1 <sect1 id="runtime-control">
2 <title>Running a compiled program</title>
4 <indexterm><primary>runtime control of Haskell programs</primary></indexterm>
5 <indexterm><primary>running, compiled program</primary></indexterm>
6 <indexterm><primary>RTS options</primary></indexterm>
8 <para>To make an executable program, the GHC system compiles your
9 code and then links it with a non-trivial runtime system (RTS),
10 which handles storage management, profiling, etc.</para>
12 <para>You have some control over the behaviour of the RTS, by giving
13 special command-line arguments to your program.</para>
15 <para>When your Haskell program starts up, its RTS extracts
16 command-line arguments bracketed between
17 <option>+RTS</option><indexterm><primary><option>+RTS</option></primary></indexterm>
19 <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
20 as its own. For example:</para>
23 % ./a.out -f +RTS -p -S -RTS -h foo bar
26 <para>The RTS will snaffle <option>-p</option> <option>-S</option>
27 for itself, and the remaining arguments <literal>-f -h foo bar</literal>
28 will be handed to your program if/when it calls
29 <function>System.getArgs</function>.</para>
31 <para>No <option>-RTS</option> option is required if the
32 runtime-system options extend to the end of the command line, as in
36 % hls -ltr /usr/etc +RTS -A5m
39 <para>If you absolutely positively want all the rest of the options
40 in a command line to go to the program (and not the RTS), use a
41 <option>––RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.</para>
43 <para>As always, for RTS options that take
44 <replaceable>size</replaceable>s: If the last character of
45 <replaceable>size</replaceable> is a K or k, multiply by 1000; if an
46 M or m, by 1,000,000; if a G or G, by 1,000,000,000. (And any
47 wraparound in the counters is <emphasis>your</emphasis>
50 <para>Giving a <literal>+RTS -f</literal>
51 <indexterm><primary><option>-f</option></primary><secondary>RTS option</secondary></indexterm> option
52 will print out the RTS options actually available in your program
53 (which vary, depending on how you compiled).</para>
55 <para>NOTE: since GHC is itself compiled by GHC, you can change RTS
56 options in the compiler using the normal
57 <literal>+RTS ... -RTS</literal>
58 combination. eg. to increase the maximum heap
59 size for a compilation to 128M, you would add
60 <literal>+RTS -M128m -RTS</literal>
61 to the command line.</para>
63 <sect2 id="rts-optinos-environment">
64 <title>Setting global RTS options</title>
66 <indexterm><primary>RTS options</primary><secondary>from the environment</secondary></indexterm>
67 <indexterm><primary>environment variable</primary><secondary>for
68 setting RTS options</secondary></indexterm>
70 <para>RTS options are also taken from the environment variable
71 <envar>GHCRTS</envar><indexterm><primary><envar>GHCRTS</envar></primary>
72 </indexterm>. For example, to set the maximum heap size
73 to 128M for all GHC-compiled programs (using an
74 <literal>sh</literal>-like shell):</para>
81 <para>RTS options taken from the <envar>GHCRTS</envar> environment
82 variable can be overriden by options given on the command
87 <sect2 id="rts-options-gc">
88 <title>RTS options to control the garbage collector</title>
90 <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
91 <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>
93 <para>There are several options to give you precise control over
94 garbage collection. Hopefully, you won't need any of these in
95 normal operation, but there are several things that can be tweaked
96 for maximum performance.</para>
101 <term><option>-A</option><replaceable>size</replaceable></Term>
102 <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
103 <indexterm><primary>allocation area, size</primary></indexterm>
105 <para>[Default: 256k] Set the allocation area size
106 used by the garbage collector. The allocation area
107 (actually generation 0 step 0) is fixed and is never resized
108 (unless you use <option>-H</option>, below).</para>
110 <para>Increasing the allocation area size may or may not
111 give better performance (a bigger allocation area means
112 worse cache behaviour but fewer garbage collections and less
115 <para>With only 1 generation (<option>-G1</option>) the
116 <option>-A</option> option specifies the minimum allocation
117 area, since the actual size of the allocation area will be
118 resized according to the amount of data in the heap (see
119 <option>-F</option>, below).</para>
124 <term><option>-c</option></term>
125 <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary>
127 <indexterm><primary>garbage collection</primary><secondary>compacting</secondary>
129 <indexterm><primary>compacting garbage collection</primary></indexterm>
132 <para>Use a compacting algorithm for collecting the oldest
133 generation. By default, the oldest generation is collected
134 using a copying algorithm; this option causes it to be
135 compacted in-place instead. The compaction algorithm is
136 slower than the copying algorithm, but the savings in memory
137 use can be considerable.</para>
139 <para>For a given heap size (using the <option>-H</option>
140 option), compaction can in fact reduce the GC cost by
141 allowing fewer GCs to be performed. This is more likely
142 when the ratio of live data to heap size is high, say
143 >30%.</para>
145 <para>NOTE: compaction doesn't currently work when a single
146 generation is requested using the <option>-G1</option>
152 <term><option>-c</option><replaceable>n</replaceable></term>
155 <para>[Default: 30] Automatically enable
156 compacting collection when the live data exceeds
157 <replaceable>n</replaceable>% of the maximum heap size
158 (see the <option>-M</option> option). Note that the maximum
159 heap size is unlimited by default, so this option has no
160 effect unless the maximum heap size is set with
161 <option>-M</option><replaceable>size</replaceable>. </para>
166 <term><option>-F</option><replaceable>factor</replaceable></Term>
168 <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
169 <indexterm><primary>heap size, factor</primary></indexterm>
171 <para>[Default: 2] This option controls the amount
172 of memory reserved for the older generations (and in the
173 case of a two space collector the size of the allocation
174 area) as a factor of the amount of live data. For example,
175 if there was 2M of live data in the oldest generation when
176 we last collected it, then by default we'll wait until it
177 grows to 4M before collecting it again.</para>
179 <para>The default seems to work well here. If you have
180 plenty of memory, it is usually better to use
181 <option>-H</option><replaceable>size</replaceable> than to
183 <option>-F</option><replaceable>factor</replaceable>.</para>
185 <para>The <option>-F</option> setting will be automatically
186 reduced by the garbage collector when the maximum heap size
187 (the <option>-M</option><replaceable>size</replaceable>
188 setting) is approaching.</para>
193 <term><option>-G</option><replaceable>generations</replaceable></Term>
194 <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
195 <indexterm><primary>generations, number
196 of</primary></indexterm>
198 <para>[Default: 2] Set the number of generations
199 used by the garbage collector. The default of 2 seems to be
200 good, but the garbage collector can support any number of
201 generations. Anything larger than about 4 is probably not a
202 good idea unless your program runs for a
203 <emphasis>long</emphasis> time, because the oldest
204 generation will hardly ever get collected.</para>
206 <para>Specifying 1 generation with <option>+RTS -G1</option>
207 gives you a simple 2-space collector, as you would expect.
208 In a 2-space collector, the <option>-A</option> option (see
209 above) specifies the <Emphasis>minimum</Emphasis> allocation
210 area size, since the allocation area will grow with the
211 amount of live data in the heap. In a multi-generational
212 collector the allocation area is a fixed size (unless you
213 use the <option>-H</option> option, see below).</para>
218 <term><option>-H</option><replaceable>size</replaceable></term>
219 <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
220 <indexterm><primary>heap size, suggested</primary></indexterm>
222 <para>[Default: 0] This option provides a
223 “suggested heap size” for the garbage collector. The
224 garbage collector will use about this much memory until the
225 program residency grows and the heap size needs to be
226 expanded to retain reasonable performance.</para>
228 <para>By default, the heap will start small, and grow and
229 shrink as necessary. This can be bad for performance, so if
230 you have plenty of memory it's worthwhile supplying a big
231 <option>-H</option><replaceable>size</replaceable>. For
232 improving GC performance, using
233 <option>-H</option><replaceable>size</replaceable> is
234 usually a better bet than
235 <option>-A</option><replaceable>size</replaceable>.</para>
240 <term><option>-k</option><replaceable>size</replaceable></term>
241 <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
242 <indexterm><primary>stack, minimum size</primary></indexterm>
244 <para>[Default: 1k] Set the initial stack size for
245 new threads. Thread stacks (including the main thread's
246 stack) live on the heap, and grow as required. The default
247 value is good for concurrent applications with lots of small
248 threads; if your program doesn't fit this model then
249 increasing this option may help performance.</para>
251 <para>The main thread is normally started with a slightly
252 larger heap to cut down on unnecessary stack growth while
253 the program is starting up.</para>
258 <term><option>-K</option><replaceable>size</replaceable></term>
259 <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
260 <indexterm><primary>stack, maximum size</primary></indexterm>
262 <para>[Default: 1M] Set the maximum stack size for
263 an individual thread to <replaceable>size</replaceable>
264 bytes. This option is there purely to stop the program
265 eating up all the available memory in the machine if it gets
266 into an infinite loop.</para>
271 <term><option>-m</option><replaceable>n</replaceable></term>
272 <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
273 <indexterm><primary>heap, minimum free</primary></indexterm>
275 <para>Minimum % <replaceable>n</replaceable> of heap
276 which must be available for allocation. The default is
282 <term><option>-M</option><replaceable>size</replaceable></term>
283 <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
284 <indexterm><primary>heap size, maximum</primary></indexterm>
286 <para>[Default: unlimited] Set the maximum heap size to
287 <replaceable>size</replaceable> bytes. The heap normally
288 grows and shrinks according to the memory requirements of
289 the program. The only reason for having this option is to
290 stop the heap growing without bound and filling up all the
291 available swap space, which at the least will result in the
292 program being summarily killed by the operating
295 <para>The maximum heap size also affects other garbage
296 collection parameters: when the amount of live data in the
297 heap exceeds a certain fraction of the maximum heap size,
298 compacting collection will be automatically enabled for the
299 oldest generation, and the <option>-F</option> parameter
300 will be reduced in order to avoid exceeding the maximum heap
306 <term><option>-s</option><replaceable>file</replaceable></term>
307 <term><option>-S</option><replaceable>file</replaceable></Term>
308 <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
309 <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
311 <para>Write modest (<option>-s</option>) or verbose
312 (<option>-S</option>) garbage-collector statistics into file
313 <replaceable>file</replaceable>. The default
314 <replaceable>file</replaceable> is
315 <Filename><replaceable>program</replaceable>.stat</Filename>. The
316 <replaceable>file</replaceable> <constant>stderr</constant>
317 is treated specially, with the output really being sent to
318 <constant>stderr</constant>.</para>
320 <para>This option is useful for watching how the storage
321 manager adjusts the heap size based on the current amount of
327 <term><option>-t</option></term>
328 <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
330 <para>Write a one-line GC stats summary after running the
331 program. This output is in the same format as that produced
332 by the <option>-Rghc-timing</option> option.</para>
339 <!-- ---------------------------------------------------------------------- -->
341 <title>RTS options for profiling and Concurrent/Parallel Haskell</title>
343 <para>The RTS options related to profiling are described in <XRef
344 LinkEnd="prof-rts-options">; and those for concurrent/parallel
345 stuff, in <XRef LinkEnd="parallel-rts-opts">.</para>
348 <!-- ---------------------------------------------------------------------- -->
349 <sect2 id="rts-options-debugging">
350 <title>RTS options for hackers, debuggers, and over-interested
353 <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>
355 <para>These RTS options might be used (a) to avoid a GHC bug,
356 (b) to see “what's really happening”, or
357 (c) because you feel like it. Not recommended for everyday
363 <term><option>-B</option></term>
364 <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
366 <para>Sound the bell at the start of each (major) garbage
369 <para>Oddly enough, people really do use this option! Our
370 pal in Durham (England), Paul Callaghan, writes: “Some
371 people here use it for a variety of
372 purposes—honestly!—e.g., confirmation that the
373 code/machine is doing something, infinite loop detection,
374 gauging cost of recently added code. Certain people can even
375 tell what stage [the program] is in by the beep
376 pattern. But the major use is for annoying others in the
377 same office…”</para>
382 <term><option>-D</option><replaceable>num</replaceable></term>
383 <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
385 <para>An RTS debugging flag; varying quantities of output
386 depending on which bits are set in
387 <replaceable>num</replaceable>. Only works if the RTS was
388 compiled with the <option>DEBUG</option> option.</para>
393 <term><option>-r</option><replaceable>file</replaceable></term>
394 <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
395 <indexterm><primary>ticky ticky profiling</primary></indexterm>
396 <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
398 <para>Produce “ticky-ticky” statistics at the
399 end of the program run. The <replaceable>file</replaceable>
400 business works just like on the <option>-S</option> RTS
401 option (above).</para>
403 <para>“Ticky-ticky” statistics are counts of
404 various program actions (updates, enters, etc.) The program
405 must have been compiled using
406 <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
407 (a.k.a. “ticky-ticky profiling”), and, for it to
408 be really useful, linked with suitable system libraries.
409 Not a trivial undertaking: consult the installation guide on
410 how to set things up for easy “ticky-ticky”
411 profiling. For more information, see <XRef
412 LinkEnd="ticky-ticky">.</para>
417 <term><option>-xc</option></term>
418 <indexterm><primary><option>-xc</option></primary><secondary>RTS
419 option</secondary></indexterm>
421 <para>(Only available when the program is compiled for
422 profiling.) When an exception is raised in the program,
423 this option causes the current cost-centre-stack to be
424 dumped to <literal>stderr</literal>.</para>
426 <para>This can be particularly useful for debugging: if your
427 program is complaining about a <literal>head []</literal>
428 error and you haven't got a clue which bit of code is
429 causing it, compiling with <literal>-prof
430 -auto-all</literal> and running with <literal>+RTS -xc
431 -RTS</literal> will tell you exactly the call stack at the
432 point the error was raised.</para>
434 <para>The output contains one line for each exception raised
435 in the program (the program might raise and catch several
436 exceptions during its execution), where each line is of the
440 < cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> >
442 <para>each <literal>cc</literal><subscript>i</subscript> is
443 a cost centre in the program (see <xref
444 linkend="cost-centres">), and the sequence represents the
445 “call stack” at the point the exception was
446 raised. The leftmost item is the innermost function in the
447 call stack, and the rightmost item is the outermost
454 <term><option>-Z</option></term>
455 <indexterm><primary><option>-Z</option></primary><secondary>RTS
456 option</secondary></indexterm>
458 <para>Turn <emphasis>off</emphasis> “update-frame
459 squeezing” at garbage-collection time. (There's no
460 particularly good reason to turn it off, except to ensure
461 the accuracy of certain data collected regarding thunk entry
469 <sect2 id="rts-hooks">
470 <title>“Hooks” to change RTS behaviour</title>
472 <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
473 <indexterm><primary>RTS hooks</primary></indexterm>
474 <indexterm><primary>RTS behaviour, changing</primary></indexterm>
476 <para>GHC lets you exercise rudimentary control over the RTS
477 settings for any given program, by compiling in a
478 “hook” that is called by the run-time system. The RTS
479 contains stub definitions for all these hooks, but by writing your
480 own version and linking it on the GHC command line, you can
481 override the defaults.</para>
483 <para>Owing to the vagaries of DLL linking, these hooks don't work
484 under Windows when the program is built dynamically.</para>
486 <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
487 </indexterm>lets you set RTS
488 options permanently for a given program. A common use for this is
489 to give your program a default heap and/or stack size that is
490 greater than the default. For example, to set <literal>-H128m
491 -K1m</literal>, place the following definition in a C source
495 char *ghc_rts_opts = "-H128m -K1m";
498 <para>Compile the C file, and include the object file on the
499 command line when you link your Haskell program.</para>
501 <para>These flags are interpreted first, before any RTS flags from
502 the <literal>GHCRTS</literal> environment variable and any flags
503 on the command line.</para>
505 <para>You can also change the messages printed when the runtime
506 system “blows up,” e.g., on stack overflow. The hooks
507 for these are as follows:</para>
512 <term><Function>void ErrorHdrHook (FILE *)</function></term>
513 <indexterm><primary><function>ErrorHdrHook</function></primary></indexterm>
515 <para>What's printed out before the message from
516 <function>error</function>.</para>
521 <term><function>void OutOfHeapHook (unsigned long, unsigned long)</function></term>
522 <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
524 <para>The heap-overflow message.</para>
529 <term><function>void StackOverflowHook (long int)</function></term>
530 <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
532 <para>The stack-overflow message.</para>
537 <term><function>void MallocFailHook (long int)</function></term>
538 <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
540 <para>The message printed if <Function>malloc</Function>
546 <term><function>void PatErrorHdrHook (FILE *)</function></term>
547 <indexterm><primary><function>PatErrorHdrHook</function></primary></indexterm>
549 <para>The message printed if a pattern-match fails (the
550 failures that were not handled by the Haskell
556 <term><function>void PreTraceHook (FILE *)</function></term>
557 <indexterm><primary><function>PreTraceHook</function></primary></indexterm>
559 <para>What's printed out before a <Function>trace</Function>
565 <term><function>void PostTraceHook (FILE *)</function></term>
566 <indexterm><primary><function>PostTraceHook</function></primary></indexterm>
568 <para>What's printed out after a <Function>trace</Function>
574 <para>For examples of the use of these hooks, see GHC's own
576 <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
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