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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-gc">
89 <title>RTS options to control the garbage collector</title>
91 <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
92 <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>
94 <para>There are several options to give you precise control over
95 garbage collection. Hopefully, you won't need any of these in
96 normal operation, but there are several things that can be tweaked
97 for maximum performance.</para>
103 <option>-A</option><replaceable>size</replaceable>
104 <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
105 <indexterm><primary>allocation area, size</primary></indexterm>
108 <para>[Default: 256k] Set the allocation area size
109 used by the garbage collector. The allocation area
110 (actually generation 0 step 0) is fixed and is never resized
111 (unless you use <option>-H</option>, below).</para>
113 <para>Increasing the allocation area size may or may not
114 give better performance (a bigger allocation area means
115 worse cache behaviour but fewer garbage collections and less
118 <para>With only 1 generation (<option>-G1</option>) the
119 <option>-A</option> option specifies the minimum allocation
120 area, since the actual size of the allocation area will be
121 resized according to the amount of data in the heap (see
122 <option>-F</option>, below).</para>
129 <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
130 <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
131 <indexterm><primary>compacting garbage collection</primary></indexterm>
134 <para>Use a compacting algorithm for collecting the oldest
135 generation. By default, the oldest generation is collected
136 using a copying algorithm; this option causes it to be
137 compacted in-place instead. The compaction algorithm is
138 slower than the copying algorithm, but the savings in memory
139 use can be considerable.</para>
141 <para>For a given heap size (using the <option>-H</option>
142 option), compaction can in fact reduce the GC cost by
143 allowing fewer GCs to be performed. This is more likely
144 when the ratio of live data to heap size is high, say
145 >30%.</para>
147 <para>NOTE: compaction doesn't currently work when a single
148 generation is requested using the <option>-G1</option>
154 <term><option>-c</option><replaceable>n</replaceable></term>
157 <para>[Default: 30] Automatically enable
158 compacting collection when the live data exceeds
159 <replaceable>n</replaceable>% of the maximum heap size
160 (see the <option>-M</option> option). Note that the maximum
161 heap size is unlimited by default, so this option has no
162 effect unless the maximum heap size is set with
163 <option>-M</option><replaceable>size</replaceable>. </para>
169 <option>-F</option><replaceable>factor</replaceable>
170 <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
171 <indexterm><primary>heap size, factor</primary></indexterm>
175 <para>[Default: 2] This option controls the amount
176 of memory reserved for the older generations (and in the
177 case of a two space collector the size of the allocation
178 area) as a factor of the amount of live data. For example,
179 if there was 2M of live data in the oldest generation when
180 we last collected it, then by default we'll wait until it
181 grows to 4M before collecting it again.</para>
183 <para>The default seems to work well here. If you have
184 plenty of memory, it is usually better to use
185 <option>-H</option><replaceable>size</replaceable> than to
187 <option>-F</option><replaceable>factor</replaceable>.</para>
189 <para>The <option>-F</option> setting will be automatically
190 reduced by the garbage collector when the maximum heap size
191 (the <option>-M</option><replaceable>size</replaceable>
192 setting) is approaching.</para>
198 <option>-G</option><replaceable>generations</replaceable>
199 <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
200 <indexterm><primary>generations, number of</primary></indexterm>
203 <para>[Default: 2] Set the number of generations
204 used by the garbage collector. The default of 2 seems to be
205 good, but the garbage collector can support any number of
206 generations. Anything larger than about 4 is probably not a
207 good idea unless your program runs for a
208 <emphasis>long</emphasis> time, because the oldest
209 generation will hardly ever get collected.</para>
211 <para>Specifying 1 generation with <option>+RTS -G1</option>
212 gives you a simple 2-space collector, as you would expect.
213 In a 2-space collector, the <option>-A</option> option (see
214 above) specifies the <emphasis>minimum</emphasis> allocation
215 area size, since the allocation area will grow with the
216 amount of live data in the heap. In a multi-generational
217 collector the allocation area is a fixed size (unless you
218 use the <option>-H</option> option, see below).</para>
224 <option>-H</option><replaceable>size</replaceable>
225 <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
226 <indexterm><primary>heap size, suggested</primary></indexterm>
229 <para>[Default: 0] This option provides a
230 “suggested heap size” for the garbage collector. The
231 garbage collector will use about this much memory until the
232 program residency grows and the heap size needs to be
233 expanded to retain reasonable performance.</para>
235 <para>By default, the heap will start small, and grow and
236 shrink as necessary. This can be bad for performance, so if
237 you have plenty of memory it's worthwhile supplying a big
238 <option>-H</option><replaceable>size</replaceable>. For
239 improving GC performance, using
240 <option>-H</option><replaceable>size</replaceable> is
241 usually a better bet than
242 <option>-A</option><replaceable>size</replaceable>.</para>
248 <option>-I</option><replaceable>seconds</replaceable>
249 <indexterm><primary><option>-H</option></primary>
250 <secondary>RTS option</secondary>
252 <indexterm><primary>idle GC</primary>
256 <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
257 <option>-threaded</option>, <xref linkend="options-linker" />), a
258 major GC is automatically performed if the runtime has been idle
259 (no Haskell computation has been running) for a period of time.
260 The amount of idle time which must pass before a GC is performed is
261 set by the <option>-I</option><replaceable>seconds</replaceable>
262 option. Specifying <option>-I0</option> disables the idle GC.</para>
264 <para>For an interactive application, it is probably a good idea to
265 use the idle GC, because this will allow finalizers to run and
266 deadlocked threads to be detected in the idle time when no Haskell
267 computation is happening. Also, it will mean that a GC is less
268 likely to happen when the application is busy, and so
269 responsiveness may be improved. However, if the amount of live data in
270 the heap is particularly large, then the idle GC can cause a
271 significant delay, and too small an interval could adversely affect
272 interactive responsiveness.</para>
274 <para>This is an experimental feature, please let us know if it
275 causes problems and/or could benefit from further tuning.</para>
281 <option>-k</option><replaceable>size</replaceable>
282 <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
283 <indexterm><primary>stack, minimum size</primary></indexterm>
286 <para>[Default: 1k] Set the initial stack size for
287 new threads. Thread stacks (including the main thread's
288 stack) live on the heap, and grow as required. The default
289 value is good for concurrent applications with lots of small
290 threads; if your program doesn't fit this model then
291 increasing this option may help performance.</para>
293 <para>The main thread is normally started with a slightly
294 larger heap to cut down on unnecessary stack growth while
295 the program is starting up.</para>
301 <option>-K</option><replaceable>size</replaceable>
302 <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
303 <indexterm><primary>stack, maximum size</primary></indexterm>
306 <para>[Default: 8M] Set the maximum stack size for
307 an individual thread to <replaceable>size</replaceable>
308 bytes. This option is there purely to stop the program
309 eating up all the available memory in the machine if it gets
310 into an infinite loop.</para>
316 <option>-m</option><replaceable>n</replaceable>
317 <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
318 <indexterm><primary>heap, minimum free</primary></indexterm>
321 <para>Minimum % <replaceable>n</replaceable> of heap
322 which must be available for allocation. The default is
329 <option>-M</option><replaceable>size</replaceable>
330 <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
331 <indexterm><primary>heap size, maximum</primary></indexterm>
334 <para>[Default: unlimited] Set the maximum heap size to
335 <replaceable>size</replaceable> bytes. The heap normally
336 grows and shrinks according to the memory requirements of
337 the program. The only reason for having this option is to
338 stop the heap growing without bound and filling up all the
339 available swap space, which at the least will result in the
340 program being summarily killed by the operating
343 <para>The maximum heap size also affects other garbage
344 collection parameters: when the amount of live data in the
345 heap exceeds a certain fraction of the maximum heap size,
346 compacting collection will be automatically enabled for the
347 oldest generation, and the <option>-F</option> parameter
348 will be reduced in order to avoid exceeding the maximum heap
355 <option>-s</option><replaceable>file</replaceable>
356 <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
359 <option>-S</option><replaceable>file</replaceable>
360 <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
363 <para>Write modest (<option>-s</option>) or verbose
364 (<option>-S</option>) garbage-collector statistics into file
365 <replaceable>file</replaceable>. The default
366 <replaceable>file</replaceable> is
367 <filename><replaceable>program</replaceable>.stat</filename>. The
368 <replaceable>file</replaceable> <constant>stderr</constant>
369 is treated specially, with the output really being sent to
370 <constant>stderr</constant>.</para>
372 <para>This option is useful for watching how the storage
373 manager adjusts the heap size based on the current amount of
380 <option>-t<replaceable>file</replaceable></option>
381 <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
384 <para>Write a one-line GC stats summary after running the
385 program. This output is in the same format as that produced
386 by the <option>-Rghc-timing</option> option.</para>
388 <para>As with <option>-s</option>, the default
389 <replaceable>file</replaceable> is
390 <filename><replaceable>program</replaceable>.stat</filename>. The
391 <replaceable>file</replaceable> <constant>stderr</constant>
392 is treated specially, with the output really being sent to
393 <constant>stderr</constant>.</para>
401 <title>RTS options for profiling and Concurrent/Parallel Haskell</title>
403 <para>The RTS options related to profiling are described in <xref
404 linkend="rts-options-heap-prof"/>; and those for concurrent/parallel
405 stuff, in <xref linkend="parallel-rts-opts"/>.</para>
408 <sect2 id="rts-options-debugging">
409 <title>RTS options for hackers, debuggers, and over-interested
412 <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>
414 <para>These RTS options might be used (a) to avoid a GHC bug,
415 (b) to see “what's really happening”, or
416 (c) because you feel like it. Not recommended for everyday
424 <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
427 <para>Sound the bell at the start of each (major) garbage
430 <para>Oddly enough, people really do use this option! Our
431 pal in Durham (England), Paul Callaghan, writes: “Some
432 people here use it for a variety of
433 purposes—honestly!—e.g., confirmation that the
434 code/machine is doing something, infinite loop detection,
435 gauging cost of recently added code. Certain people can even
436 tell what stage [the program] is in by the beep
437 pattern. But the major use is for annoying others in the
438 same office…”</para>
444 <option>-D</option><replaceable>num</replaceable>
445 <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
448 <para>An RTS debugging flag; varying quantities of output
449 depending on which bits are set in
450 <replaceable>num</replaceable>. Only works if the RTS was
451 compiled with the <option>DEBUG</option> option.</para>
457 <option>-r</option><replaceable>file</replaceable>
458 <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
459 <indexterm><primary>ticky ticky profiling</primary></indexterm>
460 <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
463 <para>Produce “ticky-ticky” statistics at the
464 end of the program run. The <replaceable>file</replaceable>
465 business works just like on the <option>-S</option> RTS
466 option (above).</para>
468 <para>“Ticky-ticky” statistics are counts of
469 various program actions (updates, enters, etc.) The program
470 must have been compiled using
471 <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
472 (a.k.a. “ticky-ticky profiling”), and, for it to
473 be really useful, linked with suitable system libraries.
474 Not a trivial undertaking: consult the installation guide on
475 how to set things up for easy “ticky-ticky”
476 profiling. For more information, see <xref
477 linkend="ticky-ticky"/>.</para>
484 <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
487 <para>(Only available when the program is compiled for
488 profiling.) When an exception is raised in the program,
489 this option causes the current cost-centre-stack to be
490 dumped to <literal>stderr</literal>.</para>
492 <para>This can be particularly useful for debugging: if your
493 program is complaining about a <literal>head []</literal>
494 error and you haven't got a clue which bit of code is
495 causing it, compiling with <literal>-prof
496 -auto-all</literal> and running with <literal>+RTS -xc
497 -RTS</literal> will tell you exactly the call stack at the
498 point the error was raised.</para>
500 <para>The output contains one line for each exception raised
501 in the program (the program might raise and catch several
502 exceptions during its execution), where each line is of the
506 < cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> >
508 <para>each <literal>cc</literal><subscript>i</subscript> is
509 a cost centre in the program (see <xref
510 linkend="cost-centres"/>), and the sequence represents the
511 “call stack” at the point the exception was
512 raised. The leftmost item is the innermost function in the
513 call stack, and the rightmost item is the outermost
522 <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
525 <para>Turn <emphasis>off</emphasis> “update-frame
526 squeezing” at garbage-collection time. (There's no
527 particularly good reason to turn it off, except to ensure
528 the accuracy of certain data collected regarding thunk entry
536 <sect2 id="rts-hooks">
537 <title>“Hooks” to change RTS behaviour</title>
539 <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
540 <indexterm><primary>RTS hooks</primary></indexterm>
541 <indexterm><primary>RTS behaviour, changing</primary></indexterm>
543 <para>GHC lets you exercise rudimentary control over the RTS
544 settings for any given program, by compiling in a
545 “hook” that is called by the run-time system. The RTS
546 contains stub definitions for all these hooks, but by writing your
547 own version and linking it on the GHC command line, you can
548 override the defaults.</para>
550 <para>Owing to the vagaries of DLL linking, these hooks don't work
551 under Windows when the program is built dynamically.</para>
553 <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
554 </indexterm>lets you set RTS
555 options permanently for a given program. A common use for this is
556 to give your program a default heap and/or stack size that is
557 greater than the default. For example, to set <literal>-H128m
558 -K1m</literal>, place the following definition in a C source
562 char *ghc_rts_opts = "-H128m -K1m";
565 <para>Compile the C file, and include the object file on the
566 command line when you link your Haskell program.</para>
568 <para>These flags are interpreted first, before any RTS flags from
569 the <literal>GHCRTS</literal> environment variable and any flags
570 on the command line.</para>
572 <para>You can also change the messages printed when the runtime
573 system “blows up,” e.g., on stack overflow. The hooks
574 for these are as follows:</para>
580 <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
581 <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
584 <para>The heap-overflow message.</para>
590 <function>void StackOverflowHook (long int)</function>
591 <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
594 <para>The stack-overflow message.</para>
600 <function>void MallocFailHook (long int)</function>
601 <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
604 <para>The message printed if <function>malloc</function>
610 <para>For examples of the use of these hooks, see GHC's own
612 <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
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