increase the resolution of the time profiler.</para>
<para>Using a value of zero disables the RTS clock
- completetly, and has the effect of disabling timers that
+ 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.
</varlistentry>
<varlistentry>
+ <term>
+ <option>-g</option><replaceable>threads</replaceable>
+ <indexterm><primary><option>-g</option></primary><secondary>RTS option</secondary></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>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term>
<option>-H</option><replaceable>size</replaceable>
<indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
</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><replaceable>file</replaceable>
+ <option>-s</option><optional><replaceable>file</replaceable></optional>
<indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
</term>
<term>
- <option>-S</option><replaceable>file</replaceable>
+ <option>-S</option><optional><replaceable>file</replaceable></optional>
<indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
</term>
<listitem>
- <para>Write modest (<option>-s</option>) or verbose
- (<option>-S</option>) garbage-collector statistics into file
- <replaceable>file</replaceable>. The default
- <replaceable>file</replaceable> is
- <filename><replaceable>program</replaceable>.stat</filename>. The
- <replaceable>file</replaceable> <constant>stderr</constant>
- is treated specially, with the output really being sent to
- <constant>stderr</constant>.</para>
-
- <para>This option is useful for watching how the storage
- manager adjusts the heap size based on the current amount of
- live data.</para>
- </listitem>
- </varlistentry>
+ <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>
+<<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>>
+</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>
+ 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>
- <varlistentry>
- <term>
- <option>-t<replaceable>file</replaceable></option>
- <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
- </term>
- <listitem>
- <para>Write a one-line GC stats summary after running the
- program. This output is in the same format as that produced
- by the <option>-Rghc-timing</option> option.</para>
-
- <para>As with <option>-s</option>, the default
- <replaceable>file</replaceable> is
- <filename><replaceable>program</replaceable>.stat</filename>. The
- <replaceable>file</replaceable> <constant>stderr</constant>
- is treated specially, with the output really being sent to
- <constant>stderr</constant>.</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
- <title>RTS options for profiling and parallelism</title>
+ <title>RTS options for concurrency and parallelism</title>
- <para>The RTS options related to profiling are described in <xref
- linkend="rts-options-heap-prof"/>, those for concurrency in
+ <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>
projects / ghc-hetmet.git / blobdiff