1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <sect1 id="runtime-control">
3 <title>Running a compiled program</title>
5 <indexterm><primary>runtime control of Haskell programs</primary></indexterm>
6 <indexterm><primary>running, compiled program</primary></indexterm>
7 <indexterm><primary>RTS options</primary></indexterm>
9 <para>To make an executable program, the GHC system compiles your
10 code and then links it with a non-trivial runtime system (RTS),
11 which handles storage management, profiling, etc.</para>
13 <para>You have some control over the behaviour of the RTS, by giving
14 special command-line arguments to your program.</para>
16 <para>When your Haskell program starts up, its RTS extracts
17 command-line arguments bracketed between
18 <option>+RTS</option><indexterm><primary><option>+RTS</option></primary></indexterm>
20 <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
21 as its own. For example:</para>
24 % ./a.out -f +RTS -p -S -RTS -h foo bar
27 <para>The RTS will snaffle <option>-p</option> <option>-S</option>
28 for itself, and the remaining arguments <literal>-f -h foo bar</literal>
29 will be handed to your program if/when it calls
30 <function>System.getArgs</function>.</para>
32 <para>No <option>-RTS</option> option is required if the
33 runtime-system options extend to the end of the command line, as in
37 % hls -ltr /usr/etc +RTS -A5m
40 <para>If you absolutely positively want all the rest of the options
41 in a command line to go to the program (and not the RTS), use a
42 <option>––RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.</para>
44 <para>As always, for RTS options that take
45 <replaceable>size</replaceable>s: If the last character of
46 <replaceable>size</replaceable> is a K or k, multiply by 1000; if an
47 M or m, by 1,000,000; if a G or G, by 1,000,000,000. (And any
48 wraparound in the counters is <emphasis>your</emphasis>
51 <para>Giving a <literal>+RTS -f</literal>
52 <indexterm><primary><option>-f</option></primary><secondary>RTS option</secondary></indexterm> option
53 will print out the RTS options actually available in your program
54 (which vary, depending on how you compiled).</para>
56 <para>NOTE: since GHC is itself compiled by GHC, you can change RTS
57 options in the compiler using the normal
58 <literal>+RTS ... -RTS</literal>
59 combination. eg. to increase the maximum heap
60 size for a compilation to 128M, you would add
61 <literal>+RTS -M128m -RTS</literal>
62 to the command line.</para>
64 <sect2 id="rts-optinos-environment">
65 <title>Setting global RTS options</title>
67 <indexterm><primary>RTS options</primary><secondary>from the environment</secondary></indexterm>
68 <indexterm><primary>environment variable</primary><secondary>for
69 setting RTS options</secondary></indexterm>
71 <para>RTS options are also taken from the environment variable
72 <envar>GHCRTS</envar><indexterm><primary><envar>GHCRTS</envar></primary>
73 </indexterm>. For example, to set the maximum heap size
74 to 128M for all GHC-compiled programs (using an
75 <literal>sh</literal>-like shell):</para>
82 <para>RTS options taken from the <envar>GHCRTS</envar> environment
83 variable can be overridden by options given on the command
88 <sect2 id="rts-options-misc">
89 <title>Miscellaneous RTS options</title>
93 <term><option>-V<replaceable>secs</replaceable></option>
94 <indexterm><primary><option>-V</option></primary><secondary>RTS
95 option</secondary></indexterm></term>
97 <para>Sets the interval that the RTS clock ticks at. The
98 runtime uses a single timer signal to count ticks; this timer
99 signal is used to control the context switch timer (<xref
100 linkend="using-concurrent" />) and the heap profiling
101 timer <xref linkend="rts-options-heap-prof" />. Also, the
102 time profiler uses the RTS timer signal directly to record
103 time profiling samples.</para>
105 <para>Normally, setting the <option>-V</option> option
106 directly is not necessary: the resolution of the RTS timer is
107 adjusted automatically if a short interval is requested with
108 the <option>-C</option> or <option>-i</option> options.
109 However, setting <option>-V</option> is required in order to
110 increase the resolution of the time profiler.</para>
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>
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>
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>
135 <term><option>-xm<replaceable>address</replaceable></option>
136 <indexterm><primary><option>-xm</option></primary><secondary>RTS
137 option</secondary></indexterm></term>
140 WARNING: this option is for working around memory
141 allocation problems only. Do not use unless GHCi fails
142 with a message like “<literal>failed to mmap() memory below 2Gb</literal>”. If you need to use this option to get GHCi working
143 on your machine, please file a bug.
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>.
165 <sect2 id="rts-options-gc">
166 <title>RTS options to control the garbage collector</title>
168 <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
169 <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>
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>
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>
185 <para>[Default: 512k] 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>
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
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>
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>
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>
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 >30%.</para>
224 <para>NOTE: compaction doesn't currently work when a single
225 generation is requested using the <option>-G1</option>
231 <term><option>-c</option><replaceable>n</replaceable></term>
234 <para>[Default: 30] Automatically enable
235 compacting collection when the live data exceeds
236 <replaceable>n</replaceable>% 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>
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>
252 <para>[Default: 2] 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>
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
264 <option>-F</option><replaceable>factor</replaceable>.</para>
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>
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>
280 <para>[Default: 2] 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>
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>
301 <option>-qg<optional><replaceable>gen</replaceable></optional></option>
302 <indexterm><primary><option>-qg</option><secondary>RTS
303 option</secondary></primary></indexterm>
306 <para>[New in GHC 6.12.1] [Default: 0]
308 generation <replaceable>gen</replaceable> and higher.
309 Omitting <replaceable>gen</replaceable> turns off the
310 parallel GC completely, reverting to sequential GC.</para>
312 <para>The default parallel GC settings are usually suitable
313 for parallel programs (i.e. those
314 using <literal>par</literal>, Strategies, or with multiple
315 threads). However, it is sometimes beneficial to enable
316 the parallel GC for a single-threaded sequential program
317 too, especially if the program has a large amount of heap
318 data and GC is a significant fraction of runtime. To use
319 the parallel GC in a sequential program, enable the
320 parallel runtime with a suitable <literal>-N</literal>
321 option, and additionally it might be beneficial to
322 restrict parallel GC to the old generation
323 with <literal>-qg1</literal>.</para>
329 <option>-qb<optional><replaceable>gen</replaceable></optional></option>
330 <indexterm><primary><option>-qb</option><secondary>RTS
331 option</secondary></primary></indexterm>
335 [New in GHC 6.12.1] [Default: 1] Use
336 load-balancing in the parallel GC in
337 generation <replaceable>gen</replaceable> and higher.
338 Omitting <replaceable>gen</replaceable> disables
339 load-balancing entirely.</para>
342 Load-balancing shares out the work of GC between the
343 available cores. This is a good idea when the heap is
344 large and we need to parallelise the GC work, however it
345 is also pessimal for the short young-generation
346 collections in a parallel program, because it can harm
347 locality by moving data from the cache of the CPU where is
348 it being used to the cache of another CPU. Hence the
349 default is to do load-balancing only in the
350 old-generation. In fact, for a parallel program it is
351 sometimes beneficial to disable load-balancing entirely
352 with <literal>-qb</literal>.
359 <option>-H</option><replaceable>size</replaceable>
360 <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
361 <indexterm><primary>heap size, suggested</primary></indexterm>
364 <para>[Default: 0] This option provides a
365 “suggested heap size” for the garbage collector. The
366 garbage collector will use about this much memory until the
367 program residency grows and the heap size needs to be
368 expanded to retain reasonable performance.</para>
370 <para>By default, the heap will start small, and grow and
371 shrink as necessary. This can be bad for performance, so if
372 you have plenty of memory it's worthwhile supplying a big
373 <option>-H</option><replaceable>size</replaceable>. For
374 improving GC performance, using
375 <option>-H</option><replaceable>size</replaceable> is
376 usually a better bet than
377 <option>-A</option><replaceable>size</replaceable>.</para>
383 <option>-I</option><replaceable>seconds</replaceable>
384 <indexterm><primary><option>-I</option></primary>
385 <secondary>RTS option</secondary>
387 <indexterm><primary>idle GC</primary>
391 <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
392 <option>-threaded</option>, <xref linkend="options-linker" />), a
393 major GC is automatically performed if the runtime has been idle
394 (no Haskell computation has been running) for a period of time.
395 The amount of idle time which must pass before a GC is performed is
396 set by the <option>-I</option><replaceable>seconds</replaceable>
397 option. Specifying <option>-I0</option> disables the idle GC.</para>
399 <para>For an interactive application, it is probably a good idea to
400 use the idle GC, because this will allow finalizers to run and
401 deadlocked threads to be detected in the idle time when no Haskell
402 computation is happening. Also, it will mean that a GC is less
403 likely to happen when the application is busy, and so
404 responsiveness may be improved. However, if the amount of live data in
405 the heap is particularly large, then the idle GC can cause a
406 significant delay, and too small an interval could adversely affect
407 interactive responsiveness.</para>
409 <para>This is an experimental feature, please let us know if it
410 causes problems and/or could benefit from further tuning.</para>
416 <option>-k</option><replaceable>size</replaceable>
417 <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
418 <indexterm><primary>stack, minimum size</primary></indexterm>
421 <para>[Default: 1k] Set the initial stack size for
422 new threads. Thread stacks (including the main thread's
423 stack) live on the heap, and grow as required. The default
424 value is good for concurrent applications with lots of small
425 threads; if your program doesn't fit this model then
426 increasing this option may help performance.</para>
428 <para>The main thread is normally started with a slightly
429 larger heap to cut down on unnecessary stack growth while
430 the program is starting up.</para>
436 <option>-K</option><replaceable>size</replaceable>
437 <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
438 <indexterm><primary>stack, maximum size</primary></indexterm>
441 <para>[Default: 8M] Set the maximum stack size for
442 an individual thread to <replaceable>size</replaceable>
443 bytes. This option is there purely to stop the program
444 eating up all the available memory in the machine if it gets
445 into an infinite loop.</para>
451 <option>-m</option><replaceable>n</replaceable>
452 <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
453 <indexterm><primary>heap, minimum free</primary></indexterm>
456 <para>Minimum % <replaceable>n</replaceable> of heap
457 which must be available for allocation. The default is
464 <option>-M</option><replaceable>size</replaceable>
465 <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
466 <indexterm><primary>heap size, maximum</primary></indexterm>
469 <para>[Default: unlimited] Set the maximum heap size to
470 <replaceable>size</replaceable> bytes. The heap normally
471 grows and shrinks according to the memory requirements of
472 the program. The only reason for having this option is to
473 stop the heap growing without bound and filling up all the
474 available swap space, which at the least will result in the
475 program being summarily killed by the operating
478 <para>The maximum heap size also affects other garbage
479 collection parameters: when the amount of live data in the
480 heap exceeds a certain fraction of the maximum heap size,
481 compacting collection will be automatically enabled for the
482 oldest generation, and the <option>-F</option> parameter
483 will be reduced in order to avoid exceeding the maximum heap
490 <option>-t</option><optional><replaceable>file</replaceable></optional>
491 <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
494 <option>-s</option><optional><replaceable>file</replaceable></optional>
495 <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
498 <option>-S</option><optional><replaceable>file</replaceable></optional>
499 <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
502 <option>--machine-readable</option>
503 <indexterm><primary><option>--machine-readable</option></primary><secondary>RTS option</secondary></indexterm>
506 <para>These options produce runtime-system statistics, such
507 as the amount of time spent executing the program and in the
508 garbage collector, the amount of memory allocated, the
509 maximum size of the heap, and so on. The three
510 variants give different levels of detail:
511 <option>-t</option> produces a single line of output in the
512 same format as GHC's <option>-Rghc-timing</option> option,
513 <option>-s</option> produces a more detailed summary at the
514 end of the program, and <option>-S</option> additionally
515 produces information about each and every garbage
518 <para>The output is placed in
519 <replaceable>file</replaceable>. If
520 <replaceable>file</replaceable> is omitted, then the output
521 is sent to <constant>stderr</constant>.</para>
524 If you use the <literal>-t</literal> flag then, when your
525 program finishes, you will see something like this:
529 <<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>>
539 The total number of bytes allocated by the program over the
545 The total number of garbage collections performed.
550 The average and maximum "residency", which is the amount of
551 live data in bytes. The runtime can only determine the
552 amount of live data during a major GC, which is why the
553 number of samples corresponds to the number of major GCs
554 (and is usually relatively small). To get a better picture
555 of the heap profile of your program, use
556 the <option>-hT</option> RTS option
557 (<xref linkend="rts-profiling" />).
562 The peak memory the RTS has allocated from the OS.
567 The amount of CPU time and elapsed wall clock time while
568 initialising the runtime system (INIT), running the program
569 itself (MUT, the mutator), and garbage collecting (GC).
575 You can also get this in a more future-proof, machine readable
576 format, with <literal>-t --machine-readable</literal>:
580 [("bytes allocated", "36169392")
582 ,("average_bytes_used", "603392")
583 ,("max_bytes_used", "1065272")
584 ,("num_byte_usage_samples", "2")
585 ,("peak_megabytes_allocated", "3")
586 ,("init_cpu_seconds", "0.00")
587 ,("init_wall_seconds", "0.00")
588 ,("mutator_cpu_seconds", "0.02")
589 ,("mutator_wall_seconds", "0.02")
590 ,("GC_cpu_seconds", "0.07")
591 ,("GC_wall_seconds", "0.07")
596 If you use the <literal>-s</literal> flag then, when your
597 program finishes, you will see something like this (the exact
598 details will vary depending on what sort of RTS you have, e.g.
599 you will only see profiling data if your RTS is compiled for
604 36,169,392 bytes allocated in the heap
605 4,057,632 bytes copied during GC
606 1,065,272 bytes maximum residency (2 sample(s))
607 54,312 bytes maximum slop
608 3 MB total memory in use (0 MB lost due to fragmentation)
610 Generation 0: 67 collections, 0 parallel, 0.04s, 0.03s elapsed
611 Generation 1: 2 collections, 0 parallel, 0.03s, 0.04s elapsed
613 SPARKS: 359207 (557 converted, 149591 pruned)
615 INIT time 0.00s ( 0.00s elapsed)
616 MUT time 0.01s ( 0.02s elapsed)
617 GC time 0.07s ( 0.07s elapsed)
618 EXIT time 0.00s ( 0.00s elapsed)
619 Total time 0.08s ( 0.09s elapsed)
621 %GC time 89.5% (75.3% elapsed)
623 Alloc rate 4,520,608,923 bytes per MUT second
625 Productivity 10.5% of total user, 9.1% of total elapsed
631 The "bytes allocated in the heap" is the total bytes allocated
632 by the program over the whole run.
637 GHC uses a copying garbage collector by default. "bytes copied
638 during GC" tells you how many bytes it had to copy during
644 The maximum space actually used by your program is the
645 "bytes maximum residency" figure. This is only checked during
646 major garbage collections, so it is only an approximation;
647 the number of samples tells you how many times it is checked.
652 The "bytes maximum slop" tells you the most space that is ever
653 wasted due to the way GHC allocates memory in blocks. Slop is
654 memory at the end of a block that was wasted. There's no way
655 to control this; we just like to see how much memory is being
661 The "total memory in use" tells you the peak memory the RTS has
662 allocated from the OS.
667 Next there is information about the garbage collections done.
668 For each generation it says how many garbage collections were
669 done, how many of those collections were done in parallel,
670 the total CPU time used for garbage collecting that generation,
671 and the total wall clock time elapsed while garbage collecting
676 <para>The <literal>SPARKS</literal> statistic refers to the
677 use of <literal>Control.Parallel.par</literal> and related
678 functionality in the program. Each spark represents a call
679 to <literal>par</literal>; a spark is "converted" when it is
680 executed in parallel; and a spark is "pruned" when it is
681 found to be already evaluated and is discarded from the pool
682 by the garbage collector. Any remaining sparks are
683 discarded at the end of execution, so "converted" plus
684 "pruned" does not necessarily add up to the total.</para>
688 Next there is the CPU time and wall clock time elapsed broken
689 down by what the runtime system was doing at the time.
690 INIT is the runtime system initialisation.
691 MUT is the mutator time, i.e. the time spent actually running
693 GC is the time spent doing garbage collection.
694 RP is the time spent doing retainer profiling.
695 PROF is the time spent doing other profiling.
696 EXIT is the runtime system shutdown time.
697 And finally, Total is, of course, the total.
700 %GC time tells you what percentage GC is of Total.
701 "Alloc rate" tells you the "bytes allocated in the heap" divided
703 "Productivity" tells you what percentage of the Total CPU and wall
704 clock elapsed times are spent in the mutator (MUT).
710 The <literal>-S</literal> flag, as well as giving the same
711 output as the <literal>-s</literal> flag, prints information
712 about each GC as it happens:
716 Alloc Copied Live GC GC TOT TOT Page Flts
717 bytes bytes bytes user elap user elap
718 528496 47728 141512 0.01 0.02 0.02 0.02 0 0 (Gen: 1)
720 524944 175944 1726384 0.00 0.00 0.08 0.11 0 0 (Gen: 0)
724 For each garbage collection, we print:
730 How many bytes we allocated this garbage collection.
735 How many bytes we copied this garbage collection.
740 How many bytes are currently live.
745 How long this garbage collection took (CPU time and elapsed
751 How long the program has been running (CPU time and elapsed
757 How many page faults occured this garbage collection.
762 How many page faults occured since the end of the last garbage
768 Which generation is being garbage collected.
780 <title>RTS options for concurrency and parallelism</title>
782 <para>The RTS options related to concurrency are described in
783 <xref linkend="using-concurrent" />, and those for parallelism in
784 <xref linkend="parallel-options"/>.</para>
787 <sect2 id="rts-profiling">
788 <title>RTS options for profiling</title>
790 <para>Most profiling runtime options are only available when you
791 compile your program for profiling (see
792 <xref linkend="prof-compiler-options" />, and
793 <xref linkend="rts-options-heap-prof" /> for the runtime options).
794 However, there is one profiling option that is available
795 for ordinary non-profiled executables:</para>
801 <indexterm><primary><option>-hT</option></primary><secondary>RTS
802 option</secondary></indexterm>
805 <para>Generates a basic heap profile, in the
806 file <literal><replaceable>prog</replaceable>.hp</literal>.
807 To produce the heap profile graph,
808 use <command>hp2ps</command> (see <xref linkend="hp2ps"
809 />). The basic heap profile is broken down by data
810 constructor, with other types of closures (functions, thunks,
811 etc.) grouped into broad categories
812 (e.g. <literal>FUN</literal>, <literal>THUNK</literal>). To
813 get a more detailed profile, use the full profiling
814 support (<xref linkend="profiling" />).</para>
820 <sect2 id="rts-eventlog">
821 <title>Tracing</title>
823 <indexterm><primary>tracing</primary></indexterm>
824 <indexterm><primary>events</primary></indexterm>
825 <indexterm><primary>eventlog files</primary></indexterm>
828 When the program is linked with the <option>-eventlog</option>
829 option (<xref linkend="options-linker" />), runtime events can
830 be logged in two ways:
836 In binary format to a file for later analysis by a
837 variety of tools. One such tool
838 is <ulink url="http://hackage.haskell.org/package/ThreadScope">ThreadScope</ulink><indexterm><primary>ThreadScope</primary></indexterm>,
839 which interprets the event log to produce a visual parallel
840 execution profile of the program.
845 As text to standard output, for debugging purposes.
853 <option>-l<optional><replaceable>type</replaceable></optional></option>
854 <indexterm><primary><option>-l</option></primary><secondary>RTS option</secondary></indexterm>
858 Log events in binary format to the
859 file <filename><replaceable>program</replaceable>.eventlog</filename>,
860 where <replaceable>type</replaceable> indicates the type
861 of events to log. Currently there is only one type
862 supported: <literal>-ls</literal>, for scheduler events.
866 The format of the log file is described by the header
867 <filename>EventLogFormat.h</filename> that comes with
868 GHC, and it can be parsed in Haskell using
869 the <ulink url="http://hackage.haskell.org/package/ghc-events">ghc-events</ulink>
870 library. To dump the contents of
871 a <literal>.eventlog</literal> file as text, use the
872 tool <literal>show-ghc-events</literal> that comes with
873 the <ulink url="http://hackage.haskell.org/package/ghc-events">ghc-events</ulink>
882 <indexterm><primary><option>-v</option></primary><secondary>RTS option</secondary></indexterm>
886 Log events as text to standard output, instead of to
887 the <literal>.eventlog</literal> file.
896 options <option>-D<replaceable>x</replaceable></option> also
897 generate events which are logged using the tracing framework.
898 By default those events are dumped as text to stdout
899 (<option>-D<replaceable>x</replaceable></option>
900 implies <option>-v</option>), but they may instead be stored in
901 the binary eventlog file by using the <option>-l</option>
906 <sect2 id="rts-options-debugging">
907 <title>RTS options for hackers, debuggers, and over-interested
910 <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>
912 <para>These RTS options might be used (a) to avoid a GHC bug,
913 (b) to see “what's really happening”, or
914 (c) because you feel like it. Not recommended for everyday
922 <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
925 <para>Sound the bell at the start of each (major) garbage
928 <para>Oddly enough, people really do use this option! Our
929 pal in Durham (England), Paul Callaghan, writes: “Some
930 people here use it for a variety of
931 purposes—honestly!—e.g., confirmation that the
932 code/machine is doing something, infinite loop detection,
933 gauging cost of recently added code. Certain people can even
934 tell what stage [the program] is in by the beep
935 pattern. But the major use is for annoying others in the
936 same office…”</para>
942 <option>-D</option><replaceable>x</replaceable>
943 <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
947 An RTS debugging flag; only availble if the program was
948 linked with the <option>-debug</option> option. Various
949 values of <replaceable>x</replaceable> are provided to
950 enable debug messages and additional runtime sanity checks
951 in different subsystems in the RTS, for
952 example <literal>+RTS -Ds -RTS</literal> enables debug
953 messages from the scheduler.
954 Use <literal>+RTS -?</literal> to find out which
955 debug flags are supported.
959 Debug messages will be sent to the binary event log file
960 instead of stdout if the <option>-l</option> option is
961 added. This might be useful for reducing the overhead of
969 <option>-r</option><replaceable>file</replaceable>
970 <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
971 <indexterm><primary>ticky ticky profiling</primary></indexterm>
972 <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
975 <para>Produce “ticky-ticky” statistics at the
976 end of the program run. The <replaceable>file</replaceable>
977 business works just like on the <option>-S</option> RTS
978 option (above).</para>
980 <para>“Ticky-ticky” statistics are counts of
981 various program actions (updates, enters, etc.) The program
982 must have been compiled using
983 <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
984 (a.k.a. “ticky-ticky profiling”), and, for it to
985 be really useful, linked with suitable system libraries.
986 Not a trivial undertaking: consult the installation guide on
987 how to set things up for easy “ticky-ticky”
988 profiling. For more information, see <xref
989 linkend="ticky-ticky"/>.</para>
996 <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
999 <para>(Only available when the program is compiled for
1000 profiling.) When an exception is raised in the program,
1001 this option causes the current cost-centre-stack to be
1002 dumped to <literal>stderr</literal>.</para>
1004 <para>This can be particularly useful for debugging: if your
1005 program is complaining about a <literal>head []</literal>
1006 error and you haven't got a clue which bit of code is
1007 causing it, compiling with <literal>-prof
1008 -auto-all</literal> and running with <literal>+RTS -xc
1009 -RTS</literal> will tell you exactly the call stack at the
1010 point the error was raised.</para>
1012 <para>The output contains one line for each exception raised
1013 in the program (the program might raise and catch several
1014 exceptions during its execution), where each line is of the
1018 < cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> >
1020 <para>each <literal>cc</literal><subscript>i</subscript> is
1021 a cost centre in the program (see <xref
1022 linkend="cost-centres"/>), and the sequence represents the
1023 “call stack” at the point the exception was
1024 raised. The leftmost item is the innermost function in the
1025 call stack, and the rightmost item is the outermost
1034 <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
1037 <para>Turn <emphasis>off</emphasis> “update-frame
1038 squeezing” at garbage-collection time. (There's no
1039 particularly good reason to turn it off, except to ensure
1040 the accuracy of certain data collected regarding thunk entry
1048 <sect2 id="rts-hooks">
1049 <title>“Hooks” to change RTS behaviour</title>
1051 <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
1052 <indexterm><primary>RTS hooks</primary></indexterm>
1053 <indexterm><primary>RTS behaviour, changing</primary></indexterm>
1055 <para>GHC lets you exercise rudimentary control over the RTS
1056 settings for any given program, by compiling in a
1057 “hook” that is called by the run-time system. The RTS
1058 contains stub definitions for all these hooks, but by writing your
1059 own version and linking it on the GHC command line, you can
1060 override the defaults.</para>
1062 <para>Owing to the vagaries of DLL linking, these hooks don't work
1063 under Windows when the program is built dynamically.</para>
1065 <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
1066 </indexterm>lets you set RTS
1067 options permanently for a given program. A common use for this is
1068 to give your program a default heap and/or stack size that is
1069 greater than the default. For example, to set <literal>-H128m
1070 -K1m</literal>, place the following definition in a C source
1074 char *ghc_rts_opts = "-H128m -K1m";
1077 <para>Compile the C file, and include the object file on the
1078 command line when you link your Haskell program.</para>
1080 <para>These flags are interpreted first, before any RTS flags from
1081 the <literal>GHCRTS</literal> environment variable and any flags
1082 on the command line.</para>
1084 <para>You can also change the messages printed when the runtime
1085 system “blows up,” e.g., on stack overflow. The hooks
1086 for these are as follows:</para>
1092 <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
1093 <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
1096 <para>The heap-overflow message.</para>
1102 <function>void StackOverflowHook (long int)</function>
1103 <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
1106 <para>The stack-overflow message.</para>
1112 <function>void MallocFailHook (long int)</function>
1113 <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
1116 <para>The message printed if <function>malloc</function>
1122 <para>For examples of the use of these hooks, see GHC's own
1123 versions in the file
1124 <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
1129 <title>Getting information about the RTS</title>
1131 <indexterm><primary>RTS</primary></indexterm>
1133 <para>It is possible to ask the RTS to give some information about
1134 itself. To do this, use the <option>--info</option> flag, e.g.</para>
1136 $ ./a.out +RTS --info
1138 ,("GHC version", "6.7")
1139 ,("RTS way", "rts_p")
1140 ,("Host platform", "x86_64-unknown-linux")
1141 ,("Host architecture", "x86_64")
1142 ,("Host OS", "linux")
1143 ,("Host vendor", "unknown")
1144 ,("Build platform", "x86_64-unknown-linux")
1145 ,("Build architecture", "x86_64")
1146 ,("Build OS", "linux")
1147 ,("Build vendor", "unknown")
1148 ,("Target platform", "x86_64-unknown-linux")
1149 ,("Target architecture", "x86_64")
1150 ,("Target OS", "linux")
1151 ,("Target vendor", "unknown")
1152 ,("Word size", "64")
1153 ,("Compiler unregisterised", "NO")
1154 ,("Tables next to code", "YES")
1157 <para>The information is formatted such that it can be read as a
1158 of type <literal>[(String, String)]</literal>. Currently the following
1159 fields are present:</para>
1164 <term><literal>GHC RTS</literal></term>
1166 <para>Is this program linked against the GHC RTS? (always
1172 <term><literal>GHC version</literal></term>
1174 <para>The version of GHC used to compile this program.</para>
1179 <term><literal>RTS way</literal></term>
1181 <para>The variant (“way”) of the runtime. The
1182 most common values are <literal>rts</literal> (vanilla),
1183 <literal>rts_thr</literal> (threaded runtime, i.e. linked using the
1184 <literal>-threaded</literal> option) and <literal>rts_p</literal>
1185 (profiling runtime, i.e. linked using the <literal>-prof</literal>
1186 option). Other variants include <literal>debug</literal>
1187 (linked using <literal>-debug</literal>),
1188 <literal>t</literal> (ticky-ticky profiling) and
1189 <literal>dyn</literal> (the RTS is
1190 linked in dynamically, i.e. a shared library, rather than statically
1191 linked into the executable itself). These can be combined,
1192 e.g. you might have <literal>rts_thr_debug_p</literal>.</para>
1198 <literal>Target platform</literal>,
1199 <literal>Target architecture</literal>,
1200 <literal>Target OS</literal>,
1201 <literal>Target vendor</literal>
1204 <para>These are the platform the program is compiled to run on.</para>
1210 <literal>Build platform</literal>,
1211 <literal>Build architecture</literal>,
1212 <literal>Build OS</literal>,
1213 <literal>Build vendor</literal>
1216 <para>These are the platform where the program was built
1217 on. (That is, the target platform of GHC itself.) Ordinarily
1218 this is identical to the target platform. (It could potentially
1219 be different if cross-compiling.)</para>
1225 <literal>Host platform</literal>,
1226 <literal>Host architecture</literal>
1227 <literal>Host OS</literal>
1228 <literal>Host vendor</literal>
1231 <para>These are the platform where GHC itself was compiled.
1232 Again, this would normally be identical to the build and
1233 target platforms.</para>
1238 <term><literal>Word size</literal></term>
1240 <para>Either <literal>"32"</literal> or <literal>"64"</literal>,
1241 reflecting the word size of the target platform.</para>
1246 <term><literal>Compiler unregistered</literal></term>
1248 <para>Was this program compiled with an “unregistered”
1249 version of GHC? (I.e., a version of GHC that has no platform-specific
1250 optimisations compiled in, usually because this is a currently
1251 unsupported platform.) This value will usually be no, unless you're
1252 using an experimental build of GHC.</para>
1257 <term><literal>Tables next to code</literal></term>
1259 <para>Putting info tables directly next to entry code is a useful
1260 performance optimisation that is not available on all platforms.
1261 This field tells you whether the program has been compiled with
1262 this optimisation. (Usually yes, except on unusual platforms.)</para>
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