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Date: Mon, 31 Jan 2011 11:45:16 +1100
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To: Adi Mutu <adi_mutu06@yahoo.com>
CC: internals@lists.php.net, tony@daylessday.org
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Subject: Re: [PHP-DEV] Re: Zend mm
From: mail_ben_schmidt@yahoo.com.au (Ben Schmidt)

On 30/01/11 4:11 AM, Adi Mutu wrote:
 > I have looked at the sources, but i'm only at the begining of
 > analyzing the php sources.....i've understand how to write extensions
 > for example, but this memory manager seems more complicated to me.....

Yes, it is. Memory management is a low-level and subtle area. It takes a
lot of careful thought to design and even to understand the algorithms
involved. It is hard to debug, because an error caused in one place may
not show itself until completely different code is executing. It is also
an area which doesn't change much. Almost all the other code in the PHP
interpreter, and therefore PHP scripts themselves, rely on the memory
manager code doing its job unintrusively and correctly, so once it is
working well, it tends to be left that way, except for fixing small bugs
as they are found, or a big and careful development effort to address
some limitation (e.g. improving the garbage collection to collect
cycles, which was something done fairly recently).

The bottom line is that this is not an area that is all that easy for
beginners to get their teeth into. But it's also an area that isn't
necessary for beginners to understand in much detail, either. Unless you
have a particular project in mind that involves the memory manager, you
probably don't need to touch it.

I don't want to discourage you, but just point out that perhaps this is
something you might look at later, or even not at all, as you may find
there are many worthwhile projects that take up all your time before you
ever get to understanding the memory manager fully.

 > It would be useful if for example, could explain me the structures
 > that are used in the Zend memory manager, how they're chained
 > etc.....and if you can tell me in words what do zend_mm_aloc_int() and
 > zend_mm_free_int() basically do...Thank you for your time,Adrian

I suspect these structures and functions are fairly easy to understand
by someone familiar with the area of memory management, which is why you
have been advised to consult the source code. So perhaps what you need
to do is not learn about PHP's memory manager, but just learn about
memory management in general. Then it should be fairly easy to
understand what PHP's memory manager is doing from its code.

I'm afraid I doubt the people here have the time to teach you this over
email. :-) So here are some references which might be of use to you.

- I believe the 'standard textbook' for learning about memory management
   is this one here:
   http://www.cs.kent.ac.uk/people/staff/rej/gcbook/gcbook.html

- This page also mentions that a new edition is due out soon:
   http://c2.com/cgi/wiki?GarbageCollectionBook

- You can also check out this site: http://www.memorymanagement.org/

- Relating this to, PHP: PHP is a garbage collected language which I
   believe uses reference counting and a clever collector to detect and
   free cycles. I haven't looked in detail into the implementation, but I
   know some documentation is available on the subject here:
   http://www.php.net/manual/en/features.gc.php

Hope this helps!

Ben.