Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:25842
Mailing-List: contact internals-help@lists.php.net; run by ezmlm
Received-SPF: error (pb1.pair.com: domain realplain.com from 209.142.136.132 cause and error)
Message-ID: <00dc01c6e231$3d091c80$0201a8c0@pc1>
To: <internals@lists.php.net>
Date: Wed, 27 Sep 2006 07:34:12 -0500
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Subject: array/HashTable filling optimization
From: php_lists@realplain.com ("Matt W")

Hi all,

I noticed awhile ago how most every use of zend[_u]_hash_init has nSize as
0.  Of course it isn't always known how many elements will be added to the
array (nTableSize), but there are places where an accurate value could be
used instead of getting the minimum default (8).  For anyone who doesn't
know, HashTable sizes are a power of 2, and when there are more than that
many elements, zend_hash_do_resize realloc's more memory to double the table
size, and then zend_hash_rehash basically goes through and "reinserts" all
the elements again.

I ran some tests to see the speed improvement from specifying the *actual*
number of elements in hash_init, thus saving that extra work (_rehash
mostly).  The "n+1" is with one more element (9, 17, 33...) that triggers
another rehash when the actual number wasn't specified (most extreme).  (I
used add_next_index_zval, so the direct zend_hash* functions would give
slightly higher % I guess, being less overhead, right?)  Results with
5.2.0-RC4:

          Elements
  n   |   n      n+1
---------------------
8     |    0%   14.2%
16    | 11.0%   20.9%
32    | 13.5%   22.1%
64    | 12.6%   21.3%
128   | 11.7%   18.5%
256   |  9.3%   16.4%
512   |  8.6%   17.0%
1024  |  7.9%   15.8%
2048  |  4.8%   33.3%
4096  |  7.8%   28.3%
8192  | 10.2%   58.4%
16384 | 24.1%   70.5%
32768 | 34.5%   80.4%
65536 | 34.8%   68.6%

I haven't looked thoroughly, but the only place I've seen that uses an
non-zero nSize in hash_init (besides some in the core engine) is the
unserialize() function. :-/  It seems the most simple and obvious place that
could be changed is in Zend/zend_variables.c:_zval_copy_ctor_func?  Just
replace

zend[_u]_hash_init(tmp_ht, 0, ...
with
zend[_u]_hash_init(tmp_ht, original_ht->nNumOfElements, ...

Other than that, some of PHP's array functions and array-returning functions
(database fetch row functions, etc.) are ones I thought of that could be
optimized like this.  Maybe create an array_init_size() macro to be used in
places where the number of elements can be easily determined?  Thoughts?
I'd volunteer to look for places that could be updated and modify them. :-)


Thanks,
Matt

P.S.  I guess the array stuff applies for objects also?  But I don't know
much about their internals...