Attempt to fix #2512 and #2063; add +RTS -xm<address> -RTS option
[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
134 <varlistentry>
135 <term><option>-xm<replaceable>address</replaceable></option>
136 <indexterm><primary><option>-xm</option></primary><secondary>RTS
137 option</secondary></indexterm></term>
138 <listitem>
139 <para>
140 WARNING: this option is for working around memory
141 allocation problems only. Do not use unless GHCi fails
142 with a message like &ldquo;<literal>failed to mmap() memory below 2Gb</literal>&rdquo;. If you need to use this option to get GHCi working
143 on your machine, please file a bug.
144 </para>
145
146 <para>
147 On 64-bit machines, the RTS needs to allocate memory in the
148 low 2Gb of the address space. Support for this across
149 different operating systems is patchy, and sometimes fails.
150 This option is there to give the RTS a hint about where it
151 should be able to allocate memory in the low 2Gb of the
152 address space. For example, <literal>+RTS -xm20000000
153 -RTS</literal> would hint that the RTS should allocate
154 starting at the 0.5Gb mark. The default is to use the OS's
155 built-in support for allocating memory in the low 2Gb if
156 available (e.g. <literal>mmap</literal>
157 with <literal>MAP_32BIT</literal> on Linux), or
158 otherwise <literal>-xm40000000</literal>.
159 </para>
160 </listitem>
161 </varlistentry>
162 </variablelist>
163 </sect2>
164
165 <sect2 id="rts-options-gc">
166 <title>RTS options to control the garbage collector</title>
167
168 <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
169 <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>
170
171 <para>There are several options to give you precise control over
172 garbage collection. Hopefully, you won't need any of these in
173 normal operation, but there are several things that can be tweaked
174 for maximum performance.</para>
175
176 <variablelist>
177
178 <varlistentry>
179 <term>
180 <option>-A</option><replaceable>size</replaceable>
181 <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
182 <indexterm><primary>allocation area, size</primary></indexterm>
183 </term>
184 <listitem>
185 <para>&lsqb;Default: 256k&rsqb; Set the allocation area size
186 used by the garbage collector. The allocation area
187 (actually generation 0 step 0) is fixed and is never resized
188 (unless you use <option>-H</option>, below).</para>
189
190 <para>Increasing the allocation area size may or may not
191 give better performance (a bigger allocation area means
192 worse cache behaviour but fewer garbage collections and less
193 promotion).</para>
194
195 <para>With only 1 generation (<option>-G1</option>) the
196 <option>-A</option> option specifies the minimum allocation
197 area, since the actual size of the allocation area will be
198 resized according to the amount of data in the heap (see
199 <option>-F</option>, below).</para>
200 </listitem>
201 </varlistentry>
202
203 <varlistentry>
204 <term>
205 <option>-c</option>
206 <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
207 <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
208 <indexterm><primary>compacting garbage collection</primary></indexterm>
209 </term>
210 <listitem>
211 <para>Use a compacting algorithm for collecting the oldest
212 generation. By default, the oldest generation is collected
213 using a copying algorithm; this option causes it to be
214 compacted in-place instead. The compaction algorithm is
215 slower than the copying algorithm, but the savings in memory
216 use can be considerable.</para>
217
218 <para>For a given heap size (using the <option>-H</option>
219 option), compaction can in fact reduce the GC cost by
220 allowing fewer GCs to be performed. This is more likely
221 when the ratio of live data to heap size is high, say
222 &gt;30&percnt;.</para>
223
224 <para>NOTE: compaction doesn't currently work when a single
225 generation is requested using the <option>-G1</option>
226 option.</para>
227 </listitem>
228 </varlistentry>
229
230 <varlistentry>
231 <term><option>-c</option><replaceable>n</replaceable></term>
232
233 <listitem>
234 <para>&lsqb;Default: 30&rsqb; Automatically enable
235 compacting collection when the live data exceeds
236 <replaceable>n</replaceable>&percnt; of the maximum heap size
237 (see the <option>-M</option> option). Note that the maximum
238 heap size is unlimited by default, so this option has no
239 effect unless the maximum heap size is set with
240 <option>-M</option><replaceable>size</replaceable>. </para>
241 </listitem>
242 </varlistentry>
243
244 <varlistentry>
245 <term>
246 <option>-F</option><replaceable>factor</replaceable>
247 <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
248 <indexterm><primary>heap size, factor</primary></indexterm>
249 </term>
250 <listitem>
251
252 <para>&lsqb;Default: 2&rsqb; This option controls the amount
253 of memory reserved for the older generations (and in the
254 case of a two space collector the size of the allocation
255 area) as a factor of the amount of live data. For example,
256 if there was 2M of live data in the oldest generation when
257 we last collected it, then by default we'll wait until it
258 grows to 4M before collecting it again.</para>
259
260 <para>The default seems to work well here. If you have
261 plenty of memory, it is usually better to use
262 <option>-H</option><replaceable>size</replaceable> than to
263 increase
264 <option>-F</option><replaceable>factor</replaceable>.</para>
265
266 <para>The <option>-F</option> setting will be automatically
267 reduced by the garbage collector when the maximum heap size
268 (the <option>-M</option><replaceable>size</replaceable>
269 setting) is approaching.</para>
270 </listitem>
271 </varlistentry>
272
273 <varlistentry>
274 <term>
275 <option>-G</option><replaceable>generations</replaceable>
276 <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
277 <indexterm><primary>generations, number of</primary></indexterm>
278 </term>
279 <listitem>
280 <para>&lsqb;Default: 2&rsqb; Set the number of generations
281 used by the garbage collector. The default of 2 seems to be
282 good, but the garbage collector can support any number of
283 generations. Anything larger than about 4 is probably not a
284 good idea unless your program runs for a
285 <emphasis>long</emphasis> time, because the oldest
286 generation will hardly ever get collected.</para>
287
288 <para>Specifying 1 generation with <option>+RTS -G1</option>
289 gives you a simple 2-space collector, as you would expect.
290 In a 2-space collector, the <option>-A</option> option (see
291 above) specifies the <emphasis>minimum</emphasis> allocation
292 area size, since the allocation area will grow with the
293 amount of live data in the heap. In a multi-generational
294 collector the allocation area is a fixed size (unless you
295 use the <option>-H</option> option, see below).</para>
296 </listitem>
297 </varlistentry>
298
299 <varlistentry>
300 <term>
301 <option>-g</option><replaceable>threads</replaceable>
302 <indexterm><primary><option>-g</option></primary><secondary>RTS option</secondary></indexterm>
303 </term>
304 <listitem>
305 <para>&lsqb;Default: 1&rsqb; &lsqb;new in GHC 6.10&rsqb; Set the number
306 of threads to use for garbage collection. This option is
307 only accepted when the program was linked with the
308 <option>-threaded</option> option; see <xref
309 linkend="options-linker" />.</para>
310
311 <para>The garbage collector is able to work in parallel when
312 given more than one OS thread. Experiments have shown
313 that this usually results in a performance improvement
314 given 3 cores or more; with 2 cores it may or may not be
315 beneficial, depending on the workload. Bigger heaps work
316 better with parallel GC, so set your <option>-H</option>
317 value high (3 or more times the maximum residency). Look
318 at the timing stats with <option>+RTS -s</option> to
319 see whether you're getting any benefit from parallel GC or
320 not. If you find parallel GC is
321 significantly <emphasis>slower</emphasis> (in elapsed
322 time) than sequential GC, please report it as a
323 bug.</para>
324
325 <para>This value is set automatically when the
326 <option>-N</option> option is used, so the only reason to
327 use <option>-g</option> would be if you wanted to use a
328 different number of threads for GC than for execution.
329 For example, if your program is strictly single-threaded
330 but you still want to benefit from parallel GC, then it
331 might make sense to use <option>-g</option> rather than
332 <option>-N</option>.</para>
333 </listitem>
334 </varlistentry>
335
336 <varlistentry>
337 <term>
338 <option>-H</option><replaceable>size</replaceable>
339 <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
340 <indexterm><primary>heap size, suggested</primary></indexterm>
341 </term>
342 <listitem>
343 <para>&lsqb;Default: 0&rsqb; This option provides a
344 &ldquo;suggested heap size&rdquo; for the garbage collector. The
345 garbage collector will use about this much memory until the
346 program residency grows and the heap size needs to be
347 expanded to retain reasonable performance.</para>
348
349 <para>By default, the heap will start small, and grow and
350 shrink as necessary. This can be bad for performance, so if
351 you have plenty of memory it's worthwhile supplying a big
352 <option>-H</option><replaceable>size</replaceable>. For
353 improving GC performance, using
354 <option>-H</option><replaceable>size</replaceable> is
355 usually a better bet than
356 <option>-A</option><replaceable>size</replaceable>.</para>
357 </listitem>
358 </varlistentry>
359
360 <varlistentry>
361 <term>
362 <option>-I</option><replaceable>seconds</replaceable>
363 <indexterm><primary><option>-I</option></primary>
364 <secondary>RTS option</secondary>
365 </indexterm>
366 <indexterm><primary>idle GC</primary>
367 </indexterm>
368 </term>
369 <listitem>
370 <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
371 <option>-threaded</option>, <xref linkend="options-linker" />), a
372 major GC is automatically performed if the runtime has been idle
373 (no Haskell computation has been running) for a period of time.
374 The amount of idle time which must pass before a GC is performed is
375 set by the <option>-I</option><replaceable>seconds</replaceable>
376 option. Specifying <option>-I0</option> disables the idle GC.</para>
377
378 <para>For an interactive application, it is probably a good idea to
379 use the idle GC, because this will allow finalizers to run and
380 deadlocked threads to be detected in the idle time when no Haskell
381 computation is happening. Also, it will mean that a GC is less
382 likely to happen when the application is busy, and so
383 responsiveness may be improved. However, if the amount of live data in
384 the heap is particularly large, then the idle GC can cause a
385 significant delay, and too small an interval could adversely affect
386 interactive responsiveness.</para>
387
388 <para>This is an experimental feature, please let us know if it
389 causes problems and/or could benefit from further tuning.</para>
390 </listitem>
391 </varlistentry>
392
393 <varlistentry>
394 <term>
395 <option>-k</option><replaceable>size</replaceable>
396 <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
397 <indexterm><primary>stack, minimum size</primary></indexterm>
398 </term>
399 <listitem>
400 <para>&lsqb;Default: 1k&rsqb; Set the initial stack size for
401 new threads. Thread stacks (including the main thread's
402 stack) live on the heap, and grow as required. The default
403 value is good for concurrent applications with lots of small
404 threads; if your program doesn't fit this model then
405 increasing this option may help performance.</para>
406
407 <para>The main thread is normally started with a slightly
408 larger heap to cut down on unnecessary stack growth while
409 the program is starting up.</para>
410 </listitem>
411 </varlistentry>
412
413 <varlistentry>
414 <term>
415 <option>-K</option><replaceable>size</replaceable>
416 <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
417 <indexterm><primary>stack, maximum size</primary></indexterm>
418 </term>
419 <listitem>
420 <para>&lsqb;Default: 8M&rsqb; Set the maximum stack size for
421 an individual thread to <replaceable>size</replaceable>
422 bytes. This option is there purely to stop the program
423 eating up all the available memory in the machine if it gets
424 into an infinite loop.</para>
425 </listitem>
426 </varlistentry>
427
428 <varlistentry>
429 <term>
430 <option>-m</option><replaceable>n</replaceable>
431 <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
432 <indexterm><primary>heap, minimum free</primary></indexterm>
433 </term>
434 <listitem>
435 <para>Minimum &percnt; <replaceable>n</replaceable> of heap
436 which must be available for allocation. The default is
437 3&percnt;.</para>
438 </listitem>
439 </varlistentry>
440
441 <varlistentry>
442 <term>
443 <option>-M</option><replaceable>size</replaceable>
444 <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
445 <indexterm><primary>heap size, maximum</primary></indexterm>
446 </term>
447 <listitem>
448 <para>&lsqb;Default: unlimited&rsqb; Set the maximum heap size to
449 <replaceable>size</replaceable> bytes. The heap normally
450 grows and shrinks according to the memory requirements of
451 the program. The only reason for having this option is to
452 stop the heap growing without bound and filling up all the
453 available swap space, which at the least will result in the
454 program being summarily killed by the operating
455 system.</para>
456
457 <para>The maximum heap size also affects other garbage
458 collection parameters: when the amount of live data in the
459 heap exceeds a certain fraction of the maximum heap size,
460 compacting collection will be automatically enabled for the
461 oldest generation, and the <option>-F</option> parameter
462 will be reduced in order to avoid exceeding the maximum heap
463 size.</para>
464 </listitem>
465 </varlistentry>
466
467 <varlistentry>
468 <term>
469 <option>-t</option><optional><replaceable>file</replaceable></optional>
470 <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
471 </term>
472 <term>
473 <option>-s</option><optional><replaceable>file</replaceable></optional>
474 <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
475 </term>
476 <term>
477 <option>-S</option><optional><replaceable>file</replaceable></optional>
478 <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
479 </term>
480 <listitem>
481 <para>These options produce runtime-system statistics, such
482 as the amount of time spent executing the program and in the
483 garbage collector, the amount of memory allocated, the
484 maximum size of the heap, and so on. The three
485 variants give different levels of detail:
486 <option>-t</option> produces a single line of output in the
487 same format as GHC's <option>-Rghc-timing</option> option,
488 <option>-s</option> produces a more detailed summary at the
489 end of the program, and <option>-S</option> additionally
490 produces information about each and every garbage
491 collection.</para>
492
493 <para>The output is placed in
494 <replaceable>file</replaceable>. If
495 <replaceable>file</replaceable> is omitted, then the output
496 is sent to <constant>stderr</constant>.</para>
497
498 <para>
499 If you use the <literal>-t</literal> flag then, when your
500 program finishes, you will see something like this:
501 </para>
502
503 <programlisting>
504 &lt;&lt;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&gt;&gt;
505 </programlisting>
506
507 <para>
508 This tells you:
509 </para>
510
511 <itemizedlist>
512 <listitem>
513 <para>
514 The total bytes allocated by the program. This may be less
515 than the peak memory use, as some may be freed.
516 </para>
517 </listitem>
518 <listitem>
519 <para>
520 The total number of garbage collections that occurred.
521 </para>
522 </listitem>
523 <listitem>
524 <para>
525 The average and maximum space used by your program.
526 This is only checked during major garbage collections, so it
527 is only an approximation; the number of samples tells you how
528 many times it is checked.
529 </para>
530 </listitem>
531 <listitem>
532 <para>
533 The peak memory the RTS has allocated from the OS.
534 </para>
535 </listitem>
536 <listitem>
537 <para>
538 The amount of CPU time and elapsed wall clock time while
539 initialising the runtime system (INIT), running the program
540 itself (MUT, the mutator), and garbage collecting (GC).
541 </para>
542 </listitem>
543 </itemizedlist>
544
545 <para>
546 If you use the <literal>-s</literal> flag then, when your
547 program finishes, you will see something like this (the exact
548 details will vary depending on what sort of RTS you have, e.g.
549 you will only see profiling data if your RTS is compiled for
550 profiling):
551 </para>
552
553 <programlisting>
554 36,169,392 bytes allocated in the heap
555 4,057,632 bytes copied during GC
556 1,065,272 bytes maximum residency (2 sample(s))
557 54,312 bytes maximum slop
558 3 MB total memory in use (0 MB lost due to fragmentation)
559
560 Generation 0: 67 collections, 0 parallel, 0.04s, 0.03s elapsed
561 Generation 1: 2 collections, 0 parallel, 0.03s, 0.04s elapsed
562
563 SPARKS: 359207 (557 converted, 149591 pruned)
564
565 INIT time 0.00s ( 0.00s elapsed)
566 MUT time 0.01s ( 0.02s elapsed)
567 GC time 0.07s ( 0.07s elapsed)
568 EXIT time 0.00s ( 0.00s elapsed)
569 Total time 0.08s ( 0.09s elapsed)
570
571 %GC time 89.5% (75.3% elapsed)
572
573 Alloc rate 4,520,608,923 bytes per MUT second
574
575 Productivity 10.5% of total user, 9.1% of total elapsed
576 </programlisting>
577
578 <itemizedlist>
579 <listitem>
580 <para>
581 The "bytes allocated in the heap" is the total bytes allocated
582 by the program. This may be less than the peak memory use, as
583 some may be freed.
584 </para>
585 </listitem>
586 <listitem>
587 <para>
588 GHC uses a copying garbage collector. "bytes copied during GC"
589 tells you how many bytes it had to copy during garbage collection.
590 </para>
591 </listitem>
592 <listitem>
593 <para>
594 The maximum space actually used by your program is the
595 "bytes maximum residency" figure. This is only checked during
596 major garbage collections, so it is only an approximation;
597 the number of samples tells you how many times it is checked.
598 </para>
599 </listitem>
600 <listitem>
601 <para>
602 The "bytes maximum slop" tells you the most space that is ever
603 wasted due to the way GHC packs data into so-called "megablocks".
604 </para>
605 </listitem>
606 <listitem>
607 <para>
608 The "total memory in use" tells you the peak memory the RTS has
609 allocated from the OS.
610 </para>
611 </listitem>
612 <listitem>
613 <para>
614 Next there is information about the garbage collections done.
615 For each generation it says how many garbage collections were
616 done, how many of those collections used multiple threads,
617 the total CPU time used for garbage collecting that generation,
618 and the total wall clock time elapsed while garbage collecting
619 that generation.
620 </para>
621 </listitem>
622 <listitem>
623 <para>The <literal>SPARKS</literal> statistic refers to the
624 use of <literal>Control.Parallel.par</literal> and related
625 functionality in the program. Each spark represents a call
626 to <literal>par</literal>; a spark is "converted" when it is
627 executed in parallel; and a spark is "pruned" when it is
628 found to be already evaluated and is discarded from the pool
629 by the garbage collector. Any remaining sparks are
630 discarded at the end of execution, so "converted" plus
631 "pruned" does not necessarily add up to the total.</para>
632 </listitem>
633 <listitem>
634 <para>
635 Next there is the CPU time and wall clock time elapsedm broken
636 down by what the runtiem system was doing at the time.
637 INIT is the runtime system initialisation.
638 MUT is the mutator time, i.e. the time spent actually running
639 your code.
640 GC is the time spent doing garbage collection.
641 RP is the time spent doing retainer profiling.
642 PROF is the time spent doing other profiling.
643 EXIT is the runtime system shutdown time.
644 And finally, Total is, of course, the total.
645 </para>
646 <para>
647 %GC time tells you what percentage GC is of Total.
648 "Alloc rate" tells you the "bytes allocated in the heap" divided
649 by the MUT CPU time.
650 "Productivity" tells you what percentage of the Total CPU and wall
651 clock elapsed times are spent in the mutator (MUT).
652 </para>
653 </listitem>
654 </itemizedlist>
655
656 <para>
657 The <literal>-S</literal> flag, as well as giving the same
658 output as the <literal>-s</literal> flag, prints information
659 about each GC as it happens:
660 </para>
661
662 <programlisting>
663 Alloc Copied Live GC GC TOT TOT Page Flts
664 bytes bytes bytes user elap user elap
665 528496 47728 141512 0.01 0.02 0.02 0.02 0 0 (Gen: 1)
666 [...]
667 524944 175944 1726384 0.00 0.00 0.08 0.11 0 0 (Gen: 0)
668 </programlisting>
669
670 <para>
671 For each garbage collection, we print:
672 </para>
673
674 <itemizedlist>
675 <listitem>
676 <para>
677 How many bytes we allocated this garbage collection.
678 </para>
679 </listitem>
680 <listitem>
681 <para>
682 How many bytes we copied this garbage collection.
683 </para>
684 </listitem>
685 <listitem>
686 <para>
687 How many bytes are currently live.
688 </para>
689 </listitem>
690 <listitem>
691 <para>
692 How long this garbage collection took (CPU time and elapsed
693 wall clock time).
694 </para>
695 </listitem>
696 <listitem>
697 <para>
698 How long the program has been running (CPU time and elapsed
699 wall clock time).
700 </para>
701 </listitem>
702 <listitem>
703 <para>
704 How many page faults occured this garbage collection.
705 </para>
706 </listitem>
707 <listitem>
708 <para>
709 How many page faults occured since the end of the last garbage
710 collection.
711 </para>
712 </listitem>
713 <listitem>
714 <para>
715 Which generation is being garbage collected.
716 </para>
717 </listitem>
718 </itemizedlist>
719
720 </listitem>
721 </varlistentry>
722 </variablelist>
723
724 </sect2>
725
726 <sect2>
727 <title>RTS options for concurrency and parallelism</title>
728
729 <para>The RTS options related to concurrency are described in
730 <xref linkend="using-concurrent" />, and those for parallelism in
731 <xref linkend="parallel-options"/>.</para>
732 </sect2>
733
734 <sect2 id="rts-profiling">
735 <title>RTS options for profiling</title>
736
737 <para>Most profiling runtime options are only available when you
738 compile your program for profiling (see
739 <xref linkend="prof-compiler-options" />, and
740 <xref linkend="rts-options-heap-prof" /> for the runtime options).
741 However, there is one profiling option that is available
742 for ordinary non-profiled executables:</para>
743
744 <variablelist>
745 <varlistentry>
746 <term>
747 <option>-hT</option>
748 <indexterm><primary><option>-hT</option></primary><secondary>RTS
749 option</secondary></indexterm>
750 </term>
751 <listitem>
752 <para>Generates a basic heap profile, in the
753 file <literal><replaceable>prog</replaceable>.hp</literal>.
754 To produce the heap profile graph,
755 use <command>hp2ps</command> (see <xref linkend="hp2ps"
756 />). The basic heap profile is broken down by data
757 constructor, with other types of closures (functions, thunks,
758 etc.) grouped into broad categories
759 (e.g. <literal>FUN</literal>, <literal>THUNK</literal>). To
760 get a more detailed profile, use the full profiling
761 support (<xref linkend="profiling" />).</para>
762 </listitem>
763 </varlistentry>
764 </variablelist>
765 </sect2>
766
767 <sect2 id="rts-options-debugging">
768 <title>RTS options for hackers, debuggers, and over-interested
769 souls</title>
770
771 <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>
772
773 <para>These RTS options might be used (a)&nbsp;to avoid a GHC bug,
774 (b)&nbsp;to see &ldquo;what's really happening&rdquo;, or
775 (c)&nbsp;because you feel like it. Not recommended for everyday
776 use!</para>
777
778 <variablelist>
779
780 <varlistentry>
781 <term>
782 <option>-B</option>
783 <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
784 </term>
785 <listitem>
786 <para>Sound the bell at the start of each (major) garbage
787 collection.</para>
788
789 <para>Oddly enough, people really do use this option! Our
790 pal in Durham (England), Paul Callaghan, writes: &ldquo;Some
791 people here use it for a variety of
792 purposes&mdash;honestly!&mdash;e.g., confirmation that the
793 code/machine is doing something, infinite loop detection,
794 gauging cost of recently added code. Certain people can even
795 tell what stage &lsqb;the program&rsqb; is in by the beep
796 pattern. But the major use is for annoying others in the
797 same office&hellip;&rdquo;</para>
798 </listitem>
799 </varlistentry>
800
801 <varlistentry>
802 <term>
803 <option>-D</option><replaceable>num</replaceable>
804 <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
805 </term>
806 <listitem>
807 <para>An RTS debugging flag; varying quantities of output
808 depending on which bits are set in
809 <replaceable>num</replaceable>. Only works if the RTS was
810 compiled with the <option>DEBUG</option> option.</para>
811 </listitem>
812 </varlistentry>
813
814 <varlistentry>
815 <term>
816 <option>-r</option><replaceable>file</replaceable>
817 <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
818 <indexterm><primary>ticky ticky profiling</primary></indexterm>
819 <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
820 </term>
821 <listitem>
822 <para>Produce &ldquo;ticky-ticky&rdquo; statistics at the
823 end of the program run. The <replaceable>file</replaceable>
824 business works just like on the <option>-S</option> RTS
825 option (above).</para>
826
827 <para>&ldquo;Ticky-ticky&rdquo; statistics are counts of
828 various program actions (updates, enters, etc.) The program
829 must have been compiled using
830 <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
831 (a.k.a. &ldquo;ticky-ticky profiling&rdquo;), and, for it to
832 be really useful, linked with suitable system libraries.
833 Not a trivial undertaking: consult the installation guide on
834 how to set things up for easy &ldquo;ticky-ticky&rdquo;
835 profiling. For more information, see <xref
836 linkend="ticky-ticky"/>.</para>
837 </listitem>
838 </varlistentry>
839
840 <varlistentry>
841 <term>
842 <option>-xc</option>
843 <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
844 </term>
845 <listitem>
846 <para>(Only available when the program is compiled for
847 profiling.) When an exception is raised in the program,
848 this option causes the current cost-centre-stack to be
849 dumped to <literal>stderr</literal>.</para>
850
851 <para>This can be particularly useful for debugging: if your
852 program is complaining about a <literal>head []</literal>
853 error and you haven't got a clue which bit of code is
854 causing it, compiling with <literal>-prof
855 -auto-all</literal> and running with <literal>+RTS -xc
856 -RTS</literal> will tell you exactly the call stack at the
857 point the error was raised.</para>
858
859 <para>The output contains one line for each exception raised
860 in the program (the program might raise and catch several
861 exceptions during its execution), where each line is of the
862 form:</para>
863
864 <screen>
865 &lt; cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> &gt;
866 </screen>
867 <para>each <literal>cc</literal><subscript>i</subscript> is
868 a cost centre in the program (see <xref
869 linkend="cost-centres"/>), and the sequence represents the
870 &ldquo;call stack&rdquo; at the point the exception was
871 raised. The leftmost item is the innermost function in the
872 call stack, and the rightmost item is the outermost
873 function.</para>
874
875 </listitem>
876 </varlistentry>
877
878 <varlistentry>
879 <term>
880 <option>-Z</option>
881 <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
882 </term>
883 <listitem>
884 <para>Turn <emphasis>off</emphasis> &ldquo;update-frame
885 squeezing&rdquo; at garbage-collection time. (There's no
886 particularly good reason to turn it off, except to ensure
887 the accuracy of certain data collected regarding thunk entry
888 counts.)</para>
889 </listitem>
890 </varlistentry>
891 </variablelist>
892
893 </sect2>
894
895 <sect2 id="rts-hooks">
896 <title>&ldquo;Hooks&rdquo; to change RTS behaviour</title>
897
898 <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
899 <indexterm><primary>RTS hooks</primary></indexterm>
900 <indexterm><primary>RTS behaviour, changing</primary></indexterm>
901
902 <para>GHC lets you exercise rudimentary control over the RTS
903 settings for any given program, by compiling in a
904 &ldquo;hook&rdquo; that is called by the run-time system. The RTS
905 contains stub definitions for all these hooks, but by writing your
906 own version and linking it on the GHC command line, you can
907 override the defaults.</para>
908
909 <para>Owing to the vagaries of DLL linking, these hooks don't work
910 under Windows when the program is built dynamically.</para>
911
912 <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
913 </indexterm>lets you set RTS
914 options permanently for a given program. A common use for this is
915 to give your program a default heap and/or stack size that is
916 greater than the default. For example, to set <literal>-H128m
917 -K1m</literal>, place the following definition in a C source
918 file:</para>
919
920 <programlisting>
921 char *ghc_rts_opts = "-H128m -K1m";
922 </programlisting>
923
924 <para>Compile the C file, and include the object file on the
925 command line when you link your Haskell program.</para>
926
927 <para>These flags are interpreted first, before any RTS flags from
928 the <literal>GHCRTS</literal> environment variable and any flags
929 on the command line.</para>
930
931 <para>You can also change the messages printed when the runtime
932 system &ldquo;blows up,&rdquo; e.g., on stack overflow. The hooks
933 for these are as follows:</para>
934
935 <variablelist>
936
937 <varlistentry>
938 <term>
939 <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
940 <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
941 </term>
942 <listitem>
943 <para>The heap-overflow message.</para>
944 </listitem>
945 </varlistentry>
946
947 <varlistentry>
948 <term>
949 <function>void StackOverflowHook (long int)</function>
950 <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
951 </term>
952 <listitem>
953 <para>The stack-overflow message.</para>
954 </listitem>
955 </varlistentry>
956
957 <varlistentry>
958 <term>
959 <function>void MallocFailHook (long int)</function>
960 <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
961 </term>
962 <listitem>
963 <para>The message printed if <function>malloc</function>
964 fails.</para>
965 </listitem>
966 </varlistentry>
967 </variablelist>
968
969 <para>For examples of the use of these hooks, see GHC's own
970 versions in the file
971 <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
972 source tree.</para>
973 </sect2>
974
975 <sect2>
976 <title>Getting information about the RTS</title>
977
978 <indexterm><primary>RTS</primary></indexterm>
979
980 <para>It is possible to ask the RTS to give some information about
981 itself. To do this, use the <option>--info</option> flag, e.g.</para>
982 <screen>
983 $ ./a.out +RTS --info
984 [("GHC RTS", "Yes")
985 ,("GHC version", "6.7")
986 ,("RTS way", "rts_p")
987 ,("Host platform", "x86_64-unknown-linux")
988 ,("Build platform", "x86_64-unknown-linux")
989 ,("Target platform", "x86_64-unknown-linux")
990 ,("Compiler unregisterised", "NO")
991 ,("Tables next to code", "YES")
992 ]
993 </screen>
994 <para>The information is formatted such that it can be read as a
995 of type <literal>[(String, String)]</literal>.</para>
996 </sect2>
997 </sect1>
998
999 <!-- Emacs stuff:
1000 ;;; Local Variables: ***
1001 ;;; mode: xml ***
1002 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***
1003 ;;; End: ***
1004 -->