<?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-misc">
    <title>Miscellaneous RTS options</title>

    <variablelist>
     <varlistentry>
       <term><option>-V<replaceable>secs</replaceable></option>
       <indexterm><primary><option>-V</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>Sets the interval that the RTS clock ticks at.  The
         runtime uses a single timer signal to count ticks; this timer
         signal is used to control the context switch timer (<xref
         linkend="using-concurrent" />) and the heap profiling
         timer <xref linkend="rts-options-heap-prof" />.  Also, the
         time profiler uses the RTS timer signal directly to record
         time profiling samples.</para>

         <para>Normally, setting the <option>-V</option> option
         directly is not necessary: the resolution of the RTS timer is
         adjusted automatically if a short interval is requested with
         the <option>-C</option> or <option>-i</option> options.
         However, setting <option>-V</option> is required in order to
         increase the resolution of the time profiler.</para>

         <para>Using a value of zero disables the RTS clock
         completely, and has the effect of disabling timers that
         depend on it: the context switch timer and the heap profiling
         timer.  Context switches will still happen, but
         deterministically and at a rate much faster than normal.
         Disabling the interval timer is useful for debugging, because
         it eliminates a source of non-determinism at runtime.</para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>--install-signal-handlers=<replaceable>yes|no</replaceable></option>
       <indexterm><primary><option>--install-signal-handlers</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>If yes (the default), the RTS installs signal handlers to catch
         things like ctrl-C. This option is primarily useful for when
         you are using the Haskell code as a DLL, and want to set your
         own signal handlers.</para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>-xm<replaceable>address</replaceable></option>
       <indexterm><primary><option>-xm</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>
           WARNING: this option is for working around memory
           allocation problems only.  Do not use unless GHCi fails
           with a message like &ldquo;<literal>failed to mmap() memory below 2Gb</literal>&rdquo;.  If you need to use this option to get GHCi working
           on your machine, please file a bug.
         </para>
         
         <para>
           On 64-bit machines, the RTS needs to allocate memory in the
           low 2Gb of the address space.  Support for this across
           different operating systems is patchy, and sometimes fails.
           This option is there to give the RTS a hint about where it
           should be able to allocate memory in the low 2Gb of the
           address space.  For example, <literal>+RTS -xm20000000
           -RTS</literal> would hint that the RTS should allocate
           starting at the 0.5Gb mark.  The default is to use the OS's
           built-in support for allocating memory in the low 2Gb if
           available (e.g. <literal>mmap</literal>
           with <literal>MAP_32BIT</literal> on Linux), or
           otherwise <literal>-xm40000000</literal>.
         </para>
       </listitem>
     </varlistentry>
    </variablelist>
  </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>-g</option><replaceable>threads</replaceable>
          <indexterm><primary><option>-g</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
        <listitem>
          <para>&lsqb;Default: 1&rsqb; &lsqb;new in GHC 6.10&rsqb; Set the number
            of threads to use for garbage collection.  This option is
            only accepted when the program was linked with the
            <option>-threaded</option> option; see <xref
            linkend="options-linker" />.</para>

          <para>The garbage collector is able to work in parallel when
            given more than one OS thread.  Experiments have shown
            that this usually results in a performance improvement
            given 3 cores or more; with 2 cores it may or may not be
            beneficial, depending on the workload.  Bigger heaps work
            better with parallel GC, so set your <option>-H</option>
            value high (3 or more times the maximum residency).  Look
            at the timing stats with <option>+RTS -s</option> to
            see whether you're getting any benefit from parallel GC or
            not.  If you find parallel GC is
            significantly <emphasis>slower</emphasis> (in elapsed
            time) than sequential GC, please report it as a
            bug.</para>

          <para>This value is set automatically when the
            <option>-N</option> option is used, so the only reason to
            use <option>-g</option> would be if you wanted to use a
            different number of threads for GC than for execution.
            For example, if your program is strictly single-threaded
            but you still want to benefit from parallel GC, then it
            might make sense to use <option>-g</option> rather than
            <option>-N</option>.</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>-I</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>
	<term>
          <option>-K</option><replaceable>size</replaceable>
          <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>stack, maximum size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 8M&rsqb; Set the maximum stack size for
          an individual thread to <replaceable>size</replaceable>
          bytes.  This option is there purely to stop the program
          eating up all the available memory in the machine if it gets
          into an infinite loop.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-m</option><replaceable>n</replaceable>
          <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap, minimum free</primary></indexterm>
        </term>
	<listitem>
	  <para>Minimum &percnt; <replaceable>n</replaceable> of heap
          which must be available for allocation.  The default is
          3&percnt;.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-M</option><replaceable>size</replaceable>
          <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, maximum</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: unlimited&rsqb; Set the maximum heap size to
          <replaceable>size</replaceable> bytes.  The heap normally
          grows and shrinks according to the memory requirements of
          the program.  The only reason for having this option is to
          stop the heap growing without bound and filling up all the
          available swap space, which at the least will result in the
          program being summarily killed by the operating
          system.</para>

	  <para>The maximum heap size also affects other garbage
	  collection parameters: when the amount of live data in the
	  heap exceeds a certain fraction of the maximum heap size,
	  compacting collection will be automatically enabled for the
	  oldest generation, and the <option>-F</option> parameter
	  will be reduced in order to avoid exceeding the maximum heap
	  size.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-t</option><optional><replaceable>file</replaceable></optional>
          <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<term>
          <option>-s</option><optional><replaceable>file</replaceable></optional>
          <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<term>
          <option>-S</option><optional><replaceable>file</replaceable></optional>
          <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<term>
          <option>--machine-readable</option>
          <indexterm><primary><option>--machine-readable</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>These options produce runtime-system statistics, such
	  as the amount of time spent executing the program and in the
	  garbage collector, the amount of memory allocated, the
	  maximum size of the heap, and so on.  The three
	  variants give different levels of detail:
	  <option>-t</option> produces a single line of output in the
	  same format as GHC's <option>-Rghc-timing</option> option,
	  <option>-s</option> produces a more detailed summary at the
	  end of the program, and <option>-S</option> additionally
	  produces information about each and every garbage
	  collection.</para>

          <para>The output is placed in
          <replaceable>file</replaceable>.  If
          <replaceable>file</replaceable> is omitted, then the output
          is sent to <constant>stderr</constant>.</para>

    <para>
        If you use the <literal>-t</literal> flag then, when your
        program finishes, you will see something like this:
    </para>

<programlisting>
&lt;&lt;ghc: 36169392 bytes, 69 GCs, 603392/1065272 avg/max bytes residency (2 samples), 3M in use, 0.00 INIT (0.00 elapsed), 0.02 MUT (0.02 elapsed), 0.07 GC (0.07 elapsed) :ghc&gt;&gt;
</programlisting>

    <para>
        This tells you:
    </para>

    <itemizedlist>
      <listitem>
        <para>
          The total bytes allocated by the program. This may be less
          than the peak memory use, as some may be freed. 
        </para>
      </listitem>
      <listitem>
        <para>
          The total number of garbage collections that occurred.
        </para>
      </listitem>
      <listitem>
        <para>
          The average and maximum space used by your program.
          This is only checked during major garbage collections, so it
          is only an approximation; the number of samples tells you how
          many times it is checked.
        </para>
      </listitem>
      <listitem>
        <para>
          The peak memory the RTS has allocated from the OS. 
        </para>
      </listitem>
      <listitem>
        <para>
          The amount of CPU time and elapsed wall clock time while
          initialising the runtime system (INIT), running the program
          itself (MUT, the mutator), and garbage collecting (GC).
        </para>
      </listitem>
    </itemizedlist>

    <para>
        You can also get this in a more future-proof, machine readable
        format, with <literal>-t --machine-readable</literal>:
    </para>

<programlisting>
 [("bytes allocated", "36169392")
 ,("num_GCs", "69")
 ,("average_bytes_used", "603392")
 ,("max_bytes_used", "1065272")
 ,("num_byte_usage_samples", "2")
 ,("peak_megabytes_allocated", "3")
 ,("init_cpu_seconds", "0.00")
 ,("init_wall_seconds", "0.00")
 ,("mutator_cpu_seconds", "0.02")
 ,("mutator_wall_seconds", "0.02")
 ,("GC_cpu_seconds", "0.07")
 ,("GC_wall_seconds", "0.07")
 ]
</programlisting>

    <para>
        If you use the <literal>-s</literal> flag then, when your
        program finishes, you will see something like this (the exact
        details will vary depending on what sort of RTS you have, e.g.
        you will only see profiling data if your RTS is compiled for
        profiling):
    </para>

<programlisting>
      36,169,392 bytes allocated in the heap
       4,057,632 bytes copied during GC
       1,065,272 bytes maximum residency (2 sample(s))
          54,312 bytes maximum slop
               3 MB total memory in use (0 MB lost due to fragmentation)

  Generation 0:    67 collections,     0 parallel,  0.04s,  0.03s elapsed
  Generation 1:     2 collections,     0 parallel,  0.03s,  0.04s elapsed

  INIT  time    0.00s  (  0.00s elapsed)
  MUT   time    0.01s  (  0.02s elapsed)
  GC    time    0.07s  (  0.07s elapsed)
  EXIT  time    0.00s  (  0.00s elapsed)
  Total time    0.08s  (  0.09s elapsed)

  %GC time      89.5%  (75.3% elapsed)

  Alloc rate    4,520,608,923 bytes per MUT second

  Productivity  10.5% of total user, 9.1% of total elapsed
</programlisting>

    <itemizedlist>
      <listitem>
        <para>
        The "bytes allocated in the heap" is the total bytes allocated
        by the program. This may be less than the peak memory use, as
        some may be freed.
        </para>
      </listitem>
      <listitem>
        <para>
        GHC uses a copying garbage collector. "bytes copied during GC" 
        tells you how many bytes it had to copy during garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
        The maximum space actually used by your program is the
        "bytes maximum residency" figure. This is only checked during
        major garbage collections, so it is only an approximation;
        the number of samples tells you how many times it is checked.
        </para>
      </listitem>
      <listitem>
        <para>
        The "bytes maximum slop" tells you the most space that is ever
        wasted due to the way GHC packs data into so-called "megablocks".
        </para>
      </listitem>
      <listitem>
        <para>
        The "total memory in use" tells you the peak memory the RTS has
        allocated from the OS.
        </para>
      </listitem>
      <listitem>
        <para>
        Next there is information about the garbage collections done.
        For each generation it says how many garbage collections were
        done, how many of those collections used multiple threads,
        the total CPU time used for garbage collecting that generation,
        and the total wall clock time elapsed while garbage collecting
        that generation.
        </para>
      </listitem>
      <listitem>
        <para>
        Next there is the CPU time and wall clock time elapsedm broken
        down by what the runtiem system was doing at the time.
        INIT is the runtime system initialisation.
        MUT is the mutator time, i.e. the time spent actually running
        your code.
        GC is the time spent doing garbage collection.
        RP is the time spent doing retainer profiling.
        PROF is the time spent doing other profiling.
        EXIT is the runtime system shutdown time.
        And finally, Total is, of course, the total.
        </para>
        <para>
        %GC time tells you what percentage GC is of Total.
        "Alloc rate" tells you the "bytes allocated in the heap" divided
        by the MUT CPU time.
        "Productivity" tells you what percentage of the Total CPU and wall
        clock elapsed times are spent in the mutator (MUT).
        </para>
      </listitem>
    </itemizedlist>

    <para>
        The <literal>-S</literal> flag, as well as giving the same
        output as the <literal>-s</literal> flag, prints information
        about each GC as it happens:
    </para>

<programlisting>
    Alloc    Copied     Live    GC    GC     TOT     TOT  Page Flts
    bytes     bytes     bytes  user  elap    user    elap
   528496     47728    141512  0.01  0.02    0.02    0.02    0    0  (Gen:  1)
[...]
   524944    175944   1726384  0.00  0.00    0.08    0.11    0    0  (Gen:  0)
</programlisting>

    <para>
        For each garbage collection, we print:
    </para>

    <itemizedlist>
      <listitem>
        <para>
          How many bytes we allocated this garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
          How many bytes we copied this garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
          How many bytes are currently live.
        </para>
      </listitem>
      <listitem>
        <para>
          How long this garbage collection took (CPU time and elapsed
          wall clock time).
        </para>
      </listitem>
      <listitem>
        <para>
          How long the program has been running (CPU time and elapsed
          wall clock time).
        </para>
      </listitem>
      <listitem>
        <para>
          How many page faults occured this garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
          How many page faults occured since the end of the last garbage
          collection.
        </para>
      </listitem>
      <listitem>
        <para>
          Which generation is being garbage collected.
        </para>
      </listitem>
    </itemizedlist>

	</listitem>
      </varlistentry>
    </variablelist>

  </sect2>

  <sect2>
    <title>RTS options for concurrency and parallelism</title>

    <para>The RTS options related to concurrency are described in
      <xref linkend="using-concurrent" />, and those for parallelism in
      <xref linkend="parallel-options"/>.</para>
  </sect2>

  <sect2 id="rts-profiling">
    <title>RTS options for profiling</title>

    <para>Most profiling runtime options are only available when you
    compile your program for profiling (see
    <xref linkend="prof-compiler-options" />, and
    <xref linkend="rts-options-heap-prof" /> for the runtime options).
    However, there is one profiling option that is available
    for ordinary non-profiled executables:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-hT</option>
          <indexterm><primary><option>-hT</option></primary><secondary>RTS
              option</secondary></indexterm>
        </term>
        <listitem>
          <para>Generates a basic heap profile, in the
            file <literal><replaceable>prog</replaceable>.hp</literal>.
            To produce the heap profile graph,
            use <command>hp2ps</command> (see <xref linkend="hp2ps"
                                                    />).  The basic heap profile is broken down by data
            constructor, with other types of closures (functions, thunks,
            etc.) grouped into broad categories
            (e.g. <literal>FUN</literal>, <literal>THUNK</literal>).  To
            get a more detailed profile, use the full profiling
            support (<xref linkend="profiling" />).</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </sect2>

  <sect2 id="rts-options-debugging">
    <title>RTS options for hackers, debuggers, and over-interested
    souls</title>

    <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>

    <para>These RTS options might be used (a)&nbsp;to avoid a GHC bug,
    (b)&nbsp;to see &ldquo;what's really happening&rdquo;, or
    (c)&nbsp;because you feel like it.  Not recommended for everyday
    use!</para>

    <variablelist>

      <varlistentry>
	<term>
          <option>-B</option>
          <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Sound the bell at the start of each (major) garbage
	  collection.</para>

	  <para>Oddly enough, people really do use this option!  Our
          pal in Durham (England), Paul Callaghan, writes: &ldquo;Some
          people here use it for a variety of
          purposes&mdash;honestly!&mdash;e.g., confirmation that the
          code/machine is doing something, infinite loop detection,
          gauging cost of recently added code. Certain people can even
          tell what stage &lsqb;the program&rsqb; is in by the beep
          pattern. But the major use is for annoying others in the
          same office&hellip;&rdquo;</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-D</option><replaceable>num</replaceable>
          <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>An RTS debugging flag; varying quantities of output
          depending on which bits are set in
          <replaceable>num</replaceable>.  Only works if the RTS was
          compiled with the <option>DEBUG</option> option.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-r</option><replaceable>file</replaceable>
          <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>ticky ticky profiling</primary></indexterm>
          <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
        </term>
	<listitem>
	  <para>Produce &ldquo;ticky-ticky&rdquo; statistics at the
          end of the program run.  The <replaceable>file</replaceable>
          business works just like on the <option>-S</option> RTS
          option (above).</para>

	  <para>&ldquo;Ticky-ticky&rdquo; statistics are counts of
          various program actions (updates, enters, etc.)  The program
          must have been compiled using
          <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
          (a.k.a. &ldquo;ticky-ticky profiling&rdquo;), and, for it to
          be really useful, linked with suitable system libraries.
          Not a trivial undertaking: consult the installation guide on
          how to set things up for easy &ldquo;ticky-ticky&rdquo;
          profiling.  For more information, see <xref
          linkend="ticky-ticky"/>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-xc</option>
          <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>(Only available when the program is compiled for
	  profiling.)  When an exception is raised in the program,
	  this option causes the current cost-centre-stack to be
	  dumped to <literal>stderr</literal>.</para>

	  <para>This can be particularly useful for debugging: if your
	  program is complaining about a <literal>head []</literal>
	  error and you haven't got a clue which bit of code is
	  causing it, compiling with <literal>-prof
	  -auto-all</literal> and running with <literal>+RTS -xc
	  -RTS</literal> will tell you exactly the call stack at the
	  point the error was raised.</para>

	  <para>The output contains one line for each exception raised
	  in the program (the program might raise and catch several
	  exceptions during its execution), where each line is of the
	  form:</para>

<screen>
&lt; cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> &gt;
</screen>
	  <para>each <literal>cc</literal><subscript>i</subscript> is
	  a cost centre in the program (see <xref
	  linkend="cost-centres"/>), and the sequence represents the
	  &ldquo;call stack&rdquo; at the point the exception was
	  raised.  The leftmost item is the innermost function in the
	  call stack, and the rightmost item is the outermost
	  function.</para>

	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-Z</option>
          <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Turn <emphasis>off</emphasis> &ldquo;update-frame
          squeezing&rdquo; at garbage-collection time.  (There's no
          particularly good reason to turn it off, except to ensure
          the accuracy of certain data collected regarding thunk entry
          counts.)</para>
	</listitem>
      </varlistentry>
    </variablelist>

  </sect2>

  <sect2 id="rts-hooks">
    <title>&ldquo;Hooks&rdquo; to change RTS behaviour</title>

    <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
    <indexterm><primary>RTS hooks</primary></indexterm>
    <indexterm><primary>RTS behaviour, changing</primary></indexterm>

    <para>GHC lets you exercise rudimentary control over the RTS
    settings for any given program, by compiling in a
    &ldquo;hook&rdquo; that is called by the run-time system.  The RTS
    contains stub definitions for all these hooks, but by writing your
    own version and linking it on the GHC command line, you can
    override the defaults.</para>

    <para>Owing to the vagaries of DLL linking, these hooks don't work
    under Windows when the program is built dynamically.</para>

    <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
      </indexterm>lets you set RTS
    options permanently for a given program.  A common use for this is
    to give your program a default heap and/or stack size that is
    greater than the default.  For example, to set <literal>-H128m
    -K1m</literal>, place the following definition in a C source
    file:</para>

<programlisting>
char *ghc_rts_opts = "-H128m -K1m";
</programlisting>

    <para>Compile the C file, and include the object file on the
    command line when you link your Haskell program.</para>

    <para>These flags are interpreted first, before any RTS flags from
    the <literal>GHCRTS</literal> environment variable and any flags
    on the command line.</para>

    <para>You can also change the messages printed when the runtime
    system &ldquo;blows up,&rdquo; e.g., on stack overflow.  The hooks
    for these are as follows:</para>

    <variablelist>

      <varlistentry>
	<term>
          <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
          <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The heap-overflow message.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <function>void StackOverflowHook (long int)</function>
          <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The stack-overflow message.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <function>void MallocFailHook (long int)</function>
          <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The message printed if <function>malloc</function>
	  fails.</para>
	</listitem>
      </varlistentry>
    </variablelist>

    <para>For examples of the use of these hooks, see GHC's own
    versions in the file
    <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
    source tree.</para>
  </sect2>

  <sect2>
    <title>Getting information about the RTS</title>

    <indexterm><primary>RTS</primary></indexterm>

    <para>It is possible to ask the RTS to give some information about
    itself. To do this, use the <option>--info</option> flag, e.g.</para>
<screen>
$ ./a.out +RTS --info
 [("GHC RTS", "YES")
 ,("GHC version", "6.7")
 ,("RTS way", "rts_p")
 ,("Host platform", "x86_64-unknown-linux")
 ,("Host architecture", "x86_64")
 ,("Host OS", "linux")
 ,("Host vendor", "unknown")
 ,("Build platform", "x86_64-unknown-linux")
 ,("Build architecture", "x86_64")
 ,("Build OS", "linux")
 ,("Build vendor", "unknown")
 ,("Target platform", "x86_64-unknown-linux")
 ,("Target architecture", "x86_64")
 ,("Target OS", "linux")
 ,("Target vendor", "unknown")
 ,("Word size", "64")
 ,("Compiler unregisterised", "NO")
 ,("Tables next to code", "YES")
 ]
</screen>
    <para>The information is formatted such that it can be read as a
    of type <literal>[(String, String)]</literal>. Currently the following
    fields are present:</para>

    <variablelist>

      <varlistentry>
        <term><literal>GHC RTS</literal></term>
        <listitem>
          <para>Is this program linked against the GHC RTS? (always
          "YES").</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>GHC version</literal></term>
        <listitem>
          <para>The version of GHC used to compile this program.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>RTS way</literal></term>
        <listitem>
          <para>The variant (&ldquo;way&rdquo;) of the runtime. The
          most common values are <literal>rts</literal> (vanilla),
          <literal>rts_thr</literal> (threaded runtime, i.e. linked using the
          <literal>-threaded</literal> option) and <literal>rts_p</literal>
          (profiling runtime, i.e. linked using the <literal>-prof</literal>
          option). Other variants include <literal>debug</literal>
          (linked using <literal>-debug</literal>),
          <literal>t</literal> (ticky-ticky profiling) and
          <literal>dyn</literal> (the RTS is
          linked in dynamically, i.e. a shared library, rather than statically
          linked into the executable itself). These can be combined,
          e.g. you might have <literal>rts_thr_debug_p</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
            <literal>Target platform</literal>,
            <literal>Target architecture</literal>,
            <literal>Target OS</literal>,
            <literal>Target vendor</literal>
        </term>
        <listitem>
          <para>These are the platform the program is compiled to run on.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
            <literal>Build platform</literal>,
            <literal>Build architecture</literal>,
            <literal>Build OS</literal>,
            <literal>Build vendor</literal>
        </term>
        <listitem>
          <para>These are the platform where the program was built
          on. (That is, the target platform of GHC itself.) Ordinarily
          this is identical to the target platform. (It could potentially
          be different if cross-compiling.)</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
            <literal>Host platform</literal>,
            <literal>Host architecture</literal>
            <literal>Host OS</literal>
            <literal>Host vendor</literal>
        </term>
        <listitem>
          <para>These are the platform where GHC itself was compiled.
          Again, this would normally be identical to the build and
          target platforms.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>Word size</literal></term>
        <listitem>
          <para>Either <literal>"32"</literal> or <literal>"64"</literal>,
          reflecting the word size of the target platform.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>Compiler unregistered</literal></term>
        <listitem>
          <para>Was this program compiled with an &ldquo;unregistered&rdquo;
          version of GHC? (I.e., a version of GHC that has no platform-specific
          optimisations compiled in, usually because this is a currently
          unsupported platform.) This value will usually be no, unless you're
          using an experimental build of GHC.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>Tables next to code</literal></term>
        <listitem>
          <para>Putting info tables directly next to entry code is a useful
          performance optimisation that is not available on all platforms.
          This field tells you whether the program has been compiled with
          this optimisation. (Usually yes, except on unusual platforms.)</para>
        </listitem>
      </varlistentry>

    </variablelist>

  </sect2>
</sect1>

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