25 (phpng)
$a = new stdclass;
for ($i=0; $i<500000; $i++) $a->$i = array("hello");
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 218.25 (php), 41.82 (hhvm), 41.75 (phpng)
$a = new SplFixedArray(1000000);
for ($i=0; $i<500000; $i++) $a[$i] = array("hello");
echo number_format(memory_get_usage(true)/1048576, 2)."\n";
// 176 (php), 30.69 (hhvm), 17.50 (phpng)
}
test();
More details and technical explanations about PHP's memory usage can be found on nikic's blog.
More details about PHPng can be found here.
Shortly: a string or an integer in PHP are not mapped to a char-array or an integer in C. A more complex structure is used to get from dynamic typing in PHP to static typing in C:
struct _zval_struct {
zvalue_value value; // value object
zend_uint refcount__gc; // number of references to variable
zend_uchar type; // type of variable
zend_uchar is_ref__gc; // reference? (&$var)
};
typedef union _zvalue_value {
long lval; // integer and boolean
double dval; // float (double)
struct {
char *val; // string (with zero bytes)
int len; // length of the string
} str;
HashTable *ht; // array (hash table)
zend_object_value obj; // object
} zvalue_value;
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