1 %************************************************************************
3 <sect1>Running a compiled program
4 <label id="runtime-control">
6 <nidx>runtime control of Haskell programs</nidx>
7 <nidx>running, compiled program</nidx>
8 <nidx>RTS options</nidx>
10 %************************************************************************
12 To make an executable program, the GHC system compiles your code and
13 then links it with a non-trivial runtime system (RTS), which handles
14 storage management, profiling, etc.
16 You have some control over the behaviour of the RTS, by giving special
17 command-line arguments to your program.
19 When your Haskell program starts up, its RTS extracts command-line
20 arguments bracketed between @+RTS@<nidx>+RTS option</nidx> and
21 @-RTS@<nidx>-RTS option</nidx> as its own. For example:
24 % ./a.out -f +RTS -p -S -RTS -h foo bar
27 The RTS will snaffle @-p -S@ for itself, and the remaining arguments
28 @-f -h foo bar@ will be handed to your program if/when it calls
31 No @-RTS@ option is required if the runtime-system options extend to
32 the end of the command line, as in this example:
35 % hls -ltr /usr/etc +RTS -A5m
38 If you absolutely positively want all the rest of the options in a
39 command line to go to the program (and not the RTS), use a
40 @--RTS@<nidx>--RTS option</nidx>.
42 As always, for RTS options that take @<size>@s: If the last
43 character of @size@ is a K or k, multiply by 1000; if an M or m, by
44 1,000,000; if a G or G, by 1,000,000,000. (And any wraparound in the
45 counters is <em>your</em> fault!)
47 Giving a @+RTS -f@<nidx>-f RTS option</nidx> option will print out the
48 RTS options actually available in your program (which vary, depending
51 %************************************************************************
53 <sect2>RTS options to control the garbage-collector
54 <label id="rts-options-gc">
56 <nidx>RTS options, garbage-collection</nidx>
58 %************************************************************************
60 There are several options to give you precise control over garbage
61 collection. Hopefully, you won't need any of these in normal
62 operation, but there are several things that can be tweaked for
66 <tag>@-A<size>@:</tag>
67 <nidx>-A<size> RTS option</nidx>
68 <nidx>allocation area, size</nidx>
70 [Default: 256k] Set the allocation area size used by the garbage
71 collector. The allocation area (actually generation 0 step 0) is
72 fixed and is never resized (unless you use <tt/-H/, below).
74 Increasing the allocation area size may or may not give better
75 performance (a bigger allocation area means worse cache behaviour but
76 fewer garbage collections and less promotion).
78 With only 1 generation (<tt/-G1/) the <tt/-A/ option specifies the
79 minimum allocation area, since the actual size of the allocation area
80 will be resized according to the amount of data in the heap (see
83 <tag>@-F<factor>@:</tag>
84 <nidx>-F<factor> RTS option</nidx>
85 <nidx>heap size, factor</nidx>
87 [Default: 2] This option controls the amount of memory reserved for
88 the older generations (and in the case of a two space collector the
89 size of the allocation area) as a factor of the amount of live data.
90 For example, if there was 2M of live data in the oldest generation
91 when we last collected it, then by default we'll wait until it grows
92 to 4M before collecting it again.
94 The default seems to work well here. If you have plenty of memory, it
95 is usually better to use <tt/-H<size>/ than to increase
96 <tt/-F<factor>/.
98 The <tt/-F/ setting will be automatically reduced by the garbage
99 collector when the maximum heap size (the <tt/-M<size>/ setting)
102 <tag>@-G<generations>@:</tag>
103 <nidx>-G<generations> RTS option</nidx>
104 <nidx>generations, number of</nidx>
106 [Default: 2] Set the number of generations used by the garbage
107 collector. The default of 2 seems to be good, but the garbage
108 collector can support any number of generations. Anything larger than
109 about 4 is probably not a good idea unless your program runs for a
110 <em/long/ time, because the oldest generation will never get
113 Specifying 1 generation with @+RTS -G1@ gives you a simple 2-space
114 collector, as you would expect. In a 2-space collector, the @-A@
115 option (see above) specifies the <em/minimum/ allocation area size,
116 since the allocation area will grow with the amount of live data in
117 the heap. In a multi-generational collector the allocation area is a
118 fixed size (unless you use the <tt/-H/ option, see below).
120 <tag>@-H<size>@:</tag>
121 <nidx>-H<size> RTS option</nidx>
122 <nidx>heap size, suggested</nidx>
124 [Default: 0] This option provides a "suggested heap size" for the
125 garbage collector. The garbage collector will use about this much
126 memory until the program residency grows and the heap size needs to be
127 expanded to retain reasonable performance.
129 By default, the heap will start small, and grow and shrink as
130 necessary. This can be bad for performance, so if you have plenty of
131 memory it's worthwhile supplying a big <tt/-H<size>/. For
132 improving GC performance, using <tt/-H<size>/ is usually a better
133 bet than <tt/-A<size>/.
135 <tag>@-k<size>@:</tag>
136 <nidx>-k<size> RTS option</nidx>
137 <nidx>stack, minimum size</nidx>
139 [Default: 1k] Set the initial stack size for new threads. Thread
140 stacks (including the main thread's stack) live on the heap, and grow
141 as required. The default value is good for concurrent applications
142 with lots of small threads; if your program doesn't fit this model
143 then increasing this option may help performance.
145 The main thread is normally started with a slightly larger heap to cut
146 down on unnecessary stack growth while the program is starting up.
148 <tag>@-K<size>@:</tag>
149 <nidx>-K<size> RTS option</nidx>
150 <nidx>stack, maximum size</nidx>
152 [Default: 1M] Set the maximum stack size for an individual thread to
153 @<size>@ bytes. This option is there purely to stop the program
154 eating up all the available memory in the machine if it gets into an
158 <nidx>-m<n> RTS option</nidx>
159 Minimum \% @<n>@ of heap which must be available for allocation.
161 <nidx>heap, minimum free</nidx>
163 <tag>@-M<size>@:</tag>
164 <nidx>-M<size> RTS option</nidx>
165 <nidx>heap size, maximum</nidx>
167 [Default: 64M] Set the maximum heap size to @<size>@ bytes. The heap
168 normally grows and shrinks according to the memory requirements of the
169 program. The only reason for having this option is to stop the heap
170 growing without bound and filling up all the available swap space,
171 which at the least will result in the program being summarily killed
172 by the operating system.
174 <tag>@-s<file>@ or @-S<file>@:</tag>
175 <nidx>-S<file> RTS option</nidx>
176 <nidx>-s<file> RTS option</nidx>
177 Write modest (@-s@) or verbose (@-S@) garbage-collector
178 statistics into file @<file>@. The default @<file>@ is
179 @<program>@@.stat@. The @<file>@ @stderr@ is treated
180 specially, with the output really being sent to @stderr@.
182 This option is useful for watching how the storage manager adjusts the
183 heap size based on the current amount of live data.
186 %For some garbage collectors (not including the default one, sadly),
187 %you can convert the @-S@ output into a residency graph (in
188 %PostScript), using the @stat2resid@<nidx>stat2resid</nidx> utility in
189 %the GHC distribution (@ghc/utils/stat2resid@).
191 % <tag>@-j<size>@:</tag>
192 % <nidx>-j<size> RTS option</nidx>
193 % Force a major garbage collection every @<size>@ bytes. (Normally
194 % used because you're keen on getting major-GC stats, notably heap residency
199 %************************************************************************
201 <sect2>RTS options for profiling and Concurrent/Parallel Haskell
204 %************************************************************************
206 The RTS options related to profiling are described in Section <ref
207 name="How to control your profiled program at runtime"
208 id="prof-rts-options">; and those for concurrent/parallel stuff, in
209 Section <ref name="RTS options for Concurrent/Parallel Haskell"
210 id="parallel-rts-opts">.
212 %************************************************************************
214 <sect2>RTS options for hackers, debuggers, and over-interested souls
216 <nidx>RTS options, hacking/debugging</nidx>
218 %************************************************************************
220 These RTS options might be used (a)~to avoid a GHC bug, (b)~to see
221 ``what's really happening'', or (c)~because you feel like it. Not
222 recommended for everyday use!
226 <nidx>-B RTS option</nidx>
227 Sound the bell at the start of each (major) garbage collection.
229 Oddly enough, people really do use this option! Our pal in Durham
230 (England), Paul Callaghan, writes: ``Some people here use it for a
231 variety of purposes---honestly!---e.g., confirmation that the
232 code/machine is doing something, infinite loop detection, gauging cost
233 of recently added code. Certain people can even tell what stage [the
234 program] is in by the beep pattern. But the major use is for annoying
235 others in the same office...''
237 <tag>@-r<file>@:</tag>
238 <nidx>-r <file> RTS option</nidx>
239 <nidx>ticky ticky profiling</nidx>
240 Produce ``ticky-ticky'' statistics at the end of the program run.
241 The @<file>@ business works just like on the @-S@ RTS option (above).
243 ``Ticky-ticky'' statistics are counts of various program actions
244 (updates, enters, etc.) The program must have been compiled using
245 @-ticky@<nidx>-ticky option</nidx> (a.k.a. ``ticky-ticky profiling''),
246 and, for it to be really useful, linked with suitable system
247 libraries. Not a trivial undertaking: consult the installation guide
248 on how to set things up for easy ``ticky-ticky'' profiling.
250 <tag>@-D<num>@:</tag>
251 <nidx>-D RTS option</nidx>
252 An RTS debugging flag; varying quantities of output depending on which
253 bits are set in @<num>@. Only works if the RTS was compiled with the
256 % Blackholing can't be turned off in new RTS --SDM
259 % <nidx>-N RTS option</nidx>
261 % Normally, the garbage collector black-holes closures which are being
262 % evaluated, as a space-saving measure. This option turns off
263 % blackholing. You shouldn't ever need to use it.
265 % Historical note: this option used to be used to work around a problem
266 % with signal handling, where a signal handler might need to evaluate
267 % blackholed closures. Signal handlers are now run in a separate
268 % thread, and don't suffer from this problem.
271 <nidx>-Z RTS option</nidx>
272 Turn <em>off</em> ``update-frame squeezing'' at garbage-collection time.
273 (There's no particularly good reason to turn it off.)
276 %************************************************************************
278 <sect2>``Hooks'' to change RTS behaviour
279 <label id="rts-hooks">
281 <nidx>hooks, RTS</nidx>
282 <nidx>RTS hooks</nidx>
283 <nidx>RTS behaviour, changing</nidx>
285 %************************************************************************
287 GHC lets you exercise rudimentary control over the RTS settings for
288 any given program, by compiling in a ``hook'' that is called by the
289 run-time system. The RTS contains stub definitions for all these
290 hooks, but by writing your own version and linking it on the GHC
291 command line, you can override the defaults.
293 The function @defaultsHook@<nidx>defaultHook</nidx> lets you change various
294 RTS options. The commonest use for this is to give your program a
295 default heap and/or stack size that is greater than the default. For
296 example, to set @-H8m -K1m@:
300 #include "RtsFlags.h"
301 void defaultsHook (void) {
302 RTSflags.GcFlags.stksSize = 1000002 / sizeof(W_);
303 RTSflags.GcFlags.heapSize = 8000002 / sizeof(W_);
307 Don't use powers of two for heap/stack sizes: these are more likely to
308 interact badly with direct-mapped caches. The full set of flags is
309 defined in @ghc/rts/RtsFlags.h@ the the GHC source tree.
311 You can also change the messages printed when the runtime system
312 ``blows up,'' e.g., on stack overflow. The hooks for these are as
316 <tag>@void ErrorHdrHook (FILE *)@:</tag>
317 <nidx>ErrorHdrHook</nidx>
318 What's printed out before the message from @error@.
320 <tag>@void OutOfHeapHook (unsigned long, unsigned long)@:</tag>
321 <nidx>OutOfHeapHook</nidx>
322 The heap-overflow message.
324 <tag>@void StackOverflowHook (long int)@:</tag>
325 <nidx>StackOverflowHook</nidx>
326 The stack-overflow message.
328 <tag>@void MallocFailHook (long int)@:</tag>
329 <nidx>MallocFailHook</nidx>
330 The message printed if @malloc@ fails.
332 <tag>@void PatErrorHdrHook (FILE *)@:</tag>
333 <nidx>PatErrorHdrHook</nidx>
334 The message printed if a pattern-match fails (the failures
335 that were not handled by the Haskell programmer).
337 <tag>@void PreTraceHook (FILE *)@:</tag>
338 <nidx>PreTraceHook</nidx>
339 What's printed out before a @trace@ message.
341 <tag>@void PostTraceHook (FILE *)@:</tag>
342 <nidx>PostTraceHook</nidx>
343 What's printed out after a @trace@ message.
346 For example, here is the ``hooks'' code used by GHC itself:
349 #define W_ unsigned long int
353 ErrorHdrHook (FILE *where)
355 fprintf(where, "\n"); /* no "Fail: " */
359 OutOfHeapHook (W_ request_size, W_ heap_size) /* both sizes in bytes */
361 fprintf(stderr, "GHC's heap exhausted;\nwhile trying to
362 allocate %lu bytes in a %lu-byte heap;\nuse the `-H<size>'
363 option to increase the total heap size.\n",
369 StackOverflowHook (I_ stack_size) /* in bytes */
371 fprintf(stderr, "GHC stack-space overflow: current size
372 %ld bytes.\nUse the `-K<size>' option to increase it.\n",
377 PatErrorHdrHook (FILE *where)
379 fprintf(where, "\n*** Pattern-matching error within GHC!\n\n
380 This is a compiler bug; please report it to
381 glasgow-haskell-bugs@dcs.gla.ac.uk.\n\nFail: ");
385 PreTraceHook (FILE *where)
387 fprintf(where, "\n"); /* not "Trace On" */
391 PostTraceHook (FILE *where)
393 fprintf(where, "\n"); /* not "Trace Off" */