gc.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
 <chapter xml:id="features.gc" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
  <title>Garbage Collection</title>

  <para>
   This section explains the merits of the new Garbage Collection (also known
   as GC) mechanism that is part of PHP 5.3.
  </para>

  <sect1 xml:id="features.gc.refcounting-basics">
   <title>Reference Counting Basics</title>
   <para>
    A PHP variable is stored in a container called a "zval". A zval container
    contains, besides the variable's type and value, two additional bits of
    information. The first is called "is_ref" and is a boolean value
    indicating whether or not the variable is part of a "reference set". With
    this bit, PHP's engine knows how to differentiate between normal variables
    and references. Since PHP allows user-land references, as created by the
    &amp; operator, a zval container also has an internal reference counting
    mechanism to optimize memory usage. This second piece of additional
    information, called "refcount", contains how many variable names (also
    called symbols) point to this one zval container. All symbols are stored in
    a symbol table, of which there is one per scope. There is a scope for the
    main script (i.e., the one requested through the browser), as well as one
    for every function or method.
   </para>
   <para>
    A zval container is created when a new variable is created with a constant
    value, such as:
    <example>
     <title>Creating a new zval container</title>
     <programlisting role="php">
<![CDATA[
<?php
$a = "new string";
?>
]]>
     </programlisting>
    </example>
   </para>
   <para>
    In this case, the new symbol name, <literal>a</literal>, is created in the current scope,
    and a new variable container is created with the type <type>string</type> and the value
    <literal>new string</literal>. The "is_ref" bit is by default set to &false; because no
    user-land reference has been created. The "refcount" is set to <literal>1</literal> as
    there is only one symbol that makes use of this variable container. Note
    that references (i.e. "is_ref" is &true;) with "refcount" <literal>1</literal>, are
    treated as if they are not references (i.e. as "is_ref" was &false;). If you have <link
    xlink:href="&url.xdebug;">Xdebug</link> installed, you can display this
    information by calling <function>xdebug_debug_zval</function>.
   </para>
   <para>
    <example>
     <title>Displaying zval information</title>
     <programlisting role="php">
<![CDATA[
<?php
$a = "new string";
xdebug_debug_zval('a');
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
a: (refcount=1, is_ref=0)='new string'
]]>
     </screen>
    </example>
   </para>
   <para>
    Assigning this variable to another variable name will increase the refcount.
   </para>
   <para>
    <example>
     <title>Increasing refcount of a zval</title>
     <programlisting role="php">
<![CDATA[
<?php
$a = "new string";
$b = $a;
xdebug_debug_zval( 'a' );
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
a: (refcount=2, is_ref=0)='new string'
]]>
     </screen>
    </example>
   </para>
   <para>
    The refcount is <literal>2</literal> here, because the same variable container is linked
    with both <varname>a</varname> and <varname>b</varname>.
    PHP is smart enough not to copy the actual variable
    container when it is not necessary. Variable containers get destroyed when
    the "refcount" reaches zero. The "refcount" gets decreased by one when any
    symbol linked to the variable container leaves the scope (e.g. when the
    function ends) or when a symbol is unassigned (e.g. by calling <function>unset</function>).
    The following example shows this:
   </para>
   <para>
    <example>
     <title>Decreasing zval refcount</title>
     <programlisting role="php">
<![CDATA[
<?php
$a = "new string";
$c = $b = $a;
xdebug_debug_zval( 'a' );
$b = 42;
xdebug_debug_zval( 'a' );
unset( $c );
xdebug_debug_zval( 'a' );
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
a: (refcount=3, is_ref=0)='new string'
a: (refcount=2, is_ref=0)='new string'
a: (refcount=1, is_ref=0)='new string'
]]>
     </screen>
    </example>
   </para>
   <para>
    If we now call <literal>unset($a);</literal>, the variable container, including the type
    and value, will be removed from memory.
   </para>

   <sect2 xml:id="features.gc.compound-types">
    <title>Compound Types</title>

    <para>
     Things get a tad more complex with compound types such as <type>array</type>s and
     <type>object</type>s. As opposed to <type>scalar</type> values, <type>array</type>s
     and <type>object</type>s store their
     properties in a symbol table of their own. This means that the following
     example creates three zval containers:
    </para>
    <para>
     <example>
      <title>Creating a <type>array</type> zval</title>
      <programlisting role="php">
<![CDATA[
<?php
$a = array( 'meaning' => 'life', 'number' => 42 );
xdebug_debug_zval( 'a' );
?>
]]>
      </programlisting>
      &example.outputs.similar;
      <screen>
<![CDATA[
a: (refcount=1, is_ref=0)=array (
   'meaning' => (refcount=1, is_ref=0)='life',
   'number' => (refcount=1, is_ref=0)=42
)
]]>
      </screen>
      <para>Or graphically</para>
      <mediaobject>
       <alt>Zvals for a simple array</alt>
       <imageobject>
        <imagedata fileref="en/features/figures/simple-array.png" format="PNG"/>
       </imageobject>
      </mediaobject>
     </example>
    </para>
    <para>
     The three zval containers are: <varname>a</varname>, <varname>meaning</varname>, and <varname>number</varname>.
     Similar rules apply for increasing and decreasing "refcounts". Below, we add another
     element to the array, and set its value to the contents of an already
     existing element:
    </para>
    <para>
     <example>
      <title>Adding already existing element to an array</title>
      <programlisting role="php">
<![CDATA[
<?php
$a = array( 'meaning' => 'life', 'number' => 42 );
$a['life'] = $a['meaning'];
xdebug_debug_zval( 'a' );
?>
]]>
      </programlisting>
      &example.outputs.similar;
      <screen>
<![CDATA[
a: (refcount=1, is_ref=0)=array (
   'meaning' => (refcount=2, is_ref=0)='life',
   'number' => (refcount=1, is_ref=0)=42,
   'life' => (refcount=2, is_ref=0)='life'
)
]]>
      </screen>
      <para>Or graphically</para>
      <mediaobject>
       <alt>Zvals for a simple array with a reference</alt>
       <imageobject>
        <imagedata fileref="en/features/figures/simple-array2.png" format="PNG"/>
       </imageobject>
      </mediaobject>
     </example>
    </para>
    <para>
     From the above Xdebug output, we see that both the old and new array
     elements now point to a zval container whose "refcount" is
     <literal>2</literal>. Although Xdebug's output shows two zval containers
     with value <literal>'life'</literal>, they are the same one. The
     <function>xdebug_debug_zval</function> function does not show this, but
     you could see it by also displaying the memory pointer.
    </para>
    <para>
     Removing an element from the array is like removing a symbol from a scope.
     By doing so, the "refcount" of a container that an array element points to
     is decreased. Again, when the "refcount" reaches zero, the variable
     container is removed from memory.  Again, an example to show this:
    </para>
    <para>
     <example>
      <title>Removing an element from an array</title>
      <programlisting role="php">
<![CDATA[
<?php
$a = array( 'meaning' => 'life', 'number' => 42 );
$a['life'] = $a['meaning'];
unset( $a['meaning'], $a['number'] );
xdebug_debug_zval( 'a' );
?>
]]>
      </programlisting>
      &example.outputs.similar;
      <screen>
<![CDATA[
a: (refcount=1, is_ref=0)=array (
   'life' => (refcount=1, is_ref=0)='life'
)
]]>
      </screen>
     </example>
    </para>
    <para>
     Now, things get interesting if we add the array itself as an element of
     the array, which we do in the next example, in which we also sneak in a
     reference operator, since otherwise PHP would create a copy:
    </para>
    <para>
     <example>
      <title>Adding the array as an element of itself</title>
      <programlisting role="php">
<![CDATA[
<?php
$a = array( 'one' );
$a[] =& $a;
xdebug_debug_zval( 'a' );
?>
]]>
      </programlisting>
      &example.outputs.similar;
      <screen>
<![CDATA[
a: (refcount=2, is_ref=1)=array (
   0 => (refcount=1, is_ref=0)='one',
   1 => (refcount=2, is_ref=1)=...
)
]]>
      </screen>
      <para>Or graphically</para>
      <mediaobject>
       <alt>Zvals for an array with a circular reference</alt>
       <imageobject>
        <imagedata fileref="en/features/figures/loop-array.png" format="PNG"/>
       </imageobject>
      </mediaobject>
     </example>
    </para>
    <para>
     You can see that the array variable (<varname>a</varname>) as well as the second element
     (<varname>1</varname>) now point to a variable container that has a "refcount" of <literal>2</literal>. The
     "..." in the display above shows that there is recursion involved, which,
     of course, in this case means that the "..." points back to the original
     array.
    </para>
    <para>
     Just like before, unsetting a variable removes the symbol, and the
     reference count of the variable container it points to is decreased by
     one. So, if we unset variable <varname>$a</varname> after running the above code, the
     reference count of the variable container that <varname>$a</varname> and element "1" point to
     gets decreased by one, from "2" to "1". This can be represented as:
    </para>
    <para>
     <example>
      <title>Unsetting <varname>$a</varname></title>
      <screen>
<![CDATA[
(refcount=1, is_ref=1)=array (
   0 => (refcount=1, is_ref=0)='one',
   1 => (refcount=1, is_ref=1)=...
)
]]>
      </screen>
      <para>Or graphically</para>
      <mediaobject>
       <alt>Zvals after removal of array with a circular reference demonstrating the memory leak</alt>
       <imageobject>
        <imagedata fileref="en/features/figures/leak-array.png" format="PNG"/>
       </imageobject>
      </mediaobject>
     </example>
    </para>
   </sect2>

   <sect2 xml:id="features.gc.cleanup-problems">
    <title>Cleanup Problems</title>
    <para>
     Although there is no longer a symbol in any scope pointing to this
     structure, it cannot be cleaned up because the array element "1" still
     points to this same array. Because there is no external symbol pointing to
     it, there is no way for a user to clean up this structure; thus you get a
     memory leak. Fortunately, PHP will clean up this data structure at the end
     of the request, but before then, this is taking up valuable space in
     memory. This situation happens often if you're implementing parsing
     algorithms or other things where you have a child point back at a "parent"
     element. The same situation can also happen with objects of course, where
     it actually is more likely to occur, as objects are always implicitly used
     "<link linkend="language.oop5.references">by reference</link>".
    </para>
    <para>
     This might not be a problem if this only happens once or twice, but if
     there are thousands, or even millions, of these memory losses, this
     obviously starts being a problem. This is especially problematic in long
     running scripts, such as daemons where the request basically never ends,
     or in large sets of unit tests. The latter caused problems while
     running the unit tests for the Template component of the eZ Components
     library. In some cases, it would require over 2 GB of memory, which the
     test server didn't quite have.
    </para>
   </sect2>
  </sect1>

  <sect1 xml:id="features.gc.collecting-cycles">
   <title>Collecting Cycles</title>
   <para>
    Traditionally, reference counting memory mechanisms, such as that used
    previously by PHP, fail to address circular reference memory leaks;
    however, as of 5.3.0, PHP implements the synchronous algorithm from the
    <link xlink:href="&url.gc-paper;">Concurrent Cycle Collection in Reference Counted Systems</link>
    paper which addresses that issue.
   </para>
   <para>
    A full explanation of how the algorithm works would be slightly beyond the
    scope of this section, but the basics are explained here. First of all,
    we have to establish a few ground rules. If a refcount is increased, it's
    still in use and therefore, not garbage. If the refcount is decreased and
    hits zero, the zval can be freed. This means that garbage cycles can only
    be created when a refcount argument is decreased to a non-zero value.
    Secondly, in a garbage cycle, it is possible to discover which parts are
    garbage by checking whether it is possible to decrease their refcount by
    one, and then checking which of the zvals have a refcount of zero.
   </para>
   <para>
     <mediaobject>
      <alt>Garbage collection algorithm</alt>
      <imageobject>
       <imagedata fileref="en/features/figures/gc-algorithm.png" format="PNG"/>
      </imageobject>
     </mediaobject>
   </para>
   <para>
    To avoid having to call the checking of garbage cycles with every possible
    decrease of a refcount, the algorithm instead puts all possible roots
    (zvals) in the "root buffer" (marking them "purple"). It also makes sure
    that each possible garbage root ends up in the buffer only once. Only when
    the root buffer is full does the collection mechanism start for all the
    different zvals inside. See step A in the figure above.
   </para>
   <para>
    In step B, the algorithm runs a depth-first search on all possible roots
    to decrease by one the refcounts of each zval it finds, making sure not to
    decrease a refcount on the same zval twice (by marking them as "grey"). In
    step C, the algorithm again runs a depth-first search from each root node,
    to check the refcount of each zval again. If it finds that the refcount is
    zero, the zval is marked "white" (blue in the figure). If it's larger than
    zero, it reverts the decreasing of the refcount by one with a depth-first
    search from that point on, and they are marked "black" again. In the last
    step (D), the algorithm walks over the root buffer removing the zval roots
    from there, and meanwhile, checks which zvals have been marked "white" in
    the previous step. Every zval marked as "white" will be freed.
   </para>
   <para>
    Now that you have a basic understanding of how the algorithm works, we
    will look back at how this integrates with PHP. By default, PHP's garbage
    collector is turned on. There is, however, a &php.ini;
    setting that allows you to change this:
    <link linkend="ini.zend.enable-gc">zend.enable_gc</link>.
   </para>
   <para>
    When the garbage collector is turned on, the cycle-finding algorithm as
    described above is executed whenever the root buffer runs full. The root
    buffer has a fixed size of 10,000 possible roots (although you can alter
    this by changing the <constant>GC_THRESHOLD_DEFAULT</constant> constant in
    <literal>Zend/zend_gc.c</literal> in the PHP source code, and re-compiling
    PHP). When the garbage collector is turned off, the cycle-finding
    algorithm will never run. However, possible roots will always be recorded
    in the root buffer, no matter whether the garbage collection mechanism has
    been activated with this configuration setting.
   </para>
   <para>
    If the root buffer becomes full with possible roots while the garbage
    collection mechanism is turned off, further possible roots will simply not
    be recorded. Those possible roots that are not recorded will never be
    analyzed by the algorithm. If they were part of a circular reference
    cycle, they would never be cleaned up and would create a memory leak.
   </para>
   <para>
    The reason why possible roots are recorded even if the mechanism has been
    disabled is because it's faster to record possible roots than to have to
    check whether the mechanism is turned on every time a possible root could
    be found. The garbage collection and analysis mechanism itself, however,
    can take a considerable amount of time.
   </para>
   <para>
    Besides changing the <link linkend="ini.zend.enable-gc">zend.enable_gc</link> configuration
    setting, it is also possible to turn the garbage collecting mechanism on
    and off by calling <function>gc_enable</function> or
    <function>gc_disable</function> respectively.  Calling those functions has
    the same effect as turning on or off the mechanism with the configuration
    setting. It is also possible to force the collection of cycles even if the
    possible root buffer is not full yet. For this, you can use the
    <function>gc_collect_cycles</function> function. This function will return
    how many cycles were collected by the algorithm.
   </para>
   <para>
    The rationale behind the ability to turn the mechanism on and off, and to
    initiate cycle collection yourself, is that some parts of your application
    could be highly time-sensitive. In those cases, you might not want the
    garbage collection mechanism to kick in. Of course, by turning off the
    garbage collection for certain parts of your application, you do risk
    creating memory leaks because some possible roots might not fit into the
    limited root buffer. Therefore, it is probably wise to call
    <function>gc_collect_cycles</function> just before you call
    <function>gc_disable</function> to free up the memory that could be lost
    through possible roots that are already recorded in the root buffer. This
    then leaves an empty buffer so that there is more space for storing
    possible roots while the cycle collecting mechanism is turned off.
   </para>
  </sect1>

  <sect1 xml:id="features.gc.performance-considerations">
   <title>Performance Considerations</title>
   <para>
    We have already mentioned in the previous section that simply collecting the
    possible roots had a very tiny performance impact, but this is when you
    compare PHP 5.2 against PHP 5.3. Although the recording of possible roots
    compared to not recording them at all, like in PHP 5.2, is slower, other
    changes to the PHP runtime in PHP 5.3 prevented this particular performance
    loss from even showing.
   </para>
   <para>
    There are two major areas in which performance is affected. The first
    area is reduced memory usage, and the second area is run-time delay when
    the garbage collection mechanism performs its memory cleanups. We will
    look at both of those issues.
   </para>

   <sect2 xml:id="features.gc.performance-considerations.reduced-mem">
    <title>Reduced Memory Usage</title>
    <para>
     First of all, the whole reason for implementing the garbage collection
     mechanism is to reduce memory usage by cleaning up circular-referenced
     variables as soon as the prerequisites are fulfilled. In PHP's
     implementation, this happens as soon as the root-buffer is full, or when
     the function <function>gc_collect_cycles</function> is called. In
     the graph below, we display the memory usage of the script below,
     in both PHP 5.2 and PHP 5.3, excluding the base memory that PHP
     itself uses when starting up.
    </para>
    <para>
     <example>
      <title>Memory usage example</title>
      <programlisting role="php">
<![CDATA[
<?php
class Foo
{
    public $var = '3.14159265359';
    public $self;
}

$baseMemory = memory_get_usage();

for ( $i = 0; $i <= 100000; $i++ )
{
    $a = new Foo;
    $a->self = $a;
    if ( $i % 500 === 0 )
    {
        echo sprintf( '%8d: ', $i ), memory_get_usage() - $baseMemory, "\n";
    }
}
?>
]]>
      </programlisting>
      <mediaobject>
       <alt>Comparison of memory usage between PHP 5.2 and PHP 5.3</alt>
       <imageobject>
        <imagedata fileref="en/features/figures/gc-benchmark.png" format="PNG"/>
       </imageobject>
      </mediaobject>
     </example>
    </para>
    <para>
     In this very academic example, we are creating an object in which a
     property is set to point back to the object itself. When the <varname>$a</varname> variable
     in the script is re-assigned in the next iteration of the loop, a memory
     leak would typically occur. In this case, two zval-containers are leaked
     (the object zval, and the property zval), but only one possible root is
     found: the variable that was unset. When the root-buffer is full after
     10,000 iterations (with a total of 10,000 possible roots), the garbage
     collection mechanism kicks in and frees the memory associated with those
     possible roots. This can very clearly be seen in the jagged memory-usage
     graph for PHP 5.3. After each 10,000 iterations, the mechanism kicks in
     and frees the memory associated with the circular referenced variables.
     The mechanism itself does not have to do a whole lot of work in this
     example, because the structure that is leaked is extremely simple. From
     the diagram, you see that the maximum memory usage in PHP 5.3 is about 9
     Mb, whereas in PHP 5.2 the memory usage keeps increasing.
    </para>
   </sect2>

   <sect2 xml:id="features.gc.performance-considerations.slowdowns">
    <title>Run-Time Slowdowns</title>
    <para>
     The second area where the garbage collection mechanism influences
     performance is the time taken when the garbage collection mechanism
     kicks in to free the "leaked" memory. In order to see how much this is,
     we slightly modify the previous script to allow for a larger number of
     iterations and the removal of the intermediate memory usage figures. The
     second script is here:
    </para>
    <para>
     <example>
      <title>GC performance influences</title>
      <programlisting role="php">
<![CDATA[
<?php
class Foo
{
    public $var = '3.14159265359';
    public $self;
}

for ( $i = 0; $i <= 1000000; $i++ )
{
    $a = new Foo;
    $a->self = $a;
}

echo memory_get_peak_usage(), "\n";
?>
]]>
      </programlisting>
     </example>
    </para>
    <para>
     We will run this script two times, once with the
     <link linkend="ini.zend.enable-gc">zend.enable_gc</link> setting turned
     on, and once with it turned off:
    </para>
    <para>
     <example>
      <title>Running the above script</title>
      <programlisting role="shell">
<![CDATA[
time php -dzend.enable_gc=0 -dmemory_limit=-1 -n example2.php
# and
time php -dzend.enable_gc=1 -dmemory_limit=-1 -n example2.php
]]>
      </programlisting>
     </example>
    </para>
    <para>
     On my machine, the first command seems to take consistently about 10.7
     seconds, whereas the second command takes about 11.4 seconds. This is a
     slowdown of about 7%. However, the maximum amount of memory used by the
     script is reduced by 98% from 931Mb to 10Mb. This benchmark is not very
     scientific, or even representative of real-life applications, but it
     does demonstrate the memory usage benefits that this garbage collection
     mechanism provides. The good thing is that the slowdown is always the
     same 7%, for this particular script, while the memory saving
     capabilities save more and more memory as more circular references are
     found during script execution.
    </para>
   </sect2>

   <sect2 xml:id="features.gc.performance-considerations.internal-stats">
    <title>PHP's Internal GC Statistics</title>
    <para>
     It is possible to coax a little bit more information about how the
     garbage collection mechanism is run from within PHP. But in order to do
     so, you will have to re-compile PHP to enable the benchmark and
     data-collecting code. You will have to set the <literal>CFLAGS</literal>
     environment variable to <literal>-DGC_BENCH=1</literal> prior to running
     <literal>./configure</literal> with your desired options. The following
     sequence should do the trick:
    </para>
    <para>
     <example>
      <title>Recompiling PHP to enable GC benchmarking</title>
      <programlisting role="shell">
<![CDATA[
export CFLAGS=-DGC_BENCH=1
./config.nice
make clean
make
]]>
      </programlisting>
     </example>
    </para>
    <para>
     When you run the above example code again with the newly built PHP
     binary, you will see the following being shown after PHP has finished
     execution:
    </para>
    <para>
     <example>
      <title>GC statistics</title>
      <programlisting role="shell">
<![CDATA[
GC Statistics
-------------
Runs:               110
Collected:          2072204
Root buffer length: 0
Root buffer peak:   10000

      Possible            Remove from  Marked
        Root    Buffered     buffer     grey
      --------  --------  -----------  ------
ZVAL   7175487   1491291    1241690   3611871
ZOBJ  28506264   1527980     677581   1025731
]]>
      </programlisting>
     </example>
    </para>
    <para>
     The most informative statistics are displayed in the first block. You
     can see here that the garbage collection mechanism ran 110 times, and in
     total, more than 2 million memory allocations were freed during those
     110 runs. As soon as the garbage collection mechanism has run at least
     one time, the "Root buffer peak" is always 10000.
    </para>
   </sect2>

   <sect2 xml:id="features.gc.performance-considerations.conclusion">
    <title>Conclusion</title>
    <para>
     In general the garbage collector in PHP will only cause a slowdown when the
     cycle collecting algorithm actually runs, whereas in normal (smaller)
     scripts there should be no performance hit at all.
    </para>
    <para>
     However, in the cases where the cycle collection mechanism does run for
     normal scripts, the memory reduction it will provide allows more of
     those scripts to run concurrently on your server, since not so much
     memory is used in total.
    </para>
    <para>
     The benefits are most apparent for longer-running scripts, such as
     lengthy test suites or daemon scripts. Also, for <link xlink:href="&url.php.gtk;">PHP-GTK</link> applications
     that generally tend to run longer than scripts for the Web, the new
     mechanism should make quite a bit of a difference regarding memory leaks
     creeping in over time.
    </para>
   </sect2>
  </sect1>
 </chapter>

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vim: et tw=78 syn=sgml
vi: ts=1 sw=1
-->