1 %************************************************************************
3 \section[runtime-control]{Controlling the run-time behaviour of your programs}
4 \index{runtime control of Haskell programs}
7 %************************************************************************
9 To make an executable program, the GHC system compiles your code and
10 then links it with a non-trivial runtime system (RTS), which handles
11 storage management, profiling, etc.
13 You have some control over the behaviour of the RTS, by giving special
14 command-line arguments to your program.
16 %You have some control over the behavior of the runtime system, either
17 %by giving special command-line arguments to your program (the usual) or by
18 %building in your own defaults at compile time (the exotic).
20 When your Haskell program starts up, its RTS extracts
21 command-line arguments bracketed between \tr{+RTS}\index{+RTS option}
22 and \tr{-RTS}\index{-RTS option} as its own. For example:
24 % ./a.out -f +RTS -p -S -RTS -h foo bar
26 The RTS will snaffle \tr{-p -S} for itself,
27 and the remaining arguments \tr{-f -h foo bar} will be handed
28 to your program when it does a @GetArgs@ I/O request.
30 No \tr{-RTS} option is required if the runtime-system options extend
31 to the end of the command line, as in this example:
33 % hls -ltr /usr/etc +RTS -H5m
35 If you absolutely positively want all the rest of the options in a
36 command line to go to the program (and not the RTS), use a
37 \tr{--RTS}\index{--RTS option}.
39 As always, for RTS options that take \tr{<size>}s: If the last
40 character of \tr{size} is a K or k, multiply by 1000; if an M or m, by
41 1,000,000; if a G or G, by 1,000,000,000. (And any wraparound in the
42 counters is {\em your} fault!)
44 Giving a \tr{+RTS -f}\index{-f RTS option} option will print out the
45 RTS options actually available in your program (which vary, depending
48 %************************************************************************
50 \subsection{Generally-available RTS options}
51 \index{RTS options, general}
53 %************************************************************************
55 The most important RTS options are:
58 \index{-H<size> RTS option}
59 Set the heap size to \pl{<size>} bytes
63 \index{-K<size> RTS option}
64 Set the stack size to \pl{<size>} bytes [default: 64K].
65 For concurrent/parallel programs, it is the stack size of the main
66 thread; generally speaking, c/p stacks are in heap.
68 Note: if your program seems to be consuming infinite stack space, it
69 is probably in a loop :-) Of course, if stacks are in the heap, make
70 that infinite {\em heap} space...
72 \item[\tr{-s<file>} or \tr{-S<file>}:]
73 \index{-S<file> RTS option}
74 \index{-s<file> RTS option}
75 Write modest (\tr{-s}) or verbose (\tr{-S}) garbage-collector
76 statistics into file \pl{<file>}. The default \pl{<file>} is
77 \pl{<program>}\tr{.stat}. The \pl{<file>} \tr{stderr} is treated
78 specially, with the output really being sent to \tr{stderr}.
80 %Note that \tr{stdout} is flushed before each garbage collection so the
81 %interleaving of \tr{stdout} and the garbage collection statistics will
84 %Note that the same program will typically allocate more space with a
85 %generational collector than with a non-generational collector.
86 The amount of heap allocation will typically increase as the total heap
87 size is reduced. The reason for this odd behaviour is that updates of
88 promoted-to-old-generation objects may require the extra allocation of a new-generation
89 object to ensure that there are never any pointers from the old
90 generation to the new generation.
92 For some garbage collectors (not including the default one, sadly),
93 you can convert the \tr{-S} output into a residency graph (in
94 PostScript), using the \tr{stat2resid}\index{stat2resid} utility in
95 the GHC distribution (\tr{ghc/utils/stat2resid}).
99 Normally, the garbage collector black-holes closures which are being
100 evaluated, as a space-saving measure. That's exactly what you want
101 for ordinary Haskell programs.
103 When signal handlers are present, however, a computation may be
104 abandoned prematurely, leaving black holes behind. If the signal
105 handler shares one of these black-holed closures, disaster can result.
106 Use the \tr{-N} option to prevent black-holing by the garbage
107 collector if you suspect that your signal handlers may share {\em any}
108 subexpressions with the top-level computation. Expect your heap usage
109 to increase, since the lifetimes of some closures may be extended.
112 %************************************************************************
114 \subsection{RTS options to control the garbage-collector}
115 \index{RTS options, garbage-collection}
117 %************************************************************************
119 Besides the \tr{-H} (set heap size) and \tr{-S}/\tr{-s} (GC stats) RTS
120 options, there are several options to give you precise control over
125 \index{-M<n> RTS option}
126 Minimum \% \pl{<n>} of heap which must be available for allocation.
129 \item[\tr{-A<size>}:]
130 \index{-A<size> RTS option}
131 Sets a limit on the size of the allocation area for generational
132 garbage collection to \pl{<size>} bytes (\tr{-A} gives default of 64k). If
133 a negative size is given the size of the allocation is fixed to
134 -\pl{<size>}. For non-generational collectors, it fixes the minimum
135 heap which must be available after a collection, overriding the
136 \tr{-M<n>} RTS option.
138 \item[\tr{-G<size>}:]
139 \index{-G<size> RTS option}
140 Sets the percentage of free space to be promoted before a major
141 collection is invoked to \pl{<size>}\%. The default is 66\%. If a
142 negative size is given it fixes the size of major generation threshold
143 to -\pl{<size>} bytes.
146 \index{-F2s RTS option}
147 Forces a program compiled for generational GC to use two-space copying
148 collection. The two-space collector may outperform the generational
149 collector for programs which have a very low heap residency. It can
150 also be used to generate a statistics file from which a basic heap
151 residency profile can be produced (see Section \ref{stat2resid}).
153 There will still be a small execution overhead imposed by the
154 generational compilation as the test for old generation updates will
155 still be executed (of course none will actually happen). This
156 overhead is typically less than 1\%.
158 \item[\tr{-j<size>}:]
159 \index{-j<size> RTS option}
160 Force a major garbage collection every \pl{<size>} bytes. (Normally
161 used because you're keen on getting major-GC stats, notably heap residency
165 %************************************************************************
167 \subsection{RTS options for profiling and Concurrent/Parallel Haskell}
169 %************************************************************************
171 The RTS options related to profiling are described in
172 \Sectionref{prof-rts-options};
173 and those for concurrent/parallel stuff, in \Sectionref{parallel-rts-opts}.
175 %************************************************************************
177 \subsection{RTS options for hackers, debuggers, and over-interested souls}
178 \index{RTS options, hacking/debugging}
180 %************************************************************************
182 These RTS options might be used (a)~to avoid a GHC bug, (b)~to see
183 ``what's really happening'', or (c)~because you feel like it. Not
184 recommended for everyday use!
188 \index{-B RTS option}
189 Sound the bell at the start of each (major) garbage collection.
190 [Why anyone would do this, I cannot imagine.]
193 Use the ``debugging mini-interpreter'' with sanity-checking; you have
194 to have an appropriately-compiled version of the prelude, etc.
195 Goes together nicely with GDB (GNU debugger)...
198 \item[\tr{-r<file>}:]
199 \index{-r <file> RTS option}
200 Produce ``ticky-ticky'' statistics at the end of the program run.
201 The \tr{<file>} business works just like on the \tr{-S} RTS option (above).
203 ``Ticky-ticky'' statistics are counts of various program actions
204 (updates, enters, etc.)
205 The program must have been compiled using
206 \tr{-fstg-reduction-counts}\index{-fstg-reduction-counts option}
207 (a.k.a. ``ticky-ticky profiling''), and, for it to be really useful,
208 linked with suitable system libraries. Not a trivial undertaking:
209 consult the installation guide on how to set things up for
210 easy ``ticky-ticky'' profiling.
213 \index{-T RTS option}
214 An RTS debugging flag; varying quantities of output depending on which bits
215 are set in \pl{<num>}.
218 \index{-Z RTS option}
219 Turn {\em off} ``update-frame squeezing'' at garbage-collection time.
220 (There's no particularly good reason to turn it off.)
223 %************************************************************************
225 \subsection[rts-hooks]{``Hooks'' to change RTS failure messages}
229 %************************************************************************
231 GHC lets you exercise rudimentary control over the messages printed
232 when the runtime system ``blows up,'' e.g., on stack overflow.
234 Simply write some of the following procedures in C and then make sure
235 they get linked in, in preference to those in the RTS library:
237 \item[\tr{void ErrorHdrHook (FILE *)}:]
239 What's printed out before the message from \tr{error}.
241 \item[\tr{void OutOfHeapHook (unsigned long, unsigned long)}:]
242 \index{OutOfHeapHook}
243 The heap-overflow message.
245 \item[\tr{void StackOverflowHook (long int)}:]
246 \index{StackOverflowHook}
247 The stack-overflow message.
249 \item[\tr{void MallocFailHook (long int)}:]
250 \index{MallocFailHook}
251 The message printed if \tr{malloc} fails.
253 \item[\tr{void PatErrorHdrHook (FILE *)}:]
254 \index{PatErrorHdrHook}
255 The message printed if a pattern-match fails (the failures
256 that were not handled by the Haskell programmer).
258 \item[\tr{void PreTraceHook (FILE *)}:]
260 What's printed out before a \tr{trace} message.
262 \item[\tr{void PostTraceHook (FILE *)}:]
263 \index{PostTraceHook}
264 What's printed out after a \tr{trace} message.
267 For example, here is the ``hooks'' code used by GHC itself:
270 #define W_ unsigned long int
277 fprintf(where, "\n"); /* no "Fail: " */
281 OutOfHeapHook (request_size, heap_size)
282 W_ request_size; /* in bytes */
283 W_ heap_size; /* in bytes */
285 fprintf(stderr, "GHC's heap exhausted;\nwhile trying to
286 allocate %lu bytes in a %lu-byte heap;\nuse the `-H<size>'
287 option to increase the total heap size.\n",
293 StackOverflowHook (stack_size)
294 I_ stack_size; /* in bytes */
296 fprintf(stderr, "GHC stack-space overflow: current size
297 %ld bytes.\nUse the `-K<size>' option to increase it.\n",
302 PatErrorHdrHook (where)
305 fprintf(where, "\n*** Pattern-matching error within GHC!\n\n
306 This is a compiler bug; please report it to
307 glasgow-haskell-bugs@dcs.glasgow.ac.uk.\n\nFail: ");
314 fprintf(where, "\n"); /* not "Trace On" */
318 PostTraceHook (where)
321 fprintf(where, "\n"); /* not "Trace Off" */
325 %************************************************************************
327 %\subsection[rts-control-shell-scripts]{Hiding the runtime-control mess with a shell script}
329 %************************************************************************