Document the --machine-readable RTS flag
[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 <term>
481 <option>--machine-readable</option>
482 <indexterm><primary><option>--machine-readable</option></primary><secondary>RTS option</secondary></indexterm>
483 </term>
484 <listitem>
485 <para>These options produce runtime-system statistics, such
486 as the amount of time spent executing the program and in the
487 garbage collector, the amount of memory allocated, the
488 maximum size of the heap, and so on. The three
489 variants give different levels of detail:
490 <option>-t</option> produces a single line of output in the
491 same format as GHC's <option>-Rghc-timing</option> option,
492 <option>-s</option> produces a more detailed summary at the
493 end of the program, and <option>-S</option> additionally
494 produces information about each and every garbage
495 collection.</para>
496
497 <para>The output is placed in
498 <replaceable>file</replaceable>. If
499 <replaceable>file</replaceable> is omitted, then the output
500 is sent to <constant>stderr</constant>.</para>
501
502 <para>
503 If you use the <literal>-t</literal> flag then, when your
504 program finishes, you will see something like this:
505 </para>
506
507 <programlisting>
508 &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;
509 </programlisting>
510
511 <para>
512 This tells you:
513 </para>
514
515 <itemizedlist>
516 <listitem>
517 <para>
518 The total bytes allocated by the program. This may be less
519 than the peak memory use, as some may be freed.
520 </para>
521 </listitem>
522 <listitem>
523 <para>
524 The total number of garbage collections that occurred.
525 </para>
526 </listitem>
527 <listitem>
528 <para>
529 The average and maximum space used by your program.
530 This is only checked during major garbage collections, so it
531 is only an approximation; the number of samples tells you how
532 many times it is checked.
533 </para>
534 </listitem>
535 <listitem>
536 <para>
537 The peak memory the RTS has allocated from the OS.
538 </para>
539 </listitem>
540 <listitem>
541 <para>
542 The amount of CPU time and elapsed wall clock time while
543 initialising the runtime system (INIT), running the program
544 itself (MUT, the mutator), and garbage collecting (GC).
545 </para>
546 </listitem>
547 </itemizedlist>
548
549 <para>
550 You can also get this in a more future-proof, machine readable
551 format, with <literal>-t --machine-readable</literal>:
552 </para>
553
554 <programlisting>
555 [("bytes allocated", "36169392")
556 ,("num_GCs", "69")
557 ,("average_bytes_used", "603392")
558 ,("max_bytes_used", "1065272")
559 ,("num_byte_usage_samples", "2")
560 ,("peak_megabytes_allocated", "3")
561 ,("init_cpu_seconds", "0.00")
562 ,("init_wall_seconds", "0.00")
563 ,("mutator_cpu_seconds", "0.02")
564 ,("mutator_wall_seconds", "0.02")
565 ,("GC_cpu_seconds", "0.07")
566 ,("GC_wall_seconds", "0.07")
567 ]
568 </programlisting>
569
570 <para>
571 If you use the <literal>-s</literal> flag then, when your
572 program finishes, you will see something like this (the exact
573 details will vary depending on what sort of RTS you have, e.g.
574 you will only see profiling data if your RTS is compiled for
575 profiling):
576 </para>
577
578 <programlisting>
579 36,169,392 bytes allocated in the heap
580 4,057,632 bytes copied during GC
581 1,065,272 bytes maximum residency (2 sample(s))
582 54,312 bytes maximum slop
583 3 MB total memory in use (0 MB lost due to fragmentation)
584
585 Generation 0: 67 collections, 0 parallel, 0.04s, 0.03s elapsed
586 Generation 1: 2 collections, 0 parallel, 0.03s, 0.04s elapsed
587
588 SPARKS: 359207 (557 converted, 149591 pruned)
589
590 INIT time 0.00s ( 0.00s elapsed)
591 MUT time 0.01s ( 0.02s elapsed)
592 GC time 0.07s ( 0.07s elapsed)
593 EXIT time 0.00s ( 0.00s elapsed)
594 Total time 0.08s ( 0.09s elapsed)
595
596 %GC time 89.5% (75.3% elapsed)
597
598 Alloc rate 4,520,608,923 bytes per MUT second
599
600 Productivity 10.5% of total user, 9.1% of total elapsed
601 </programlisting>
602
603 <itemizedlist>
604 <listitem>
605 <para>
606 The "bytes allocated in the heap" is the total bytes allocated
607 by the program. This may be less than the peak memory use, as
608 some may be freed.
609 </para>
610 </listitem>
611 <listitem>
612 <para>
613 GHC uses a copying garbage collector. "bytes copied during GC"
614 tells you how many bytes it had to copy during garbage collection.
615 </para>
616 </listitem>
617 <listitem>
618 <para>
619 The maximum space actually used by your program is the
620 "bytes maximum residency" figure. This is only checked during
621 major garbage collections, so it is only an approximation;
622 the number of samples tells you how many times it is checked.
623 </para>
624 </listitem>
625 <listitem>
626 <para>
627 The "bytes maximum slop" tells you the most space that is ever
628 wasted due to the way GHC packs data into so-called "megablocks".
629 </para>
630 </listitem>
631 <listitem>
632 <para>
633 The "total memory in use" tells you the peak memory the RTS has
634 allocated from the OS.
635 </para>
636 </listitem>
637 <listitem>
638 <para>
639 Next there is information about the garbage collections done.
640 For each generation it says how many garbage collections were
641 done, how many of those collections used multiple threads,
642 the total CPU time used for garbage collecting that generation,
643 and the total wall clock time elapsed while garbage collecting
644 that generation.
645 </para>
646 </listitem>
647 <listitem>
648 <para>The <literal>SPARKS</literal> statistic refers to the
649 use of <literal>Control.Parallel.par</literal> and related
650 functionality in the program. Each spark represents a call
651 to <literal>par</literal>; a spark is "converted" when it is
652 executed in parallel; and a spark is "pruned" when it is
653 found to be already evaluated and is discarded from the pool
654 by the garbage collector. Any remaining sparks are
655 discarded at the end of execution, so "converted" plus
656 "pruned" does not necessarily add up to the total.</para>
657 </listitem>
658 <listitem>
659 <para>
660 Next there is the CPU time and wall clock time elapsedm broken
661 down by what the runtiem system was doing at the time.
662 INIT is the runtime system initialisation.
663 MUT is the mutator time, i.e. the time spent actually running
664 your code.
665 GC is the time spent doing garbage collection.
666 RP is the time spent doing retainer profiling.
667 PROF is the time spent doing other profiling.
668 EXIT is the runtime system shutdown time.
669 And finally, Total is, of course, the total.
670 </para>
671 <para>
672 %GC time tells you what percentage GC is of Total.
673 "Alloc rate" tells you the "bytes allocated in the heap" divided
674 by the MUT CPU time.
675 "Productivity" tells you what percentage of the Total CPU and wall
676 clock elapsed times are spent in the mutator (MUT).
677 </para>
678 </listitem>
679 </itemizedlist>
680
681 <para>
682 The <literal>-S</literal> flag, as well as giving the same
683 output as the <literal>-s</literal> flag, prints information
684 about each GC as it happens:
685 </para>
686
687 <programlisting>
688 Alloc Copied Live GC GC TOT TOT Page Flts
689 bytes bytes bytes user elap user elap
690 528496 47728 141512 0.01 0.02 0.02 0.02 0 0 (Gen: 1)
691 [...]
692 524944 175944 1726384 0.00 0.00 0.08 0.11 0 0 (Gen: 0)
693 </programlisting>
694
695 <para>
696 For each garbage collection, we print:
697 </para>
698
699 <itemizedlist>
700 <listitem>
701 <para>
702 How many bytes we allocated this garbage collection.
703 </para>
704 </listitem>
705 <listitem>
706 <para>
707 How many bytes we copied this garbage collection.
708 </para>
709 </listitem>
710 <listitem>
711 <para>
712 How many bytes are currently live.
713 </para>
714 </listitem>
715 <listitem>
716 <para>
717 How long this garbage collection took (CPU time and elapsed
718 wall clock time).
719 </para>
720 </listitem>
721 <listitem>
722 <para>
723 How long the program has been running (CPU time and elapsed
724 wall clock time).
725 </para>
726 </listitem>
727 <listitem>
728 <para>
729 How many page faults occured this garbage collection.
730 </para>
731 </listitem>
732 <listitem>
733 <para>
734 How many page faults occured since the end of the last garbage
735 collection.
736 </para>
737 </listitem>
738 <listitem>
739 <para>
740 Which generation is being garbage collected.
741 </para>
742 </listitem>
743 </itemizedlist>
744
745 </listitem>
746 </varlistentry>
747 </variablelist>
748
749 </sect2>
750
751 <sect2>
752 <title>RTS options for concurrency and parallelism</title>
753
754 <para>The RTS options related to concurrency are described in
755 <xref linkend="using-concurrent" />, and those for parallelism in
756 <xref linkend="parallel-options"/>.</para>
757 </sect2>
758
759 <sect2 id="rts-profiling">
760 <title>RTS options for profiling</title>
761
762 <para>Most profiling runtime options are only available when you
763 compile your program for profiling (see
764 <xref linkend="prof-compiler-options" />, and
765 <xref linkend="rts-options-heap-prof" /> for the runtime options).
766 However, there is one profiling option that is available
767 for ordinary non-profiled executables:</para>
768
769 <variablelist>
770 <varlistentry>
771 <term>
772 <option>-hT</option>
773 <indexterm><primary><option>-hT</option></primary><secondary>RTS
774 option</secondary></indexterm>
775 </term>
776 <listitem>
777 <para>Generates a basic heap profile, in the
778 file <literal><replaceable>prog</replaceable>.hp</literal>.
779 To produce the heap profile graph,
780 use <command>hp2ps</command> (see <xref linkend="hp2ps"
781 />). The basic heap profile is broken down by data
782 constructor, with other types of closures (functions, thunks,
783 etc.) grouped into broad categories
784 (e.g. <literal>FUN</literal>, <literal>THUNK</literal>). To
785 get a more detailed profile, use the full profiling
786 support (<xref linkend="profiling" />).</para>
787 </listitem>
788 </varlistentry>
789 </variablelist>
790 </sect2>
791
792 <sect2 id="rts-options-debugging">
793 <title>RTS options for hackers, debuggers, and over-interested
794 souls</title>
795
796 <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>
797
798 <para>These RTS options might be used (a)&nbsp;to avoid a GHC bug,
799 (b)&nbsp;to see &ldquo;what's really happening&rdquo;, or
800 (c)&nbsp;because you feel like it. Not recommended for everyday
801 use!</para>
802
803 <variablelist>
804
805 <varlistentry>
806 <term>
807 <option>-B</option>
808 <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
809 </term>
810 <listitem>
811 <para>Sound the bell at the start of each (major) garbage
812 collection.</para>
813
814 <para>Oddly enough, people really do use this option! Our
815 pal in Durham (England), Paul Callaghan, writes: &ldquo;Some
816 people here use it for a variety of
817 purposes&mdash;honestly!&mdash;e.g., confirmation that the
818 code/machine is doing something, infinite loop detection,
819 gauging cost of recently added code. Certain people can even
820 tell what stage &lsqb;the program&rsqb; is in by the beep
821 pattern. But the major use is for annoying others in the
822 same office&hellip;&rdquo;</para>
823 </listitem>
824 </varlistentry>
825
826 <varlistentry>
827 <term>
828 <option>-D</option><replaceable>num</replaceable>
829 <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
830 </term>
831 <listitem>
832 <para>An RTS debugging flag; varying quantities of output
833 depending on which bits are set in
834 <replaceable>num</replaceable>. Only works if the RTS was
835 compiled with the <option>DEBUG</option> option.</para>
836 </listitem>
837 </varlistentry>
838
839 <varlistentry>
840 <term>
841 <option>-r</option><replaceable>file</replaceable>
842 <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
843 <indexterm><primary>ticky ticky profiling</primary></indexterm>
844 <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
845 </term>
846 <listitem>
847 <para>Produce &ldquo;ticky-ticky&rdquo; statistics at the
848 end of the program run. The <replaceable>file</replaceable>
849 business works just like on the <option>-S</option> RTS
850 option (above).</para>
851
852 <para>&ldquo;Ticky-ticky&rdquo; statistics are counts of
853 various program actions (updates, enters, etc.) The program
854 must have been compiled using
855 <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
856 (a.k.a. &ldquo;ticky-ticky profiling&rdquo;), and, for it to
857 be really useful, linked with suitable system libraries.
858 Not a trivial undertaking: consult the installation guide on
859 how to set things up for easy &ldquo;ticky-ticky&rdquo;
860 profiling. For more information, see <xref
861 linkend="ticky-ticky"/>.</para>
862 </listitem>
863 </varlistentry>
864
865 <varlistentry>
866 <term>
867 <option>-xc</option>
868 <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
869 </term>
870 <listitem>
871 <para>(Only available when the program is compiled for
872 profiling.) When an exception is raised in the program,
873 this option causes the current cost-centre-stack to be
874 dumped to <literal>stderr</literal>.</para>
875
876 <para>This can be particularly useful for debugging: if your
877 program is complaining about a <literal>head []</literal>
878 error and you haven't got a clue which bit of code is
879 causing it, compiling with <literal>-prof
880 -auto-all</literal> and running with <literal>+RTS -xc
881 -RTS</literal> will tell you exactly the call stack at the
882 point the error was raised.</para>
883
884 <para>The output contains one line for each exception raised
885 in the program (the program might raise and catch several
886 exceptions during its execution), where each line is of the
887 form:</para>
888
889 <screen>
890 &lt; cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> &gt;
891 </screen>
892 <para>each <literal>cc</literal><subscript>i</subscript> is
893 a cost centre in the program (see <xref
894 linkend="cost-centres"/>), and the sequence represents the
895 &ldquo;call stack&rdquo; at the point the exception was
896 raised. The leftmost item is the innermost function in the
897 call stack, and the rightmost item is the outermost
898 function.</para>
899
900 </listitem>
901 </varlistentry>
902
903 <varlistentry>
904 <term>
905 <option>-Z</option>
906 <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
907 </term>
908 <listitem>
909 <para>Turn <emphasis>off</emphasis> &ldquo;update-frame
910 squeezing&rdquo; at garbage-collection time. (There's no
911 particularly good reason to turn it off, except to ensure
912 the accuracy of certain data collected regarding thunk entry
913 counts.)</para>
914 </listitem>
915 </varlistentry>
916 </variablelist>
917
918 </sect2>
919
920 <sect2 id="rts-hooks">
921 <title>&ldquo;Hooks&rdquo; to change RTS behaviour</title>
922
923 <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
924 <indexterm><primary>RTS hooks</primary></indexterm>
925 <indexterm><primary>RTS behaviour, changing</primary></indexterm>
926
927 <para>GHC lets you exercise rudimentary control over the RTS
928 settings for any given program, by compiling in a
929 &ldquo;hook&rdquo; that is called by the run-time system. The RTS
930 contains stub definitions for all these hooks, but by writing your
931 own version and linking it on the GHC command line, you can
932 override the defaults.</para>
933
934 <para>Owing to the vagaries of DLL linking, these hooks don't work
935 under Windows when the program is built dynamically.</para>
936
937 <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
938 </indexterm>lets you set RTS
939 options permanently for a given program. A common use for this is
940 to give your program a default heap and/or stack size that is
941 greater than the default. For example, to set <literal>-H128m
942 -K1m</literal>, place the following definition in a C source
943 file:</para>
944
945 <programlisting>
946 char *ghc_rts_opts = "-H128m -K1m";
947 </programlisting>
948
949 <para>Compile the C file, and include the object file on the
950 command line when you link your Haskell program.</para>
951
952 <para>These flags are interpreted first, before any RTS flags from
953 the <literal>GHCRTS</literal> environment variable and any flags
954 on the command line.</para>
955
956 <para>You can also change the messages printed when the runtime
957 system &ldquo;blows up,&rdquo; e.g., on stack overflow. The hooks
958 for these are as follows:</para>
959
960 <variablelist>
961
962 <varlistentry>
963 <term>
964 <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
965 <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
966 </term>
967 <listitem>
968 <para>The heap-overflow message.</para>
969 </listitem>
970 </varlistentry>
971
972 <varlistentry>
973 <term>
974 <function>void StackOverflowHook (long int)</function>
975 <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
976 </term>
977 <listitem>
978 <para>The stack-overflow message.</para>
979 </listitem>
980 </varlistentry>
981
982 <varlistentry>
983 <term>
984 <function>void MallocFailHook (long int)</function>
985 <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
986 </term>
987 <listitem>
988 <para>The message printed if <function>malloc</function>
989 fails.</para>
990 </listitem>
991 </varlistentry>
992 </variablelist>
993
994 <para>For examples of the use of these hooks, see GHC's own
995 versions in the file
996 <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
997 source tree.</para>
998 </sect2>
999
1000 <sect2>
1001 <title>Getting information about the RTS</title>
1002
1003 <indexterm><primary>RTS</primary></indexterm>
1004
1005 <para>It is possible to ask the RTS to give some information about
1006 itself. To do this, use the <option>--info</option> flag, e.g.</para>
1007 <screen>
1008 $ ./a.out +RTS --info
1009 [("GHC RTS", "Yes")
1010 ,("GHC version", "6.7")
1011 ,("RTS way", "rts_p")
1012 ,("Host platform", "x86_64-unknown-linux")
1013 ,("Build platform", "x86_64-unknown-linux")
1014 ,("Target platform", "x86_64-unknown-linux")
1015 ,("Compiler unregisterised", "NO")
1016 ,("Tables next to code", "YES")
1017 ]
1018 </screen>
1019 <para>The information is formatted such that it can be read as a
1020 of type <literal>[(String, String)]</literal>.</para>
1021 </sect2>
1022 </sect1>
1023
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