runtime control of Haskell programs running, compiled program RTS options To make an executable program, the GHC system compiles your code and then links it with a non-trivial runtime system (RTS), which handles storage management, profiling, etc. You have some control over the behaviour of the RTS, by giving special command-line arguments to your program. When your Haskell program starts up, its RTS extracts command-line arguments bracketed between +RTS+RTS option and -RTS-RTS option as its own. For example: % ./a.out -f +RTS -p -S -RTS -h foo bar The RTS will snaffle -p -S for itself, and the remaining arguments -f -h foo bar will be handed to your program if/when it calls System.getArgs. No -RTS option is required if the runtime-system options extend to the end of the command line, as in this example: % hls -ltr /usr/etc +RTS -A5m If you absolutely positively want all the rest of the options in a command line to go to the program (and not the RTS), use a --RTS--RTS option. As always, for RTS options that take <size>s: If the last character of size is a K or k, multiply by 1000; if an M or m, by 1,000,000; if a G or G, by 1,000,000,000. (And any wraparound in the counters is your fault!) Giving a +RTS -f-f RTS option option will print out the RTS options actually available in your program (which vary, depending on how you compiled). NOTE: to send RTS options to the compiler itself, you need to prefix the option with -optCrts, eg. to increase the maximum heap size for a compilation to 128M, you would add -optCrts-M128m to the command line. The compiler understands some options directly without needing -optCrts: these are -H and -K. RTS options, garbage-collection There are several options to give you precise control over garbage collection. Hopefully, you won't need any of these in normal operation, but there are several things that can be tweaked for maximum performance. -A<size>: -A<size> RTS option allocation area, size [Default: 256k] Set the allocation area size used by the garbage collector. The allocation area (actually generation 0 step 0) is fixed and is never resized (unless you use -H, below). Increasing the allocation area size may or may not give better performance (a bigger allocation area means worse cache behaviour but fewer garbage collections and less promotion). With only 1 generation (-G1) the -A option specifies the minimum allocation area, since the actual size of the allocation area will be resized according to the amount of data in the heap (see -F, below). -F<factor>: -F<factor> RTS option heap size, factor [Default: 2] This option controls the amount of memory reserved for the older generations (and in the case of a two space collector the size of the allocation area) as a factor of the amount of live data. For example, if there was 2M of live data in the oldest generation when we last collected it, then by default we'll wait until it grows to 4M before collecting it again. The default seems to work well here. If you have plenty of memory, it is usually better to use -H<size> than to increase -F<factor>. The -F setting will be automatically reduced by the garbage collector when the maximum heap size (the -M<size> setting) is approaching. -G<generations>: -G<generations> RTS option generations, number of [Default: 2] Set the number of generations used by the garbage collector. The default of 2 seems to be good, but the garbage collector can support any number of generations. Anything larger than about 4 is probably not a good idea unless your program runs for a long time, because the oldest generation will never get collected. Specifying 1 generation with +RTS -G1 gives you a simple 2-space collector, as you would expect. In a 2-space collector, the -A option (see above) specifies the minimum allocation area size, since the allocation area will grow with the amount of live data in the heap. In a multi-generational collector the allocation area is a fixed size (unless you use the -H option, see below). -H<size>: -H<size> RTS option heap size, suggested [Default: 0] This option provides a "suggested heap size" for the garbage collector. The garbage collector will use about this much memory until the program residency grows and the heap size needs to be expanded to retain reasonable performance. By default, the heap will start small, and grow and shrink as necessary. This can be bad for performance, so if you have plenty of memory it's worthwhile supplying a big -H<size>. For improving GC performance, using -H<size> is usually a better bet than -A<size>. -k<size>: -k<size> RTS option stack, minimum size [Default: 1k] Set the initial stack size for new threads. Thread stacks (including the main thread's stack) live on the heap, and grow as required. The default value is good for concurrent applications with lots of small threads; if your program doesn't fit this model then increasing this option may help performance. The main thread is normally started with a slightly larger heap to cut down on unnecessary stack growth while the program is starting up. -K<size>: -K<size> RTS option stack, maximum size [Default: 1M] Set the maximum stack size for an individual thread to <size> bytes. This option is there purely to stop the program eating up all the available memory in the machine if it gets into an infinite loop. -m<n>: -m<n> RTS option Minimum % <n> of heap which must be available for allocation. The default is 3%. heap, minimum free -M<size>: -M<size> RTS option heap size, maximum [Default: 64M] Set the maximum heap size to <size> bytes. The heap normally grows and shrinks according to the memory requirements of the program. The only reason for having this option is to stop the heap growing without bound and filling up all the available swap space, which at the least will result in the program being summarily killed by the operating system. -s<file> or -S<file>: -S<file> RTS option -s<file> RTS option Write modest (-s) or verbose (-S) garbage-collector statistics into file <file>. The default <file> is <program>@.stat. The <file> stderr is treated specially, with the output really being sent to stderr. This option is useful for watching how the storage manager adjusts the heap size based on the current amount of live data. The RTS options related to profiling are described in ; and those for concurrent/parallel stuff, in . RTS options, hacking/debugging These RTS options might be used (a) to avoid a GHC bug, (b) to see “what's really happening”, or (c) because you feel like it. Not recommended for everyday use! -B: -B RTS option Sound the bell at the start of each (major) garbage collection. Oddly enough, people really do use this option! Our pal in Durham (England), Paul Callaghan, writes: “Some people here use it for a variety of purposes—honestly!—e.g., confirmation that the code/machine is doing something, infinite loop detection, gauging cost of recently added code. Certain people can even tell what stage [the program] is in by the beep pattern. But the major use is for annoying others in the same office…” -r<file>: -r <file> RTS option ticky ticky profiling Produce “ticky-ticky” statistics at the end of the program run. The <file> business works just like on the -S RTS option (above). “Ticky-ticky” statistics are counts of various program actions (updates, enters, etc.) The program must have been compiled using -ticky-ticky option (a.k.a. “ticky-ticky profiling”), and, for it to be really useful, linked with suitable system libraries. Not a trivial undertaking: consult the installation guide on how to set things up for easy “ticky-ticky” profiling. For more information, see . -D<num>: -D RTS option An RTS debugging flag; varying quantities of output depending on which bits are set in <num>. Only works if the RTS was compiled with the DEBUG option. -Z: -Z RTS option Turn off “update-frame squeezing” at garbage-collection time. (There's no particularly good reason to turn it off, except to ensure the accuracy of certain data collected regarding thunk entry counts.) hooks, RTS RTS hooks RTS behaviour, changing GHC lets you exercise rudimentary control over the RTS settings for any given program, by compiling in a “hook” that is called by the run-time system. The RTS contains stub definitions for all these hooks, but by writing your own version and linking it on the GHC command line, you can override the defaults. Owing to the vagaries of DLL linking, these hooks don't work under Windows. The function defaultsHookdefaultHook lets you change various RTS options. The commonest use for this is to give your program a default heap and/or stack size that is greater than the default. For example, to set -H8m -K1m: #include "Rts.h" #include "RtsFlags.h" void defaultsHook (void) { RTSflags.GcFlags.stksSize = 1000002 / sizeof(W_); RTSflags.GcFlags.heapSize = 8000002 / sizeof(W_); } Don't use powers of two for heap/stack sizes: these are more likely to interact badly with direct-mapped caches. The full set of flags is defined in ghc/rts/RtsFlags.h the the GHC source tree. You can also change the messages printed when the runtime system “blows up,” e.g., on stack overflow. The hooks for these are as follows: void ErrorHdrHook (FILE *): ErrorHdrHook What's printed out before the message from error. void OutOfHeapHook (unsigned long, unsigned long): OutOfHeapHook The heap-overflow message. void StackOverflowHook (long int): StackOverflowHook The stack-overflow message. void MallocFailHook (long int): MallocFailHook The message printed if malloc fails. void PatErrorHdrHook (FILE *): PatErrorHdrHook The message printed if a pattern-match fails (the failures that were not handled by the Haskell programmer). void PreTraceHook (FILE *): PreTraceHook What's printed out before a trace message. void PostTraceHook (FILE *): PostTraceHook What's printed out after a trace message. For example, here is the “hooks” code used by GHC itself: #include <stdio.h> #define W_ unsigned long int #define I_ long int void ErrorHdrHook (FILE *where) { fprintf(where, "\n"); /* no "Fail: " */ } void OutOfHeapHook (W_ request_size, W_ heap_size) /* both sizes in bytes */ { fprintf(stderr, "GHC's heap exhausted;\nwhile trying to allocate %lu bytes in a %lu-byte heap;\nuse the `-H<size>' option to increase the total heap size.\n", request_size, heap_size); } void StackOverflowHook (I_ stack_size) /* in bytes */ { fprintf(stderr, "GHC stack-space overflow: current size %ld bytes.\nUse the `-K<size>' option to increase it.\n", stack_size); } void PatErrorHdrHook (FILE *where) { fprintf(where, "\n*** Pattern-matching error within GHC!\n\n This is a compiler bug; please report it to glasgow-haskell-bugs@haskell.org.\n\nFail: "); } void PreTraceHook (FILE *where) { fprintf(where, "\n"); /* not "Trace On" */ } void PostTraceHook (FILE *where) { fprintf(where, "\n"); /* not "Trace Off" */ }