document paralel GC option: +RTS -g
[ghc.git] / docs / users_guide / runtime_control.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <sect1 id="runtime-control">
3 <title>Running a compiled program</title>
4
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>
8
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>
12
13 <para>You have some control over the behaviour of the RTS, by giving
14 special command-line arguments to your program.</para>
15
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>
19 and
20 <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
21 as its own. For example:</para>
22
23 <screen>
24 % ./a.out -f +RTS -p -S -RTS -h foo bar
25 </screen>
26
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>
31
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
34 this example:</para>
35
36 <screen>
37 % hls -ltr /usr/etc +RTS -A5m
38 </screen>
39
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>&ndash;&ndash;RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.</para>
43
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>
49 fault!)</para>
50
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>
55
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>
63
64 <sect2 id="rts-optinos-environment">
65 <title>Setting global RTS options</title>
66
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>
70
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>
76
77 <screen>
78 GHCRTS='-M128m'
79 export GHCRTS
80 </screen>
81
82 <para>RTS options taken from the <envar>GHCRTS</envar> environment
83 variable can be overridden by options given on the command
84 line.</para>
85
86 </sect2>
87
88 <sect2 id="rts-options-misc">
89 <title>Miscellaneous RTS options</title>
90
91 <variablelist>
92 <varlistentry>
93 <term><option>-V<replaceable>secs</replaceable></option>
94 <indexterm><primary><option>-V</option></primary><secondary>RTS
95 option</secondary></indexterm></term>
96 <listitem>
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>
104
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>
111
112 <para>Using a value of zero disables the RTS clock
113 completely, and has the effect of disabling timers that
114 depend on it: the context switch timer and the heap profiling
115 timer. Context switches will still happen, but
116 deterministically and at a rate much faster than normal.
117 Disabling the interval timer is useful for debugging, because
118 it eliminates a source of non-determinism at runtime.</para>
119 </listitem>
120 </varlistentry>
121
122 <varlistentry>
123 <term><option>--install-signal-handlers=<replaceable>yes|no</replaceable></option>
124 <indexterm><primary><option>--install-signal-handlers</option></primary><secondary>RTS
125 option</secondary></indexterm></term>
126 <listitem>
127 <para>If yes (the default), the RTS installs signal handlers to catch
128 things like ctrl-C. This option is primarily useful for when
129 you are using the Haskell code as a DLL, and want to set your
130 own signal handlers.</para>
131 </listitem>
132 </varlistentry>
133 </variablelist>
134 </sect2>
135
136 <sect2 id="rts-options-gc">
137 <title>RTS options to control the garbage collector</title>
138
139 <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
140 <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>
141
142 <para>There are several options to give you precise control over
143 garbage collection. Hopefully, you won't need any of these in
144 normal operation, but there are several things that can be tweaked
145 for maximum performance.</para>
146
147 <variablelist>
148
149 <varlistentry>
150 <term>
151 <option>-A</option><replaceable>size</replaceable>
152 <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
153 <indexterm><primary>allocation area, size</primary></indexterm>
154 </term>
155 <listitem>
156 <para>&lsqb;Default: 256k&rsqb; Set the allocation area size
157 used by the garbage collector. The allocation area
158 (actually generation 0 step 0) is fixed and is never resized
159 (unless you use <option>-H</option>, below).</para>
160
161 <para>Increasing the allocation area size may or may not
162 give better performance (a bigger allocation area means
163 worse cache behaviour but fewer garbage collections and less
164 promotion).</para>
165
166 <para>With only 1 generation (<option>-G1</option>) the
167 <option>-A</option> option specifies the minimum allocation
168 area, since the actual size of the allocation area will be
169 resized according to the amount of data in the heap (see
170 <option>-F</option>, below).</para>
171 </listitem>
172 </varlistentry>
173
174 <varlistentry>
175 <term>
176 <option>-c</option>
177 <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
178 <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
179 <indexterm><primary>compacting garbage collection</primary></indexterm>
180 </term>
181 <listitem>
182 <para>Use a compacting algorithm for collecting the oldest
183 generation. By default, the oldest generation is collected
184 using a copying algorithm; this option causes it to be
185 compacted in-place instead. The compaction algorithm is
186 slower than the copying algorithm, but the savings in memory
187 use can be considerable.</para>
188
189 <para>For a given heap size (using the <option>-H</option>
190 option), compaction can in fact reduce the GC cost by
191 allowing fewer GCs to be performed. This is more likely
192 when the ratio of live data to heap size is high, say
193 &gt;30&percnt;.</para>
194
195 <para>NOTE: compaction doesn't currently work when a single
196 generation is requested using the <option>-G1</option>
197 option.</para>
198 </listitem>
199 </varlistentry>
200
201 <varlistentry>
202 <term><option>-c</option><replaceable>n</replaceable></term>
203
204 <listitem>
205 <para>&lsqb;Default: 30&rsqb; Automatically enable
206 compacting collection when the live data exceeds
207 <replaceable>n</replaceable>&percnt; of the maximum heap size
208 (see the <option>-M</option> option). Note that the maximum
209 heap size is unlimited by default, so this option has no
210 effect unless the maximum heap size is set with
211 <option>-M</option><replaceable>size</replaceable>. </para>
212 </listitem>
213 </varlistentry>
214
215 <varlistentry>
216 <term>
217 <option>-F</option><replaceable>factor</replaceable>
218 <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
219 <indexterm><primary>heap size, factor</primary></indexterm>
220 </term>
221 <listitem>
222
223 <para>&lsqb;Default: 2&rsqb; This option controls the amount
224 of memory reserved for the older generations (and in the
225 case of a two space collector the size of the allocation
226 area) as a factor of the amount of live data. For example,
227 if there was 2M of live data in the oldest generation when
228 we last collected it, then by default we'll wait until it
229 grows to 4M before collecting it again.</para>
230
231 <para>The default seems to work well here. If you have
232 plenty of memory, it is usually better to use
233 <option>-H</option><replaceable>size</replaceable> than to
234 increase
235 <option>-F</option><replaceable>factor</replaceable>.</para>
236
237 <para>The <option>-F</option> setting will be automatically
238 reduced by the garbage collector when the maximum heap size
239 (the <option>-M</option><replaceable>size</replaceable>
240 setting) is approaching.</para>
241 </listitem>
242 </varlistentry>
243
244 <varlistentry>
245 <term>
246 <option>-G</option><replaceable>generations</replaceable>
247 <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
248 <indexterm><primary>generations, number of</primary></indexterm>
249 </term>
250 <listitem>
251 <para>&lsqb;Default: 2&rsqb; Set the number of generations
252 used by the garbage collector. The default of 2 seems to be
253 good, but the garbage collector can support any number of
254 generations. Anything larger than about 4 is probably not a
255 good idea unless your program runs for a
256 <emphasis>long</emphasis> time, because the oldest
257 generation will hardly ever get collected.</para>
258
259 <para>Specifying 1 generation with <option>+RTS -G1</option>
260 gives you a simple 2-space collector, as you would expect.
261 In a 2-space collector, the <option>-A</option> option (see
262 above) specifies the <emphasis>minimum</emphasis> allocation
263 area size, since the allocation area will grow with the
264 amount of live data in the heap. In a multi-generational
265 collector the allocation area is a fixed size (unless you
266 use the <option>-H</option> option, see below).</para>
267 </listitem>
268 </varlistentry>
269
270 <varlistentry>
271 <term>
272 <option>-g</option><replaceable>threads</replaceable>
273 <indexterm><primary><option>-g</option></primary><secondary>RTS option</secondary></indexterm>
274 </term>
275 <listitem>
276 <para>&lsqb;Default: 1&rsqb; &lsqb;new in GHC 6.10&rsqb; Set the number
277 of threads to use for garbage collection. This option is
278 only accepted when the program was linked with the
279 <option>-threaded</option> option; see <xref
280 linkend="options-linker" />.</para>
281
282 <para>The garbage collector is able to work in parallel when
283 given more than one OS thread. Experiments have shown
284 that this usually results in a performance improvement
285 given 3 cores or more; with 2 cores it may or may not be
286 beneficial, depending on the workload. Bigger heaps work
287 better with parallel GC, so set your <option>-H</option>
288 value high (3 or more times the maximum residency). Look
289 at the timing stats with <option>+RTS -s</option> to
290 see whether you're getting any benefit from parallel GC or
291 not. If you find parallel GC is
292 significantly <emphasis>slower</emphasis> (in elapsed
293 time) than sequential GC, please report it as a
294 bug.</para>
295
296 <para>This value is set automatically when the
297 <option>-N</option> option is used, so the only reason to
298 use <option>-g</option> would be if you wanted to use a
299 different number of threads for GC than for execution.
300 For example, if your program is strictly single-threaded
301 but you still want to benefit from parallel GC, then it
302 might make sense to use <option>-g</option> rather than
303 <option>-N</option>.</para>
304 </listitem>
305 </varlistentry>
306
307 <varlistentry>
308 <term>
309 <option>-H</option><replaceable>size</replaceable>
310 <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
311 <indexterm><primary>heap size, suggested</primary></indexterm>
312 </term>
313 <listitem>
314 <para>&lsqb;Default: 0&rsqb; This option provides a
315 &ldquo;suggested heap size&rdquo; for the garbage collector. The
316 garbage collector will use about this much memory until the
317 program residency grows and the heap size needs to be
318 expanded to retain reasonable performance.</para>
319
320 <para>By default, the heap will start small, and grow and
321 shrink as necessary. This can be bad for performance, so if
322 you have plenty of memory it's worthwhile supplying a big
323 <option>-H</option><replaceable>size</replaceable>. For
324 improving GC performance, using
325 <option>-H</option><replaceable>size</replaceable> is
326 usually a better bet than
327 <option>-A</option><replaceable>size</replaceable>.</para>
328 </listitem>
329 </varlistentry>
330
331 <varlistentry>
332 <term>
333 <option>-I</option><replaceable>seconds</replaceable>
334 <indexterm><primary><option>-I</option></primary>
335 <secondary>RTS option</secondary>
336 </indexterm>
337 <indexterm><primary>idle GC</primary>
338 </indexterm>
339 </term>
340 <listitem>
341 <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
342 <option>-threaded</option>, <xref linkend="options-linker" />), a
343 major GC is automatically performed if the runtime has been idle
344 (no Haskell computation has been running) for a period of time.
345 The amount of idle time which must pass before a GC is performed is
346 set by the <option>-I</option><replaceable>seconds</replaceable>
347 option. Specifying <option>-I0</option> disables the idle GC.</para>
348
349 <para>For an interactive application, it is probably a good idea to
350 use the idle GC, because this will allow finalizers to run and
351 deadlocked threads to be detected in the idle time when no Haskell
352 computation is happening. Also, it will mean that a GC is less
353 likely to happen when the application is busy, and so
354 responsiveness may be improved. However, if the amount of live data in
355 the heap is particularly large, then the idle GC can cause a
356 significant delay, and too small an interval could adversely affect
357 interactive responsiveness.</para>
358
359 <para>This is an experimental feature, please let us know if it
360 causes problems and/or could benefit from further tuning.</para>
361 </listitem>
362 </varlistentry>
363
364 <varlistentry>
365 <term>
366 <option>-k</option><replaceable>size</replaceable>
367 <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
368 <indexterm><primary>stack, minimum size</primary></indexterm>
369 </term>
370 <listitem>
371 <para>&lsqb;Default: 1k&rsqb; Set the initial stack size for
372 new threads. Thread stacks (including the main thread's
373 stack) live on the heap, and grow as required. The default
374 value is good for concurrent applications with lots of small
375 threads; if your program doesn't fit this model then
376 increasing this option may help performance.</para>
377
378 <para>The main thread is normally started with a slightly
379 larger heap to cut down on unnecessary stack growth while
380 the program is starting up.</para>
381 </listitem>
382 </varlistentry>
383
384 <varlistentry>
385 <term>
386 <option>-K</option><replaceable>size</replaceable>
387 <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
388 <indexterm><primary>stack, maximum size</primary></indexterm>
389 </term>
390 <listitem>
391 <para>&lsqb;Default: 8M&rsqb; Set the maximum stack size for
392 an individual thread to <replaceable>size</replaceable>
393 bytes. This option is there purely to stop the program
394 eating up all the available memory in the machine if it gets
395 into an infinite loop.</para>
396 </listitem>
397 </varlistentry>
398
399 <varlistentry>
400 <term>
401 <option>-m</option><replaceable>n</replaceable>
402 <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
403 <indexterm><primary>heap, minimum free</primary></indexterm>
404 </term>
405 <listitem>
406 <para>Minimum &percnt; <replaceable>n</replaceable> of heap
407 which must be available for allocation. The default is
408 3&percnt;.</para>
409 </listitem>
410 </varlistentry>
411
412 <varlistentry>
413 <term>
414 <option>-M</option><replaceable>size</replaceable>
415 <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
416 <indexterm><primary>heap size, maximum</primary></indexterm>
417 </term>
418 <listitem>
419 <para>&lsqb;Default: unlimited&rsqb; Set the maximum heap size to
420 <replaceable>size</replaceable> bytes. The heap normally
421 grows and shrinks according to the memory requirements of
422 the program. The only reason for having this option is to
423 stop the heap growing without bound and filling up all the
424 available swap space, which at the least will result in the
425 program being summarily killed by the operating
426 system.</para>
427
428 <para>The maximum heap size also affects other garbage
429 collection parameters: when the amount of live data in the
430 heap exceeds a certain fraction of the maximum heap size,
431 compacting collection will be automatically enabled for the
432 oldest generation, and the <option>-F</option> parameter
433 will be reduced in order to avoid exceeding the maximum heap
434 size.</para>
435 </listitem>
436 </varlistentry>
437
438 <varlistentry>
439 <term>
440 <option>-s</option><replaceable>file</replaceable>
441 <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
442 </term>
443 <term>
444 <option>-S</option><replaceable>file</replaceable>
445 <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
446 </term>
447 <listitem>
448 <para>Write modest (<option>-s</option>) or verbose
449 (<option>-S</option>) garbage-collector statistics into file
450 <replaceable>file</replaceable>. The default
451 <replaceable>file</replaceable> is
452 <filename><replaceable>program</replaceable>.stat</filename>. The
453 <replaceable>file</replaceable> <constant>stderr</constant>
454 is treated specially, with the output really being sent to
455 <constant>stderr</constant>.</para>
456
457 <para>This option is useful for watching how the storage
458 manager adjusts the heap size based on the current amount of
459 live data.</para>
460 </listitem>
461 </varlistentry>
462
463 <varlistentry>
464 <term>
465 <option>-t<replaceable>file</replaceable></option>
466 <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
467 </term>
468 <listitem>
469 <para>Write a one-line GC stats summary after running the
470 program. This output is in the same format as that produced
471 by the <option>-Rghc-timing</option> option.</para>
472
473 <para>As with <option>-s</option>, the default
474 <replaceable>file</replaceable> is
475 <filename><replaceable>program</replaceable>.stat</filename>. The
476 <replaceable>file</replaceable> <constant>stderr</constant>
477 is treated specially, with the output really being sent to
478 <constant>stderr</constant>.</para>
479 </listitem>
480 </varlistentry>
481 </variablelist>
482
483 </sect2>
484
485 <sect2>
486 <title>RTS options for profiling and parallelism</title>
487
488 <para>The RTS options related to profiling are described in <xref
489 linkend="rts-options-heap-prof"/>, those for concurrency in
490 <xref linkend="using-concurrent" />, and those for parallelism in
491 <xref linkend="parallel-options"/>.</para>
492 </sect2>
493
494 <sect2 id="rts-options-debugging">
495 <title>RTS options for hackers, debuggers, and over-interested
496 souls</title>
497
498 <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>
499
500 <para>These RTS options might be used (a)&nbsp;to avoid a GHC bug,
501 (b)&nbsp;to see &ldquo;what's really happening&rdquo;, or
502 (c)&nbsp;because you feel like it. Not recommended for everyday
503 use!</para>
504
505 <variablelist>
506
507 <varlistentry>
508 <term>
509 <option>-B</option>
510 <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
511 </term>
512 <listitem>
513 <para>Sound the bell at the start of each (major) garbage
514 collection.</para>
515
516 <para>Oddly enough, people really do use this option! Our
517 pal in Durham (England), Paul Callaghan, writes: &ldquo;Some
518 people here use it for a variety of
519 purposes&mdash;honestly!&mdash;e.g., confirmation that the
520 code/machine is doing something, infinite loop detection,
521 gauging cost of recently added code. Certain people can even
522 tell what stage &lsqb;the program&rsqb; is in by the beep
523 pattern. But the major use is for annoying others in the
524 same office&hellip;&rdquo;</para>
525 </listitem>
526 </varlistentry>
527
528 <varlistentry>
529 <term>
530 <option>-D</option><replaceable>num</replaceable>
531 <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
532 </term>
533 <listitem>
534 <para>An RTS debugging flag; varying quantities of output
535 depending on which bits are set in
536 <replaceable>num</replaceable>. Only works if the RTS was
537 compiled with the <option>DEBUG</option> option.</para>
538 </listitem>
539 </varlistentry>
540
541 <varlistentry>
542 <term>
543 <option>-r</option><replaceable>file</replaceable>
544 <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
545 <indexterm><primary>ticky ticky profiling</primary></indexterm>
546 <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
547 </term>
548 <listitem>
549 <para>Produce &ldquo;ticky-ticky&rdquo; statistics at the
550 end of the program run. The <replaceable>file</replaceable>
551 business works just like on the <option>-S</option> RTS
552 option (above).</para>
553
554 <para>&ldquo;Ticky-ticky&rdquo; statistics are counts of
555 various program actions (updates, enters, etc.) The program
556 must have been compiled using
557 <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
558 (a.k.a. &ldquo;ticky-ticky profiling&rdquo;), and, for it to
559 be really useful, linked with suitable system libraries.
560 Not a trivial undertaking: consult the installation guide on
561 how to set things up for easy &ldquo;ticky-ticky&rdquo;
562 profiling. For more information, see <xref
563 linkend="ticky-ticky"/>.</para>
564 </listitem>
565 </varlistentry>
566
567 <varlistentry>
568 <term>
569 <option>-xc</option>
570 <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
571 </term>
572 <listitem>
573 <para>(Only available when the program is compiled for
574 profiling.) When an exception is raised in the program,
575 this option causes the current cost-centre-stack to be
576 dumped to <literal>stderr</literal>.</para>
577
578 <para>This can be particularly useful for debugging: if your
579 program is complaining about a <literal>head []</literal>
580 error and you haven't got a clue which bit of code is
581 causing it, compiling with <literal>-prof
582 -auto-all</literal> and running with <literal>+RTS -xc
583 -RTS</literal> will tell you exactly the call stack at the
584 point the error was raised.</para>
585
586 <para>The output contains one line for each exception raised
587 in the program (the program might raise and catch several
588 exceptions during its execution), where each line is of the
589 form:</para>
590
591 <screen>
592 &lt; cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> &gt;
593 </screen>
594 <para>each <literal>cc</literal><subscript>i</subscript> is
595 a cost centre in the program (see <xref
596 linkend="cost-centres"/>), and the sequence represents the
597 &ldquo;call stack&rdquo; at the point the exception was
598 raised. The leftmost item is the innermost function in the
599 call stack, and the rightmost item is the outermost
600 function.</para>
601
602 </listitem>
603 </varlistentry>
604
605 <varlistentry>
606 <term>
607 <option>-Z</option>
608 <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
609 </term>
610 <listitem>
611 <para>Turn <emphasis>off</emphasis> &ldquo;update-frame
612 squeezing&rdquo; at garbage-collection time. (There's no
613 particularly good reason to turn it off, except to ensure
614 the accuracy of certain data collected regarding thunk entry
615 counts.)</para>
616 </listitem>
617 </varlistentry>
618 </variablelist>
619
620 </sect2>
621
622 <sect2 id="rts-hooks">
623 <title>&ldquo;Hooks&rdquo; to change RTS behaviour</title>
624
625 <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
626 <indexterm><primary>RTS hooks</primary></indexterm>
627 <indexterm><primary>RTS behaviour, changing</primary></indexterm>
628
629 <para>GHC lets you exercise rudimentary control over the RTS
630 settings for any given program, by compiling in a
631 &ldquo;hook&rdquo; that is called by the run-time system. The RTS
632 contains stub definitions for all these hooks, but by writing your
633 own version and linking it on the GHC command line, you can
634 override the defaults.</para>
635
636 <para>Owing to the vagaries of DLL linking, these hooks don't work
637 under Windows when the program is built dynamically.</para>
638
639 <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
640 </indexterm>lets you set RTS
641 options permanently for a given program. A common use for this is
642 to give your program a default heap and/or stack size that is
643 greater than the default. For example, to set <literal>-H128m
644 -K1m</literal>, place the following definition in a C source
645 file:</para>
646
647 <programlisting>
648 char *ghc_rts_opts = "-H128m -K1m";
649 </programlisting>
650
651 <para>Compile the C file, and include the object file on the
652 command line when you link your Haskell program.</para>
653
654 <para>These flags are interpreted first, before any RTS flags from
655 the <literal>GHCRTS</literal> environment variable and any flags
656 on the command line.</para>
657
658 <para>You can also change the messages printed when the runtime
659 system &ldquo;blows up,&rdquo; e.g., on stack overflow. The hooks
660 for these are as follows:</para>
661
662 <variablelist>
663
664 <varlistentry>
665 <term>
666 <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
667 <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
668 </term>
669 <listitem>
670 <para>The heap-overflow message.</para>
671 </listitem>
672 </varlistentry>
673
674 <varlistentry>
675 <term>
676 <function>void StackOverflowHook (long int)</function>
677 <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
678 </term>
679 <listitem>
680 <para>The stack-overflow message.</para>
681 </listitem>
682 </varlistentry>
683
684 <varlistentry>
685 <term>
686 <function>void MallocFailHook (long int)</function>
687 <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
688 </term>
689 <listitem>
690 <para>The message printed if <function>malloc</function>
691 fails.</para>
692 </listitem>
693 </varlistentry>
694 </variablelist>
695
696 <para>For examples of the use of these hooks, see GHC's own
697 versions in the file
698 <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
699 source tree.</para>
700 </sect2>
701
702 <sect2>
703 <title>Getting information about the RTS</title>
704
705 <indexterm><primary>RTS</primary></indexterm>
706
707 <para>It is possible to ask the RTS to give some information about
708 itself. To do this, use the <option>--info</option> flag, e.g.</para>
709 <screen>
710 $ ./a.out +RTS --info
711 [("GHC RTS", "Yes")
712 ,("GHC version", "6.7")
713 ,("RTS way", "rts_p")
714 ,("Host platform", "x86_64-unknown-linux")
715 ,("Build platform", "x86_64-unknown-linux")
716 ,("Target platform", "x86_64-unknown-linux")
717 ,("Compiler unregisterised", "NO")
718 ,("Tables next to code", "YES")
719 ]
720 </screen>
721 <para>The information is formatted such that it can be read as a
722 of type <literal>[(String, String)]</literal>.</para>
723 </sect2>
724 </sect1>
725
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