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 “<literal>failed to mmap() memory below 2Gb</literal>”. 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>
<varlistentry>
<term>
- <option>-g</option><replaceable>threads</replaceable>
- <indexterm><primary><option>-g</option></primary><secondary>RTS option</secondary></indexterm>
+ <option>-q1</option>
+ <indexterm><primary><option>-q1</option><secondary>RTS
+ option</secondary></primary></indexterm>
</term>
<listitem>
- <para>[Default: 1] [new in GHC 6.10] 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>
+ <para>[New in GHC 6.12.1] Disable the parallel GC.
+ The parallel GC is turned on automatically when parallel
+ execution is enabled with the <option>-N</option> option;
+ this option is available to turn it off if
+ necessary.</para>
+
+ <para>Experiments have shown that parallel GC 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>In GHC 6.10.1 it was possible to use a different
+ number of threads for GC than for execution, because the GC
+ used its own pool of threads. Now, the GC uses the same
+ threads as the mutator (for executing the program).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <option>-qg<replaceable>n</replaceable></option>
+ <indexterm><primary><option>-qg</option><secondary>RTS
+ option</secondary></primary></indexterm>
+ </term>
+ <listitem>
+ <para>
+ [Default: 1] [New in GHC 6.12.1]
+ Enable the parallel GC only in
+ generation <replaceable>n</replaceable> and greater.
+ Parallel GC is often not worthwhile for collections in
+ generation 0 (the young generation), so it is enabled by
+ default only for collections in generation 1 (and higher,
+ if applicable).
+ </para>
</listitem>
</varlistentry>
<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
</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.
Generation 0: 67 collections, 0 parallel, 0.04s, 0.03s elapsed
Generation 1: 2 collections, 0 parallel, 0.03s, 0.04s elapsed
+ SPARKS: 359207 (557 converted, 149591 pruned)
+
INIT time 0.00s ( 0.00s elapsed)
MUT time 0.01s ( 0.02s elapsed)
GC time 0.07s ( 0.07s elapsed)
</para>
</listitem>
<listitem>
+ <para>The <literal>SPARKS</literal> statistic refers to the
+ use of <literal>Control.Parallel.par</literal> and related
+ functionality in the program. Each spark represents a call
+ to <literal>par</literal>; a spark is "converted" when it is
+ executed in parallel; and a spark is "pruned" when it is
+ found to be already evaluated and is discarded from the pool
+ by the garbage collector. Any remaining sparks are
+ discarded at the end of execution, so "converted" plus
+ "pruned" does not necessarily add up to the total.</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.
<xref linkend="parallel-options"/>.</para>
</sect2>
- <sect2>
+ <sect2 id="rts-profiling">
<title>RTS options for profiling</title>
<para>Most profiling runtime options are only available when you
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