REMOVE : Re: Zend engine's hashtable performance tweaks

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----- Original Message -----
From: Marcin Babij [mailto:marcin.babij@nasza-klasa.pl]
To: internals@lists.php.net,apc-dev@lists.php.net
Sent: Mon, 31 Jan 2011 12:35:53 +0100
Subject: Re: [PHP-DEV] Re: Zend engine's hashtable performance tweaks

Hello,
I've managed to change my patch to merge against trunk, there were some
problems with interned strings optimization. I've created bug report with
new patch:
http://bugs.php.net/bug.php?id=53866

I've changed hashing function again, it looks like working better than
previous, but nothings proven.

What's to be done (this is listed also in bug report):

http://lxr.php.net/xref/PHP_TRUNK/ext/standard/html_tables/html_table_gen.php#722
should be changed and html_tables.h regenerated, but this will need to
rewrite hashtable engine from C to PHP,

• APC should be fixed.

Hi Marcin,

Can you mail the PHP Internals list and also the apc-dev@lists.php.net
list when you have logged the bug? I mentioned your patches to Gopal,
the APC maintainer, and he was interested to see what was needed.

Regards,

Chris

Hello,
I merge patch with trunk, do some things already proposed, and I'll
create bug report with patch for trunk in 1, 2 weeks most.

hi,

Thanks for the patches :)

Can you open a bug report please (and attach the patches to it)? I'm
sure this patch will be updated a couple of times before it reaches
the repository.

Cheers,

Hi Marcin,

Did you log a bug for this?

Regards,

Chris

On Fri, Dec 31, 2010 at 4:58 PM, Marcin Babij
marcin.babij@nasza-klasa.pl wrote:

Sorry for no attachments in previous message, I think my attachments
weren't
redirected with message by lists.php.net email confirmation system.
I send
them again, and for sure I attach links to public copy of them over
HTTP:
https://gist.github.com/761094 -
php-5.3.4-hashtable-optimization.patch
https://gist.github.com/761096 -
apc-3.1.6-hashtable-optimization.patch

Hello,
I work for social network company, where we were running
optimization
project. One of it's results is patch to Zend engine's Hashtable,
which we
want to share and ask you for comments and improvements.

Why we do this?
We run profiling on our production servers and found out that
zend_hash_*
functions take 10-20% CPU time of request. So there is some room
for easy
improvements.

What was done?

• Hash function in zend_hash.h was rebuilt and became much faster,
  without
  losing the most important properties.
• Hashtable implementation was changed from Simple chaining to Open
  addressing with linear probing, but with linked bucket, not
  included in
  hash array, which causes:
  -- Bucket structure to lose 2 pointers.
  -- Searching works similar, but don't have to jump with pointers
  stored in
  different memory locations, inserting, deleting and rehashing don't
  need
  to update linked list, but must search for first empty bucket,
  which is
  fast, because it scans continuous memory.
  -- Load factor decreases from 1.0 to 0.5-0.75 to make less
  collisions and
  faster hashtable, which in turn increases memory footprint a little.
• Open addressing doesn't change significantly performance, but
  next thing
  was to create new array (arEmpty), which is of size nTableSize
  bytes,
  which keeps track of used/empty buckets and makes inserting and
  rehashing
  much faster. In future it can be tested as bit-array with size of
  nTableSize/8 bytes.
• More macros were added to replace repetitive constructs.
• New constants were added to allow:
  -- Creating new hashtables of size at least X (where 4 and 8 are
  reasonable), which makes no rehashing and reallocing memory while
  changing
  size to 2 and then to 4.
  -- For small tables it's better to extend them by a factor of 4
  times, not
  2, to make rehashing cost smaller for most hashtables, of cost of
  little
  higher memory consumption.
  -- For large tables it's better to have other load factor, closer
  to 1,
  while for small tables it's better to use load factor closer to 0.5.
• APC was patched to take changes in Bucket structure into account.

How was it tested?
It was tested with make test, where one more (comparing to original
sources) test fails, but it's most probably because
http://bugs.php.net/bug.php?id=48858 - IMO test is badly
constructed (is
too simple) and any change of hashing function makes it fail. Also
it was
tested on our testing environment and production servers
against>30mln
requests to our site, with 120requests/s at peak on Xeon @ 2.50GHz
with
8GB RAM running Debian Linux.

What is the gain?
After tests CPU usage dropped by about 4% -6%.
Memory footprint goes up just by few percent.

What can be done in future?

• Make new constants configurable by php.ini.
• Test if changing arEmpty from byte-array to bit-array helps on
  performance.
• Tweak default constants' values using some real-live benchmarks.
• Prove (or modify and prove) hash function to have property, that
  it has
  no collisions if two keys don't differ on no more than 6 bytes,
  which will
  lead to memcmp omit first (or last) 6 bytes of key. Also simpler
  thing may
  be proven, that is it has no collisions if two keys are not longer
  than 6
  bytes, which will make most string keys omit memcpy at all.

The patch was created and tested against php-5.3.0, apc-3.1.3p1,
then
merged with php-5.3.4, apc-3.1.6 without conflicts, and for these
last
versions patches are attached. Also, it shouldn't conflict with
http://wiki.php.net/rfc/performanceimprovements .

--

--
Email: christopher.jones@oracle.com
Tel: +1 650 506 8630
Blog: http://blogs.oracle.com/opal/