<commit_before><?xml version="1.0" encoding="iso-8859-1"?>
<sect1 id="runtime-control">
  <title>Running a compiled program</title>

  <indexterm><primary>runtime control of Haskell programs</primary></indexterm>
  <indexterm><primary>running, compiled program</primary></indexterm>
  <indexterm><primary>RTS options</primary></indexterm>

  <para>To make an executable program, the GHC system compiles your
  code and then links it with a non-trivial runtime system (RTS),
  which handles storage management, profiling, etc.</para>

  <para>You have some control over the behaviour of the RTS, by giving
  special command-line arguments to your program.</para>

  <para>When your Haskell program starts up, its RTS extracts
  command-line arguments bracketed between
  <option>+RTS</option><indexterm><primary><option>+RTS</option></primary></indexterm>
  and
  <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
  as its own.  For example:</para>

<screen>
% ./a.out -f +RTS -p -S -RTS -h foo bar
</screen>

  <para>The RTS will snaffle <option>-p</option> <option>-S</option>
  for itself, and the remaining arguments <literal>-f -h foo bar</literal>
  will be handed to your program if/when it calls
  <function>System.getArgs</function>.</para>

  <para>No <option>-RTS</option> option is required if the
  runtime-system options extend to the end of the command line, as in
  this example:</para>

<screen>
% hls -ltr /usr/etc +RTS -A5m
</screen>

  <para>If you absolutely positively want all the rest of the options
  in a command line to go to the program (and not the RTS), use a
  <option>&ndash;&ndash;RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.</para>

  <para>As always, for RTS options that take
  <replaceable>size</replaceable>s: If the last character of
  <replaceable>size</replaceable> is a K or k, multiply by 1000; if an
  M or m, by 1,000,000; if a G or G, by 1,000,000,000.  (And any
  wraparound in the counters is <emphasis>your</emphasis>
  fault!)</para>

  <para>Giving a <literal>+RTS -f</literal>
  <indexterm><primary><option>-f</option></primary><secondary>RTS option</secondary></indexterm> option
  will print out the RTS options actually available in your program
  (which vary, depending on how you compiled).</para>

  <para>NOTE: since GHC is itself compiled by GHC, you can change RTS
  options in the compiler using the normal
  <literal>+RTS ... -RTS</literal>
  combination.  eg. to increase the maximum heap
  size for a compilation to 128M, you would add
  <literal>+RTS -M128m -RTS</literal>
  to the command line.</para>

  <sect2 id="rts-optinos-environment">
    <title>Setting global RTS options</title>

    <indexterm><primary>RTS options</primary><secondary>from the environment</secondary></indexterm>
    <indexterm><primary>environment variable</primary><secondary>for
    setting RTS options</secondary></indexterm>

    <para>RTS options are also taken from the environment variable
    <envar>GHCRTS</envar><indexterm><primary><envar>GHCRTS</envar></primary>
      </indexterm>.  For example, to set the maximum heap size
    to 128M for all GHC-compiled programs (using an
    <literal>sh</literal>-like shell):</para>

<screen>
   GHCRTS='-M128m'
   export GHCRTS
</screen>

    <para>RTS options taken from the <envar>GHCRTS</envar> environment
    variable can be overridden by options given on the command
    line.</para>

  </sect2>

  <sect2 id="rts-options-gc">
    <title>RTS options to control the garbage collector</title>

    <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
    <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>

    <para>There are several options to give you precise control over
    garbage collection.  Hopefully, you won't need any of these in
    normal operation, but there are several things that can be tweaked
    for maximum performance.</para>

    <variablelist>

      <varlistentry>
	<term>
          <option>-A</option><replaceable>size</replaceable>
          <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>allocation area, size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 256k&rsqb; Set the allocation area size
          used by the garbage collector.  The allocation area
          (actually generation 0 step 0) is fixed and is never resized
          (unless you use <option>-H</option>, below).</para>

	  <para>Increasing the allocation area size may or may not
          give better performance (a bigger allocation area means
          worse cache behaviour but fewer garbage collections and less
          promotion).</para>

	  <para>With only 1 generation (<option>-G1</option>) the
          <option>-A</option> option specifies the minimum allocation
          area, since the actual size of the allocation area will be
          resized according to the amount of data in the heap (see
          <option>-F</option>, below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-c</option>
          <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
          <indexterm><primary>compacting garbage collection</primary></indexterm>
        </term>
	<listitem>
	  <para>Use a compacting algorithm for collecting the oldest
	  generation.  By default, the oldest generation is collected
	  using a copying algorithm; this option causes it to be
	  compacted in-place instead.  The compaction algorithm is
	  slower than the copying algorithm, but the savings in memory
	  use can be considerable.</para>

	  <para>For a given heap size (using the <option>-H</option>
	  option), compaction can in fact reduce the GC cost by
	  allowing fewer GCs to be performed.  This is more likely
	  when the ratio of live data to heap size is high, say
	  &gt;30&percnt;.</para>

	  <para>NOTE: compaction doesn't currently work when a single
	  generation is requested using the <option>-G1</option>
	  option.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-c</option><replaceable>n</replaceable></term>

	<listitem>
	  <para>&lsqb;Default: 30&rsqb; Automatically enable
	  compacting collection when the live data exceeds
	  <replaceable>n</replaceable>&percnt; of the maximum heap size
	  (see the <option>-M</option> option).  Note that the maximum
	  heap size is unlimited by default, so this option has no
	  effect unless the maximum heap size is set with
	  <option>-M</option><replaceable>size</replaceable>. </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-F</option><replaceable>factor</replaceable>
          <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, factor</primary></indexterm>
        </term>
	<listitem>

	  <para>&lsqb;Default: 2&rsqb; This option controls the amount
          of memory reserved for the older generations (and in the
          case of a two space collector the size of the allocation
          area) as a factor of the amount of live data.  For example,
          if there was 2M of live data in the oldest generation when
          we last collected it, then by default we'll wait until it
          grows to 4M before collecting it again.</para>

	  <para>The default seems to work well here.  If you have
          plenty of memory, it is usually better to use
          <option>-H</option><replaceable>size</replaceable> than to
          increase
          <option>-F</option><replaceable>factor</replaceable>.</para>

	  <para>The <option>-F</option> setting will be automatically
          reduced by the garbage collector when the maximum heap size
          (the <option>-M</option><replaceable>size</replaceable>
          setting) is approaching.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-G</option><replaceable>generations</replaceable>
          <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>generations, number of</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 2&rsqb; Set the number of generations
          used by the garbage collector.  The default of 2 seems to be
          good, but the garbage collector can support any number of
          generations.  Anything larger than about 4 is probably not a
          good idea unless your program runs for a
          <emphasis>long</emphasis> time, because the oldest
          generation will hardly ever get collected.</para>

	  <para>Specifying 1 generation with <option>+RTS -G1</option>
          gives you a simple 2-space collector, as you would expect.
          In a 2-space collector, the <option>-A</option> option (see
          above) specifies the <emphasis>minimum</emphasis> allocation
          area size, since the allocation area will grow with the
          amount of live data in the heap.  In a multi-generational
          collector the allocation area is a fixed size (unless you
          use the <option>-H</option> option, see below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-H</option><replaceable>size</replaceable>
          <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, suggested</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 0&rsqb; This option provides a
          &ldquo;suggested heap size&rdquo; for the garbage collector.  The
          garbage collector will use about this much memory until the
          program residency grows and the heap size needs to be
          expanded to retain reasonable performance.</para>

	  <para>By default, the heap will start small, and grow and
          shrink as necessary.  This can be bad for performance, so if
          you have plenty of memory it's worthwhile supplying a big
          <option>-H</option><replaceable>size</replaceable>.  For
          improving GC performance, using
          <option>-H</option><replaceable>size</replaceable> is
          usually a better bet than
          <option>-A</option><replaceable>size</replaceable>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
         <option>-k</option><replaceable>size</replaceable>
         <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
         <indexterm><primary>stack, minimum size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 1k&rsqb; Set the initial stack size for
          new threads.  Thread stacks (including the main thread's
          stack) live on the heap, and grow as required.  The default
          value is good for concurrent applications with lots of small
          threads; if your program doesn't fit this model then
          increasing this option may help performance.</para>

	  <para>The main thread is normally started with a slightly
          larger heap to cut down on unnecessary stack growth while
          the program is starting up.</para>
	</listitem>
      </varlistentry>

      <varlistentry><commit_msg>[project @ 2005-05-16 14:40:06 by simonmar] document +RTS -I<commit_after><?xml version="1.0" encoding="iso-8859-1"?>
<sect1 id="runtime-control">
  <title>Running a compiled program</title>

  <indexterm><primary>runtime control of Haskell programs</primary></indexterm>
  <indexterm><primary>running, compiled program</primary></indexterm>
  <indexterm><primary>RTS options</primary></indexterm>

  <para>To make an executable program, the GHC system compiles your
  code and then links it with a non-trivial runtime system (RTS),
  which handles storage management, profiling, etc.</para>

  <para>You have some control over the behaviour of the RTS, by giving
  special command-line arguments to your program.</para>

  <para>When your Haskell program starts up, its RTS extracts
  command-line arguments bracketed between
  <option>+RTS</option><indexterm><primary><option>+RTS</option></primary></indexterm>
  and
  <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
  as its own.  For example:</para>

<screen>
% ./a.out -f +RTS -p -S -RTS -h foo bar
</screen>

  <para>The RTS will snaffle <option>-p</option> <option>-S</option>
  for itself, and the remaining arguments <literal>-f -h foo bar</literal>
  will be handed to your program if/when it calls
  <function>System.getArgs</function>.</para>

  <para>No <option>-RTS</option> option is required if the
  runtime-system options extend to the end of the command line, as in
  this example:</para>

<screen>
% hls -ltr /usr/etc +RTS -A5m
</screen>

  <para>If you absolutely positively want all the rest of the options
  in a command line to go to the program (and not the RTS), use a
  <option>&ndash;&ndash;RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.</para>

  <para>As always, for RTS options that take
  <replaceable>size</replaceable>s: If the last character of
  <replaceable>size</replaceable> is a K or k, multiply by 1000; if an
  M or m, by 1,000,000; if a G or G, by 1,000,000,000.  (And any
  wraparound in the counters is <emphasis>your</emphasis>
  fault!)</para>

  <para>Giving a <literal>+RTS -f</literal>
  <indexterm><primary><option>-f</option></primary><secondary>RTS option</secondary></indexterm> option
  will print out the RTS options actually available in your program
  (which vary, depending on how you compiled).</para>

  <para>NOTE: since GHC is itself compiled by GHC, you can change RTS
  options in the compiler using the normal
  <literal>+RTS ... -RTS</literal>
  combination.  eg. to increase the maximum heap
  size for a compilation to 128M, you would add
  <literal>+RTS -M128m -RTS</literal>
  to the command line.</para>

  <sect2 id="rts-optinos-environment">
    <title>Setting global RTS options</title>

    <indexterm><primary>RTS options</primary><secondary>from the environment</secondary></indexterm>
    <indexterm><primary>environment variable</primary><secondary>for
    setting RTS options</secondary></indexterm>

    <para>RTS options are also taken from the environment variable
    <envar>GHCRTS</envar><indexterm><primary><envar>GHCRTS</envar></primary>
      </indexterm>.  For example, to set the maximum heap size
    to 128M for all GHC-compiled programs (using an
    <literal>sh</literal>-like shell):</para>

<screen>
   GHCRTS='-M128m'
   export GHCRTS
</screen>

    <para>RTS options taken from the <envar>GHCRTS</envar> environment
    variable can be overridden by options given on the command
    line.</para>

  </sect2>

  <sect2 id="rts-options-gc">
    <title>RTS options to control the garbage collector</title>

    <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
    <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>

    <para>There are several options to give you precise control over
    garbage collection.  Hopefully, you won't need any of these in
    normal operation, but there are several things that can be tweaked
    for maximum performance.</para>

    <variablelist>

      <varlistentry>
	<term>
          <option>-A</option><replaceable>size</replaceable>
          <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>allocation area, size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 256k&rsqb; Set the allocation area size
          used by the garbage collector.  The allocation area
          (actually generation 0 step 0) is fixed and is never resized
          (unless you use <option>-H</option>, below).</para>

	  <para>Increasing the allocation area size may or may not
          give better performance (a bigger allocation area means
          worse cache behaviour but fewer garbage collections and less
          promotion).</para>

	  <para>With only 1 generation (<option>-G1</option>) the
          <option>-A</option> option specifies the minimum allocation
          area, since the actual size of the allocation area will be
          resized according to the amount of data in the heap (see
          <option>-F</option>, below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-c</option>
          <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
          <indexterm><primary>compacting garbage collection</primary></indexterm>
        </term>
	<listitem>
	  <para>Use a compacting algorithm for collecting the oldest
	  generation.  By default, the oldest generation is collected
	  using a copying algorithm; this option causes it to be
	  compacted in-place instead.  The compaction algorithm is
	  slower than the copying algorithm, but the savings in memory
	  use can be considerable.</para>

	  <para>For a given heap size (using the <option>-H</option>
	  option), compaction can in fact reduce the GC cost by
	  allowing fewer GCs to be performed.  This is more likely
	  when the ratio of live data to heap size is high, say
	  &gt;30&percnt;.</para>

	  <para>NOTE: compaction doesn't currently work when a single
	  generation is requested using the <option>-G1</option>
	  option.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-c</option><replaceable>n</replaceable></term>

	<listitem>
	  <para>&lsqb;Default: 30&rsqb; Automatically enable
	  compacting collection when the live data exceeds
	  <replaceable>n</replaceable>&percnt; of the maximum heap size
	  (see the <option>-M</option> option).  Note that the maximum
	  heap size is unlimited by default, so this option has no
	  effect unless the maximum heap size is set with
	  <option>-M</option><replaceable>size</replaceable>. </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-F</option><replaceable>factor</replaceable>
          <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, factor</primary></indexterm>
        </term>
	<listitem>

	  <para>&lsqb;Default: 2&rsqb; This option controls the amount
          of memory reserved for the older generations (and in the
          case of a two space collector the size of the allocation
          area) as a factor of the amount of live data.  For example,
          if there was 2M of live data in the oldest generation when
          we last collected it, then by default we'll wait until it
          grows to 4M before collecting it again.</para>

	  <para>The default seems to work well here.  If you have
          plenty of memory, it is usually better to use
          <option>-H</option><replaceable>size</replaceable> than to
          increase
          <option>-F</option><replaceable>factor</replaceable>.</para>

	  <para>The <option>-F</option> setting will be automatically
          reduced by the garbage collector when the maximum heap size
          (the <option>-M</option><replaceable>size</replaceable>
          setting) is approaching.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-G</option><replaceable>generations</replaceable>
          <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>generations, number of</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 2&rsqb; Set the number of generations
          used by the garbage collector.  The default of 2 seems to be
          good, but the garbage collector can support any number of
          generations.  Anything larger than about 4 is probably not a
          good idea unless your program runs for a
          <emphasis>long</emphasis> time, because the oldest
          generation will hardly ever get collected.</para>

	  <para>Specifying 1 generation with <option>+RTS -G1</option>
          gives you a simple 2-space collector, as you would expect.
          In a 2-space collector, the <option>-A</option> option (see
          above) specifies the <emphasis>minimum</emphasis> allocation
          area size, since the allocation area will grow with the
          amount of live data in the heap.  In a multi-generational
          collector the allocation area is a fixed size (unless you
          use the <option>-H</option> option, see below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-H</option><replaceable>size</replaceable>
          <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, suggested</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 0&rsqb; This option provides a
          &ldquo;suggested heap size&rdquo; for the garbage collector.  The
          garbage collector will use about this much memory until the
          program residency grows and the heap size needs to be
          expanded to retain reasonable performance.</para>

	  <para>By default, the heap will start small, and grow and
          shrink as necessary.  This can be bad for performance, so if
          you have plenty of memory it's worthwhile supplying a big
          <option>-H</option><replaceable>size</replaceable>.  For
          improving GC performance, using
          <option>-H</option><replaceable>size</replaceable> is
          usually a better bet than
          <option>-A</option><replaceable>size</replaceable>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-I</option><replaceable>seconds</replaceable>
	  <indexterm><primary><option>-H</option></primary>
	    <secondary>RTS option</secondary>
	  </indexterm>
	  <indexterm><primary>idle GC</primary>
	  </indexterm>
	  </term>
	<listitem>
	  <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
	    <option>-threaded</option>, <xref linkend="options-linker" />), a
	    major GC is automatically performed if the runtime has been idle
	    (no Haskell computation has been running) for a period of time.
	    The amount of idle time which must pass before a GC is performed is
	    set by the <option>-I</option><replaceable>seconds</replaceable>
	    option.  Specifying <option>-I0</option> disables the idle GC.</para>

	  <para>For an interactive application, it is probably a good idea to
	    use the idle GC, because this will allow finalizers to run and
	    deadlocked threads to be detected in the idle time when no Haskell
	    computation is happening.  Also, it will mean that a GC is less
	    likely to happen when the application is busy, and so
	    responsiveness may be improved.   However, if the amount of live data in
	    the heap is particularly large, then the idle GC can cause a
	    significant delay, and too small an interval could adversely affect
	    interactive responsiveness.</para>

	  <para>This is an experimental feature, please let us know if it
	    causes problems and/or could benefit from further tuning.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
         <option>-k</option><replaceable>size</replaceable>
         <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
         <indexterm><primary>stack, minimum size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 1k&rsqb; Set the initial stack size for
          new threads.  Thread stacks (including the main thread's
          stack) live on the heap, and grow as required.  The default
          value is good for concurrent applications with lots of small
          threads; if your program doesn't fit this model then
          increasing this option may help performance.</para>

	  <para>The main thread is normally started with a slightly
          larger heap to cut down on unnecessary stack growth while
          the program is starting up.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
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