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>@-G<generations>@:</tag>
67 <nidx>-G<generations> RTS option</nidx>
68 <nidx>generations, number of</nidx>
70 [Default: 2] Set the number of generations used by the garbage
71 collector. The default of 2 seems to be good, but the garbage
72 collector can support any number of generations. Anything larger than
73 about 4 is probably not a good idea unless your program runs for a
74 <em/long/ time, because the oldest generation will never get
77 Specifying 1 generation with @+RTS -G1@ gives you a simple 2-space
78 collector, as you would expect. In a 2-space collector, the @-A@
79 option (see below) specifies the <em/minimum/ allocation area size,
80 since the allocation area will grow with the amount of live data in
81 the heap. In a multi-generational collector the allocation area is a
84 <tag>@-A<size>@:</tag>
85 <nidx>-A<size> RTS option</nidx>
86 <nidx>allocation area, size</nidx>
88 [Default: 256k] Set the allocation area size used by the garbage
89 collector. The allocation area (actually generation 0 step 0) is
90 fixed and is never resized.
92 Increasing the allocation area size may or may not give better
93 performance (a bigger allocation area means worse cache behaviour but
94 fewer garbage collections and less promotion).
96 <tag>@-k<size>@:</tag>
97 <nidx>-k<size> RTS option</nidx>
98 <nidx>stack, minimum size</nidx>
100 [Default: 1k] Set the initial stack size for new threads. Thread
101 stacks (including the main thread's stack) live on the heap, and grow
102 as required. The default value is good for concurrent applications
103 with lots of small threads; if your program doesn't fit this model
104 then increasing this option may help performance.
106 The main thread is normally started with a slightly larger heap to cut
107 down on unnecessary stack growth while the program is starting up.
109 <tag>@-K<size>@:</tag>
110 <nidx>-K<size> RTS option</nidx>
111 <nidx>stack, maximum size</nidx>
113 [Default: 1M] Set the maximum stack size for an individual thread to
114 @<size>@ bytes. This option is there purely to stop the program
115 eating up all the available memory in the machine if it gets into an
119 <nidx>-m<n> RTS option</nidx>
120 Minimum \% @<n>@ of heap which must be available for allocation.
122 <nidx>heap, minimum free</nidx>
124 <tag>@-M<size>@:</tag>
125 <nidx>-M<size> RTS option</nidx>
126 <nidx>heap size, maximum</nidx>
128 [Default: 64M] Set the maximum heap size to @<size>@ bytes. The heap
129 normally grows and shrinks according to the memory requirements of the
130 program. The only reason for having this option is to stop the heap
131 growing without bound and filling up all the available swap space,
132 which at the least will result in the program being summarily killed
133 by the operating system.
135 <tag>@-s<file>@ or @-S<file>@:</tag>
136 <nidx>-S<file> RTS option</nidx>
137 <nidx>-s<file> RTS option</nidx>
138 Write modest (@-s@) or verbose (@-S@) garbage-collector
139 statistics into file @<file>@. The default @<file>@ is
140 @<program>@@.stat@. The @<file>@ @stderr@ is treated
141 specially, with the output really being sent to @stderr@.
143 This option is useful for watching how the storage manager adjusts the
144 heap size based on the current amount of live data.
147 %For some garbage collectors (not including the default one, sadly),
148 %you can convert the @-S@ output into a residency graph (in
149 %PostScript), using the @stat2resid@<nidx>stat2resid</nidx> utility in
150 %the GHC distribution (@ghc/utils/stat2resid@).
153 % <nidx>-F2s RTS option</nidx>
155 % Forces a program compiled for generational GC to use two-space copying
156 % collection. The two-space collector may outperform the generational
157 % collector for programs which have a very low heap residency. It can
158 % also be used to generate a statistics file from which a basic heap
159 % residency profile can be produced (see Section <ref name="stat2resid -
160 % residency info from GC stats" id="stat2resid">).
162 % There will still be a small execution overhead imposed by the
163 % generational compilation as the test for old generation updates will
164 % still be executed (of course none will actually happen). This
165 % overhead is typically less than 1\%.
167 % <tag>@-j<size>@:</tag>
168 % <nidx>-j<size> RTS option</nidx>
169 % Force a major garbage collection every @<size>@ bytes. (Normally
170 % used because you're keen on getting major-GC stats, notably heap residency
175 %************************************************************************
177 <sect2>RTS options for profiling and Concurrent/Parallel Haskell
180 %************************************************************************
182 The RTS options related to profiling are described in Section <ref
183 name="How to control your profiled program at runtime"
184 id="prof-rts-options">; and those for concurrent/parallel stuff, in
185 Section <ref name="RTS options for Concurrent/Parallel Haskell"
186 id="parallel-rts-opts">.
188 %************************************************************************
190 <sect2>RTS options for hackers, debuggers, and over-interested souls
192 <nidx>RTS options, hacking/debugging</nidx>
194 %************************************************************************
196 These RTS options might be used (a)~to avoid a GHC bug, (b)~to see
197 ``what's really happening'', or (c)~because you feel like it. Not
198 recommended for everyday use!
202 <nidx>-B RTS option</nidx>
203 Sound the bell at the start of each (major) garbage collection.
205 Oddly enough, people really do use this option! Our pal in Durham
206 (England), Paul Callaghan, writes: ``Some people here use it for a
207 variety of purposes---honestly!---e.g., confirmation that the
208 code/machine is doing something, infinite loop detection, gauging cost
209 of recently added code. Certain people can even tell what stage [the
210 program] is in by the beep pattern. But the major use is for annoying
211 others in the same office...''
213 <tag>@-r<file>@:</tag>
214 <nidx>-r <file> RTS option</nidx>
215 <nidx>ticky ticky profiling</nidx>
216 Produce ``ticky-ticky'' statistics at the end of the program run.
217 The @<file>@ business works just like on the @-S@ RTS option (above).
219 ``Ticky-ticky'' statistics are counts of various program actions
220 (updates, enters, etc.) The program must have been compiled using
221 @-fstg-reduction-counts@<nidx>-fstg-reduction-counts option</nidx>
222 (a.k.a. ``ticky-ticky profiling''), and, for it to be really useful,
223 linked with suitable system libraries. Not a trivial undertaking:
224 consult the installation guide on how to set things up for easy
225 ``ticky-ticky'' profiling.
227 <tag>@-D<num>@:</tag>
228 <nidx>-D RTS option</nidx>
229 An RTS debugging flag; varying quantities of output depending on which
230 bits are set in @<num>@. Only works if the RTS was compiled with the
234 <nidx>-N RTS option</nidx>
236 Normally, the garbage collector black-holes closures which are being
237 evaluated, as a space-saving measure. This option turns off
238 blackholing. You shouldn't ever need to use it.
240 Historical note: this option used to be used to work around a problem
241 with signal handling, where a signal handler might need to evaluate
242 blackholed closures. Signal handlers are now run in a separate
243 thread, and don't suffer from this problem.
246 <nidx>-Z RTS option</nidx>
247 Turn <em>off</em> ``update-frame squeezing'' at garbage-collection time.
248 (There's no particularly good reason to turn it off.)
251 %************************************************************************
253 <sect2>``Hooks'' to change RTS behaviour
254 <label id="rts-hooks">
256 <nidx>hooks, RTS</nidx>
257 <nidx>RTS hooks</nidx>
258 <nidx>RTS behaviour, changing</nidx>
260 %************************************************************************
262 GHC lets you exercise rudimentary control over the RTS settings for
263 any given program, by compiling in a ``hook'' that is called by the
264 run-time system. The RTS contains stub definitions for all these
265 hooks, but by writing your own version and linking it on the GHC
266 command line, you can override the defaults.
268 The function @defaultsHook@<nidx>defaultHook</nidx> lets you change various
269 RTS options. The commonest use for this is to give your program a
270 default heap and/or stack size that is greater than the default. For
271 example, to set @-H8m -K1m@:
275 #include "RtsFlags.h"
276 void defaultsHook (void) {
277 RTSflags.GcFlags.stksSize = 1000002 / sizeof(W_);
278 RTSflags.GcFlags.heapSize = 8000002 / sizeof(W_);
282 Don't use powers of two for heap/stack sizes: these are more likely to
283 interact badly with direct-mapped caches. The full set of flags is
284 defined in @ghc/rts/RtsFlags.h@ the the GHC source tree.
286 You can also change the messages printed when the runtime system
287 ``blows up,'' e.g., on stack overflow. The hooks for these are as
291 <tag>@void ErrorHdrHook (FILE *)@:</tag>
292 <nidx>ErrorHdrHook</nidx>
293 What's printed out before the message from @error@.
295 <tag>@void OutOfHeapHook (unsigned long, unsigned long)@:</tag>
296 <nidx>OutOfHeapHook</nidx>
297 The heap-overflow message.
299 <tag>@void StackOverflowHook (long int)@:</tag>
300 <nidx>StackOverflowHook</nidx>
301 The stack-overflow message.
303 <tag>@void MallocFailHook (long int)@:</tag>
304 <nidx>MallocFailHook</nidx>
305 The message printed if @malloc@ fails.
307 <tag>@void PatErrorHdrHook (FILE *)@:</tag>
308 <nidx>PatErrorHdrHook</nidx>
309 The message printed if a pattern-match fails (the failures
310 that were not handled by the Haskell programmer).
312 <tag>@void PreTraceHook (FILE *)@:</tag>
313 <nidx>PreTraceHook</nidx>
314 What's printed out before a @trace@ message.
316 <tag>@void PostTraceHook (FILE *)@:</tag>
317 <nidx>PostTraceHook</nidx>
318 What's printed out after a @trace@ message.
321 For example, here is the ``hooks'' code used by GHC itself:
324 #define W_ unsigned long int
328 ErrorHdrHook (FILE *where)
330 fprintf(where, "\n"); /* no "Fail: " */
334 OutOfHeapHook (W_ request_size, W_ heap_size) /* both sizes in bytes */
336 fprintf(stderr, "GHC's heap exhausted;\nwhile trying to
337 allocate %lu bytes in a %lu-byte heap;\nuse the `-H<size>'
338 option to increase the total heap size.\n",
344 StackOverflowHook (I_ stack_size) /* in bytes */
346 fprintf(stderr, "GHC stack-space overflow: current size
347 %ld bytes.\nUse the `-K<size>' option to increase it.\n",
352 PatErrorHdrHook (FILE *where)
354 fprintf(where, "\n*** Pattern-matching error within GHC!\n\n
355 This is a compiler bug; please report it to
356 glasgow-haskell-bugs@dcs.gla.ac.uk.\n\nFail: ");
360 PreTraceHook (FILE *where)
362 fprintf(where, "\n"); /* not "Trace On" */
366 PostTraceHook (FILE *where)
368 fprintf(where, "\n"); /* not "Trace Off" */