Array Insert Time Jump - php

Question

During deep researching about hash and zval structure and how arrays are based on it, faced with strange insert time.
Here is example:
$array = array();
$someValueToInsert = 100;
for ($i = 0; $i < 10000; ++$i) {
$time = microtime(true);
array_push($array, $someValueToInsert);
echo $i . " : " . (int)((microtime(true) - $time) * 100000000) . "</br>";
}
So, I found that every 1024, 2024, 4048... element will be inserted using much more time(>~x10).
It doesn't depends will I use array_push, array_unshift, or simply $array[] = someValueToInsert.
I'm thinking about that in Hash structure:
typedef struct _hashtable {
...
uint nNumOfElements;
...
} HashTable;
nNumOfElements has default max value, but it doesn't the answer why does it took more time to insert in special counters(1024, 2048...).
Any thoughts ?

Answer 1

While I would suggest double checking my answer on the PHP internals list, I believe the answer lay in zend_hash_do_resize(). When more elements are needed in the hash table, this function is called and the extant hash table is doubled in size. Since the table starts life at 1024, this doubling explains the results you've observed. Code:
} else if (ht->nTableSize < HT_MAX_SIZE) { /* Let's double the table size */
void *old_data = HT_GET_DATA_ADDR(ht);
Bucket *old_buckets = ht->arData;
HANDLE_BLOCK_INTERRUPTIONS();
ht->nTableSize += ht->nTableSize;
ht->nTableMask = -ht->nTableSize;
HT_SET_DATA_ADDR(ht, pemalloc(HT_SIZE(ht), ht->u.flags & HASH_FLAG_PERSISTENT));
memcpy(ht->arData, old_buckets, sizeof(Bucket) * ht->nNumUsed);
pefree(old_data, ht->u.flags & HASH_FLAG_PERSISTENT);
zend_hash_rehash(ht);
HANDLE_UNBLOCK_INTERRUPTIONS();
I am uncertain if the remalloc is the performance hit, or if the rehashing is the hit, or the fact that the whole block is uninterruptable. Would be interesting to put a profiler on it. I think some might have already done that for PHP 7.
Side note, the Thread Safe version does things differently. I'm not overly familiar with that code, so there may be a different issue going on if your using ZTS.

Answer 2

I think it is related to implementation of dynamic arrays.
See here "Geometric expansion and amortized cost" http://en.wikipedia.org/wiki/Dynamic_array
To avoid incurring the cost of resizing many times, dynamic arrays resize by a large amount, **such as doubling in size**, and use the reserved space for future expansion
You can read about arrays in PHP here as well https://nikic.github.io/2011/12/12/How-big-are-PHP-arrays-really-Hint-BIG.html
It is a standard practice for dynamic arrays. E.g. check here C++ dynamic array, increasing capacity
capacity = capacity * 2; // doubles the capacity of the array