1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <sect1 id="runtime-control">
3 <title>Running a compiled program</title>
5 <indexterm><primary>runtime control of Haskell programs</primary></indexterm>
6 <indexterm><primary>running, compiled program</primary></indexterm>
7 <indexterm><primary>RTS options</primary></indexterm>
9 <para>To make an executable program, the GHC system compiles your
10 code and then links it with a non-trivial runtime system (RTS),
11 which handles storage management, profiling, etc.</para>
13 <para>You have some control over the behaviour of the RTS, by giving
14 special command-line arguments to your program.</para>
16 <para>When your Haskell program starts up, its RTS extracts
17 command-line arguments bracketed between
18 <option>+RTS</option><indexterm><primary><option>+RTS</option></primary></indexterm>
20 <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
21 as its own. For example:</para>
24 % ./a.out -f +RTS -p -S -RTS -h foo bar
27 <para>The RTS will snaffle <option>-p</option> <option>-S</option>
28 for itself, and the remaining arguments <literal>-f -h foo bar</literal>
29 will be handed to your program if/when it calls
30 <function>System.getArgs</function>.</para>
32 <para>No <option>-RTS</option> option is required if the
33 runtime-system options extend to the end of the command line, as in
37 % hls -ltr /usr/etc +RTS -A5m
40 <para>If you absolutely positively want all the rest of the options
41 in a command line to go to the program (and not the RTS), use a
42 <option>––RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.</para>
44 <para>As always, for RTS options that take
45 <replaceable>size</replaceable>s: If the last character of
46 <replaceable>size</replaceable> is a K or k, multiply by 1000; if an
47 M or m, by 1,000,000; if a G or G, by 1,000,000,000. (And any
48 wraparound in the counters is <emphasis>your</emphasis>
51 <para>Giving a <literal>+RTS -f</literal>
52 <indexterm><primary><option>-f</option></primary><secondary>RTS option</secondary></indexterm> option
53 will print out the RTS options actually available in your program
54 (which vary, depending on how you compiled).</para>
56 <para>NOTE: since GHC is itself compiled by GHC, you can change RTS
57 options in the compiler using the normal
58 <literal>+RTS ... -RTS</literal>
59 combination. eg. to increase the maximum heap
60 size for a compilation to 128M, you would add
61 <literal>+RTS -M128m -RTS</literal>
62 to the command line.</para>
64 <sect2 id="rts-optinos-environment">
65 <title>Setting global RTS options</title>
67 <indexterm><primary>RTS options</primary><secondary>from the environment</secondary></indexterm>
68 <indexterm><primary>environment variable</primary><secondary>for
69 setting RTS options</secondary></indexterm>
71 <para>RTS options are also taken from the environment variable
72 <envar>GHCRTS</envar><indexterm><primary><envar>GHCRTS</envar></primary>
73 </indexterm>. For example, to set the maximum heap size
74 to 128M for all GHC-compiled programs (using an
75 <literal>sh</literal>-like shell):</para>
82 <para>RTS options taken from the <envar>GHCRTS</envar> environment
83 variable can be overridden by options given on the command
88 <sect2 id="rts-options-misc">
89 <title>Miscellaneous RTS options</title>
93 <term><option>-V<replaceable>secs</replaceable></option>
94 <indexterm><primary><option>-V</option></primary><secondary>RTS
95 option</secondary></indexterm></term>
97 <para>Sets the interval that the RTS clock ticks at. The
98 runtime uses a single timer signal to count ticks; this timer
99 signal is used to control the context switch timer (<xref
100 linkend="using-concurrent" />) and the heap profiling
101 timer <xref linkend="rts-options-heap-prof" />. Also, the
102 time profiler uses the RTS timer signal directly to record
103 time profiling samples.</para>
105 <para>Normally, setting the <option>-V</option> option
106 directly is not necessary: the resolution of the RTS timer is
107 adjusted automatically if a short interval is requested with
108 the <option>-C</option> or <option>-i</option> options.
109 However, setting <option>-V</option> is required in order to
110 increase the resolution of the time profiler.</para>
115 <term><option>--install-signal-handlers=<replaceable>yes|no</replaceable></option>
116 <indexterm><primary><option>--install-signal-handlers</option></primary><secondary>RTS
117 option</secondary></indexterm></term>
119 <para>If yes (the default), the RTS installs signal handlers to catch
120 things like ctrl-C. This option is primarily useful for when
121 you are using the Haskell code as a DLL, and want to set your
122 own signal handlers.</para>
128 <sect2 id="rts-options-gc">
129 <title>RTS options to control the garbage collector</title>
131 <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
132 <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>
134 <para>There are several options to give you precise control over
135 garbage collection. Hopefully, you won't need any of these in
136 normal operation, but there are several things that can be tweaked
137 for maximum performance.</para>
143 <option>-A</option><replaceable>size</replaceable>
144 <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
145 <indexterm><primary>allocation area, size</primary></indexterm>
148 <para>[Default: 256k] Set the allocation area size
149 used by the garbage collector. The allocation area
150 (actually generation 0 step 0) is fixed and is never resized
151 (unless you use <option>-H</option>, below).</para>
153 <para>Increasing the allocation area size may or may not
154 give better performance (a bigger allocation area means
155 worse cache behaviour but fewer garbage collections and less
158 <para>With only 1 generation (<option>-G1</option>) the
159 <option>-A</option> option specifies the minimum allocation
160 area, since the actual size of the allocation area will be
161 resized according to the amount of data in the heap (see
162 <option>-F</option>, below).</para>
169 <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
170 <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
171 <indexterm><primary>compacting garbage collection</primary></indexterm>
174 <para>Use a compacting algorithm for collecting the oldest
175 generation. By default, the oldest generation is collected
176 using a copying algorithm; this option causes it to be
177 compacted in-place instead. The compaction algorithm is
178 slower than the copying algorithm, but the savings in memory
179 use can be considerable.</para>
181 <para>For a given heap size (using the <option>-H</option>
182 option), compaction can in fact reduce the GC cost by
183 allowing fewer GCs to be performed. This is more likely
184 when the ratio of live data to heap size is high, say
185 >30%.</para>
187 <para>NOTE: compaction doesn't currently work when a single
188 generation is requested using the <option>-G1</option>
194 <term><option>-c</option><replaceable>n</replaceable></term>
197 <para>[Default: 30] Automatically enable
198 compacting collection when the live data exceeds
199 <replaceable>n</replaceable>% of the maximum heap size
200 (see the <option>-M</option> option). Note that the maximum
201 heap size is unlimited by default, so this option has no
202 effect unless the maximum heap size is set with
203 <option>-M</option><replaceable>size</replaceable>. </para>
209 <option>-F</option><replaceable>factor</replaceable>
210 <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
211 <indexterm><primary>heap size, factor</primary></indexterm>
215 <para>[Default: 2] This option controls the amount
216 of memory reserved for the older generations (and in the
217 case of a two space collector the size of the allocation
218 area) as a factor of the amount of live data. For example,
219 if there was 2M of live data in the oldest generation when
220 we last collected it, then by default we'll wait until it
221 grows to 4M before collecting it again.</para>
223 <para>The default seems to work well here. If you have
224 plenty of memory, it is usually better to use
225 <option>-H</option><replaceable>size</replaceable> than to
227 <option>-F</option><replaceable>factor</replaceable>.</para>
229 <para>The <option>-F</option> setting will be automatically
230 reduced by the garbage collector when the maximum heap size
231 (the <option>-M</option><replaceable>size</replaceable>
232 setting) is approaching.</para>
238 <option>-G</option><replaceable>generations</replaceable>
239 <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
240 <indexterm><primary>generations, number of</primary></indexterm>
243 <para>[Default: 2] Set the number of generations
244 used by the garbage collector. The default of 2 seems to be
245 good, but the garbage collector can support any number of
246 generations. Anything larger than about 4 is probably not a
247 good idea unless your program runs for a
248 <emphasis>long</emphasis> time, because the oldest
249 generation will hardly ever get collected.</para>
251 <para>Specifying 1 generation with <option>+RTS -G1</option>
252 gives you a simple 2-space collector, as you would expect.
253 In a 2-space collector, the <option>-A</option> option (see
254 above) specifies the <emphasis>minimum</emphasis> allocation
255 area size, since the allocation area will grow with the
256 amount of live data in the heap. In a multi-generational
257 collector the allocation area is a fixed size (unless you
258 use the <option>-H</option> option, see below).</para>
264 <option>-H</option><replaceable>size</replaceable>
265 <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
266 <indexterm><primary>heap size, suggested</primary></indexterm>
269 <para>[Default: 0] This option provides a
270 “suggested heap size” for the garbage collector. The
271 garbage collector will use about this much memory until the
272 program residency grows and the heap size needs to be
273 expanded to retain reasonable performance.</para>
275 <para>By default, the heap will start small, and grow and
276 shrink as necessary. This can be bad for performance, so if
277 you have plenty of memory it's worthwhile supplying a big
278 <option>-H</option><replaceable>size</replaceable>. For
279 improving GC performance, using
280 <option>-H</option><replaceable>size</replaceable> is
281 usually a better bet than
282 <option>-A</option><replaceable>size</replaceable>.</para>
288 <option>-I</option><replaceable>seconds</replaceable>
289 <indexterm><primary><option>-I</option></primary>
290 <secondary>RTS option</secondary>
292 <indexterm><primary>idle GC</primary>
296 <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
297 <option>-threaded</option>, <xref linkend="options-linker" />), a
298 major GC is automatically performed if the runtime has been idle
299 (no Haskell computation has been running) for a period of time.
300 The amount of idle time which must pass before a GC is performed is
301 set by the <option>-I</option><replaceable>seconds</replaceable>
302 option. Specifying <option>-I0</option> disables the idle GC.</para>
304 <para>For an interactive application, it is probably a good idea to
305 use the idle GC, because this will allow finalizers to run and
306 deadlocked threads to be detected in the idle time when no Haskell
307 computation is happening. Also, it will mean that a GC is less
308 likely to happen when the application is busy, and so
309 responsiveness may be improved. However, if the amount of live data in
310 the heap is particularly large, then the idle GC can cause a
311 significant delay, and too small an interval could adversely affect
312 interactive responsiveness.</para>
314 <para>This is an experimental feature, please let us know if it
315 causes problems and/or could benefit from further tuning.</para>
321 <option>-k</option><replaceable>size</replaceable>
322 <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
323 <indexterm><primary>stack, minimum size</primary></indexterm>
326 <para>[Default: 1k] Set the initial stack size for
327 new threads. Thread stacks (including the main thread's
328 stack) live on the heap, and grow as required. The default
329 value is good for concurrent applications with lots of small
330 threads; if your program doesn't fit this model then
331 increasing this option may help performance.</para>
333 <para>The main thread is normally started with a slightly
334 larger heap to cut down on unnecessary stack growth while
335 the program is starting up.</para>
341 <option>-K</option><replaceable>size</replaceable>
342 <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
343 <indexterm><primary>stack, maximum size</primary></indexterm>
346 <para>[Default: 8M] Set the maximum stack size for
347 an individual thread to <replaceable>size</replaceable>
348 bytes. This option is there purely to stop the program
349 eating up all the available memory in the machine if it gets
350 into an infinite loop.</para>
356 <option>-m</option><replaceable>n</replaceable>
357 <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
358 <indexterm><primary>heap, minimum free</primary></indexterm>
361 <para>Minimum % <replaceable>n</replaceable> of heap
362 which must be available for allocation. The default is
369 <option>-M</option><replaceable>size</replaceable>
370 <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
371 <indexterm><primary>heap size, maximum</primary></indexterm>
374 <para>[Default: unlimited] Set the maximum heap size to
375 <replaceable>size</replaceable> bytes. The heap normally
376 grows and shrinks according to the memory requirements of
377 the program. The only reason for having this option is to
378 stop the heap growing without bound and filling up all the
379 available swap space, which at the least will result in the
380 program being summarily killed by the operating
383 <para>The maximum heap size also affects other garbage
384 collection parameters: when the amount of live data in the
385 heap exceeds a certain fraction of the maximum heap size,
386 compacting collection will be automatically enabled for the
387 oldest generation, and the <option>-F</option> parameter
388 will be reduced in order to avoid exceeding the maximum heap
395 <option>-s</option><replaceable>file</replaceable>
396 <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
399 <option>-S</option><replaceable>file</replaceable>
400 <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
403 <para>Write modest (<option>-s</option>) or verbose
404 (<option>-S</option>) garbage-collector statistics into file
405 <replaceable>file</replaceable>. The default
406 <replaceable>file</replaceable> is
407 <filename><replaceable>program</replaceable>.stat</filename>. The
408 <replaceable>file</replaceable> <constant>stderr</constant>
409 is treated specially, with the output really being sent to
410 <constant>stderr</constant>.</para>
412 <para>This option is useful for watching how the storage
413 manager adjusts the heap size based on the current amount of
420 <option>-t<replaceable>file</replaceable></option>
421 <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
424 <para>Write a one-line GC stats summary after running the
425 program. This output is in the same format as that produced
426 by the <option>-Rghc-timing</option> option.</para>
428 <para>As with <option>-s</option>, the default
429 <replaceable>file</replaceable> is
430 <filename><replaceable>program</replaceable>.stat</filename>. The
431 <replaceable>file</replaceable> <constant>stderr</constant>
432 is treated specially, with the output really being sent to
433 <constant>stderr</constant>.</para>
441 <title>RTS options for profiling and parallelism</title>
443 <para>The RTS options related to profiling are described in <xref
444 linkend="rts-options-heap-prof"/>, those for concurrency in
445 <xref linkend="using-concurrent" />, and those for parallelism in
446 <xref linkend="parallel-options"/>.</para>
449 <sect2 id="rts-options-debugging">
450 <title>RTS options for hackers, debuggers, and over-interested
453 <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>
455 <para>These RTS options might be used (a) to avoid a GHC bug,
456 (b) to see “what's really happening”, or
457 (c) because you feel like it. Not recommended for everyday
465 <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
468 <para>Sound the bell at the start of each (major) garbage
471 <para>Oddly enough, people really do use this option! Our
472 pal in Durham (England), Paul Callaghan, writes: “Some
473 people here use it for a variety of
474 purposes—honestly!—e.g., confirmation that the
475 code/machine is doing something, infinite loop detection,
476 gauging cost of recently added code. Certain people can even
477 tell what stage [the program] is in by the beep
478 pattern. But the major use is for annoying others in the
479 same office…”</para>
485 <option>-D</option><replaceable>num</replaceable>
486 <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
489 <para>An RTS debugging flag; varying quantities of output
490 depending on which bits are set in
491 <replaceable>num</replaceable>. Only works if the RTS was
492 compiled with the <option>DEBUG</option> option.</para>
498 <option>-r</option><replaceable>file</replaceable>
499 <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
500 <indexterm><primary>ticky ticky profiling</primary></indexterm>
501 <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
504 <para>Produce “ticky-ticky” statistics at the
505 end of the program run. The <replaceable>file</replaceable>
506 business works just like on the <option>-S</option> RTS
507 option (above).</para>
509 <para>“Ticky-ticky” statistics are counts of
510 various program actions (updates, enters, etc.) The program
511 must have been compiled using
512 <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
513 (a.k.a. “ticky-ticky profiling”), and, for it to
514 be really useful, linked with suitable system libraries.
515 Not a trivial undertaking: consult the installation guide on
516 how to set things up for easy “ticky-ticky”
517 profiling. For more information, see <xref
518 linkend="ticky-ticky"/>.</para>
525 <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
528 <para>(Only available when the program is compiled for
529 profiling.) When an exception is raised in the program,
530 this option causes the current cost-centre-stack to be
531 dumped to <literal>stderr</literal>.</para>
533 <para>This can be particularly useful for debugging: if your
534 program is complaining about a <literal>head []</literal>
535 error and you haven't got a clue which bit of code is
536 causing it, compiling with <literal>-prof
537 -auto-all</literal> and running with <literal>+RTS -xc
538 -RTS</literal> will tell you exactly the call stack at the
539 point the error was raised.</para>
541 <para>The output contains one line for each exception raised
542 in the program (the program might raise and catch several
543 exceptions during its execution), where each line is of the
547 < cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> >
549 <para>each <literal>cc</literal><subscript>i</subscript> is
550 a cost centre in the program (see <xref
551 linkend="cost-centres"/>), and the sequence represents the
552 “call stack” at the point the exception was
553 raised. The leftmost item is the innermost function in the
554 call stack, and the rightmost item is the outermost
563 <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
566 <para>Turn <emphasis>off</emphasis> “update-frame
567 squeezing” at garbage-collection time. (There's no
568 particularly good reason to turn it off, except to ensure
569 the accuracy of certain data collected regarding thunk entry
577 <sect2 id="rts-hooks">
578 <title>“Hooks” to change RTS behaviour</title>
580 <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
581 <indexterm><primary>RTS hooks</primary></indexterm>
582 <indexterm><primary>RTS behaviour, changing</primary></indexterm>
584 <para>GHC lets you exercise rudimentary control over the RTS
585 settings for any given program, by compiling in a
586 “hook” that is called by the run-time system. The RTS
587 contains stub definitions for all these hooks, but by writing your
588 own version and linking it on the GHC command line, you can
589 override the defaults.</para>
591 <para>Owing to the vagaries of DLL linking, these hooks don't work
592 under Windows when the program is built dynamically.</para>
594 <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
595 </indexterm>lets you set RTS
596 options permanently for a given program. A common use for this is
597 to give your program a default heap and/or stack size that is
598 greater than the default. For example, to set <literal>-H128m
599 -K1m</literal>, place the following definition in a C source
603 char *ghc_rts_opts = "-H128m -K1m";
606 <para>Compile the C file, and include the object file on the
607 command line when you link your Haskell program.</para>
609 <para>These flags are interpreted first, before any RTS flags from
610 the <literal>GHCRTS</literal> environment variable and any flags
611 on the command line.</para>
613 <para>You can also change the messages printed when the runtime
614 system “blows up,” e.g., on stack overflow. The hooks
615 for these are as follows:</para>
621 <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
622 <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
625 <para>The heap-overflow message.</para>
631 <function>void StackOverflowHook (long int)</function>
632 <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
635 <para>The stack-overflow message.</para>
641 <function>void MallocFailHook (long int)</function>
642 <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
645 <para>The message printed if <function>malloc</function>
651 <para>For examples of the use of these hooks, see GHC's own
653 <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
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