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@).
192 % <nidx>-F2s RTS option</nidx>
194 % Forces a program compiled for generational GC to use two-space copying
195 % collection. The two-space collector may outperform the generational
196 % collector for programs which have a very low heap residency. It can
197 % also be used to generate a statistics file from which a basic heap
198 % residency profile can be produced (see Section <ref name="stat2resid -
199 % residency info from GC stats" id="stat2resid">).
201 % There will still be a small execution overhead imposed by the
202 % generational compilation as the test for old generation updates will
203 % still be executed (of course none will actually happen). This
204 % overhead is typically less than 1\%.
206 % <tag>@-j<size>@:</tag>
207 % <nidx>-j<size> RTS option</nidx>
208 % Force a major garbage collection every @<size>@ bytes. (Normally
209 % used because you're keen on getting major-GC stats, notably heap residency
214 %************************************************************************
216 <sect2>RTS options for profiling and Concurrent/Parallel Haskell
219 %************************************************************************
221 The RTS options related to profiling are described in Section <ref
222 name="How to control your profiled program at runtime"
223 id="prof-rts-options">; and those for concurrent/parallel stuff, in
224 Section <ref name="RTS options for Concurrent/Parallel Haskell"
225 id="parallel-rts-opts">.
227 %************************************************************************
229 <sect2>RTS options for hackers, debuggers, and over-interested souls
231 <nidx>RTS options, hacking/debugging</nidx>
233 %************************************************************************
235 These RTS options might be used (a)~to avoid a GHC bug, (b)~to see
236 ``what's really happening'', or (c)~because you feel like it. Not
237 recommended for everyday use!
241 <nidx>-B RTS option</nidx>
242 Sound the bell at the start of each (major) garbage collection.
244 Oddly enough, people really do use this option! Our pal in Durham
245 (England), Paul Callaghan, writes: ``Some people here use it for a
246 variety of purposes---honestly!---e.g., confirmation that the
247 code/machine is doing something, infinite loop detection, gauging cost
248 of recently added code. Certain people can even tell what stage [the
249 program] is in by the beep pattern. But the major use is for annoying
250 others in the same office...''
252 <tag>@-r<file>@:</tag>
253 <nidx>-r <file> RTS option</nidx>
254 <nidx>ticky ticky profiling</nidx>
255 Produce ``ticky-ticky'' statistics at the end of the program run.
256 The @<file>@ business works just like on the @-S@ RTS option (above).
258 ``Ticky-ticky'' statistics are counts of various program actions
259 (updates, enters, etc.) The program must have been compiled using
260 @-fstg-reduction-counts@<nidx>-fstg-reduction-counts option</nidx>
261 (a.k.a. ``ticky-ticky profiling''), and, for it to be really useful,
262 linked with suitable system libraries. Not a trivial undertaking:
263 consult the installation guide on how to set things up for easy
264 ``ticky-ticky'' profiling.
266 <tag>@-D<num>@:</tag>
267 <nidx>-D RTS option</nidx>
268 An RTS debugging flag; varying quantities of output depending on which
269 bits are set in @<num>@. Only works if the RTS was compiled with the
273 <nidx>-N RTS option</nidx>
275 Normally, the garbage collector black-holes closures which are being
276 evaluated, as a space-saving measure. This option turns off
277 blackholing. You shouldn't ever need to use it.
279 Historical note: this option used to be used to work around a problem
280 with signal handling, where a signal handler might need to evaluate
281 blackholed closures. Signal handlers are now run in a separate
282 thread, and don't suffer from this problem.
285 <nidx>-Z RTS option</nidx>
286 Turn <em>off</em> ``update-frame squeezing'' at garbage-collection time.
287 (There's no particularly good reason to turn it off.)
290 %************************************************************************
292 <sect2>``Hooks'' to change RTS behaviour
293 <label id="rts-hooks">
295 <nidx>hooks, RTS</nidx>
296 <nidx>RTS hooks</nidx>
297 <nidx>RTS behaviour, changing</nidx>
299 %************************************************************************
301 GHC lets you exercise rudimentary control over the RTS settings for
302 any given program, by compiling in a ``hook'' that is called by the
303 run-time system. The RTS contains stub definitions for all these
304 hooks, but by writing your own version and linking it on the GHC
305 command line, you can override the defaults.
307 The function @defaultsHook@<nidx>defaultHook</nidx> lets you change various
308 RTS options. The commonest use for this is to give your program a
309 default heap and/or stack size that is greater than the default. For
310 example, to set @-H8m -K1m@:
314 #include "RtsFlags.h"
315 void defaultsHook (void) {
316 RTSflags.GcFlags.stksSize = 1000002 / sizeof(W_);
317 RTSflags.GcFlags.heapSize = 8000002 / sizeof(W_);
321 Don't use powers of two for heap/stack sizes: these are more likely to
322 interact badly with direct-mapped caches. The full set of flags is
323 defined in @ghc/rts/RtsFlags.h@ the the GHC source tree.
325 You can also change the messages printed when the runtime system
326 ``blows up,'' e.g., on stack overflow. The hooks for these are as
330 <tag>@void ErrorHdrHook (FILE *)@:</tag>
331 <nidx>ErrorHdrHook</nidx>
332 What's printed out before the message from @error@.
334 <tag>@void OutOfHeapHook (unsigned long, unsigned long)@:</tag>
335 <nidx>OutOfHeapHook</nidx>
336 The heap-overflow message.
338 <tag>@void StackOverflowHook (long int)@:</tag>
339 <nidx>StackOverflowHook</nidx>
340 The stack-overflow message.
342 <tag>@void MallocFailHook (long int)@:</tag>
343 <nidx>MallocFailHook</nidx>
344 The message printed if @malloc@ fails.
346 <tag>@void PatErrorHdrHook (FILE *)@:</tag>
347 <nidx>PatErrorHdrHook</nidx>
348 The message printed if a pattern-match fails (the failures
349 that were not handled by the Haskell programmer).
351 <tag>@void PreTraceHook (FILE *)@:</tag>
352 <nidx>PreTraceHook</nidx>
353 What's printed out before a @trace@ message.
355 <tag>@void PostTraceHook (FILE *)@:</tag>
356 <nidx>PostTraceHook</nidx>
357 What's printed out after a @trace@ message.
360 For example, here is the ``hooks'' code used by GHC itself:
363 #define W_ unsigned long int
367 ErrorHdrHook (FILE *where)
369 fprintf(where, "\n"); /* no "Fail: " */
373 OutOfHeapHook (W_ request_size, W_ heap_size) /* both sizes in bytes */
375 fprintf(stderr, "GHC's heap exhausted;\nwhile trying to
376 allocate %lu bytes in a %lu-byte heap;\nuse the `-H<size>'
377 option to increase the total heap size.\n",
383 StackOverflowHook (I_ stack_size) /* in bytes */
385 fprintf(stderr, "GHC stack-space overflow: current size
386 %ld bytes.\nUse the `-K<size>' option to increase it.\n",
391 PatErrorHdrHook (FILE *where)
393 fprintf(where, "\n*** Pattern-matching error within GHC!\n\n
394 This is a compiler bug; please report it to
395 glasgow-haskell-bugs@dcs.gla.ac.uk.\n\nFail: ");
399 PreTraceHook (FILE *where)
401 fprintf(where, "\n"); /* not "Trace On" */
405 PostTraceHook (FILE *where)
407 fprintf(where, "\n"); /* not "Trace Off" */