Running a compiled programruntime control of Haskell programsrunning, compiled programRTS optionsTo 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
and
as its own. For example:
% ./a.out -f +RTS -p -S -RTS -h foo bar
The RTS will snaffle
for itself, and the remaining arguments -f -h foo bar
will be handed to your program if/when it calls
System.getArgs.No 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
.As always, for RTS options that take
sizes: 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 -fRTS option option
will print out the RTS options actually available in your program
(which vary, depending on how you compiled).NOTE: since GHC is itself compiled by GHC, you can change RTS
options in the compiler using the normal
+RTS ... -RTS
combination. eg. to increase the maximum heap
size for a compilation to 128M, you would add
+RTS -M128m -RTS
to the command line.Setting global RTS optionsRTS optionsfrom the environmentenvironment variablefor
setting RTS optionsRTS options are also taken from the environment variable
GHCRTSGHCRTS. For example, to set the maximum heap size
to 128M for all GHC-compiled programs (using an
sh-like shell):
GHCRTS='-M128m'
export GHCRTS
RTS options taken from the GHCRTS environment
variable can be overriden by options given on the command
line.RTS options to control the garbage collectorgarbage collectoroptionsRTS optionsgarbage collectionThere 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.sizeRTS optionallocation 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 , 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 () the
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
, below).RTS optiongarbage collectioncompactingcompacting garbage collectionUse a compacting algorithm for collecting the oldest
generation. By default, the oldest generation is collected
using a copying algorithm; this option causes it to be
compacted in-place instead. The compaction algorithm is
slower than the copying algorithm, but the savings in memory
use can be considerable.For a given heap size (using the
option), compaction can in fact reduce the GC cost by
allowing fewer GCs to be performed. This is more likely
when the ratio of live data to heap size is high, say
>30%.NOTE: compaction doesn't currently work when a single
generation is requested using the
option.n[Default: 30] Automatically enable
compacting collection when the live data exceeds
n% of the maximum heap size
(see the option). Note that the maximum
heap size is unlimited by default, so this option has no
effect unless the maximum heap size is set with
size. factorRTS optionheap 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
size than to
increase
factor.The setting will be automatically
reduced by the garbage collector when the maximum heap size
(the size
setting) is approaching.generationsRTS optiongenerations, 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 hardly ever get collected.Specifying 1 generation with
gives you a simple 2-space collector, as you would expect.
In a 2-space collector, the 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 option, see below).sizeRTS optionheap 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
size. For
improving GC performance, using
size is
usually a better bet than
size.sizeRTS optionstack, 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.sizeRTS optionstack, 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.nRTS optionheap, minimum freeMinimum % n of heap
which must be available for allocation. The default is
3%.sizeRTS optionheap size, maximum[Default: unlimited] 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.The maximum heap size also affects other garbage
collection parameters: when the amount of live data in the
heap exceeds a certain fraction of the maximum heap size,
compacting collection will be automatically enabled for the
oldest generation, and the parameter
will be reduced in order to avoid exceeding the maximum heap
size.filefileRTS optionRTS optionWrite modest () or verbose
() garbage-collector statistics into file
file. The default
file is
program.stat. The
filestderr
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.RTS optionWrite a one-line GC stats summary after running the
program. This output is in the same format as that produced
by the option.RTS options for profiling and Concurrent/Parallel HaskellThe RTS options related to profiling are described in ; and those for concurrent/parallel
stuff, in .RTS options for hackers, debuggers, and over-interested
soulsRTS options, hacking/debuggingThese 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!RTS optionSound 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…”num-DRTS optionAn RTS debugging flag; varying quantities of output
depending on which bits are set in
num. Only works if the RTS was
compiled with the option.fileRTS optionticky ticky profilingprofilingticky tickyProduce “ticky-ticky” statistics at the
end of the program run. The file
business works just like on the RTS
option (above).“Ticky-ticky” statistics are counts of
various program actions (updates, enters, etc.) The program
must have been compiled using
(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 .RTS
option(Only available when the program is compiled for
profiling.) When an exception is raised in the program,
this option causes the current cost-centre-stack to be
dumped to stderr.This can be particularly useful for debugging: if your
program is complaining about a head []
error and you haven't got a clue which bit of code is
causing it, compiling with -prof
-auto-all and running with +RTS -xc
-RTS will tell you exactly the call stack at the
point the error was raised.The output contains one line for each exception raised
in the program (the program might raise and catch several
exceptions during its execution), where each line is of the
form:
< cc1, ..., ccn >
each cci is
a cost centre in the program (see ), and the sequence represents the
“call stack” at the point the exception was
raised. The leftmost item is the innermost function in the
call stack, and the rightmost item is the outermost
function.RTS
optionTurn 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” to change RTS behaviourhooksRTSRTS hooksRTS behaviour, changingGHC 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 when the program is built dynamically.The hook ghc_rts_optsghc_rts_optslets you set RTS
options permanently for a given program. A common 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 -H128m
-K1m, place the following definition in a C source
file:
char *ghc_rts_opts = "-H128m -K1m";
Compile the C file, and include the object file on the
command line when you link your Haskell program.These flags are interpreted first, before any RTS flags from
the GHCRTS environment variable and any flags
on the command line.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 *)ErrorHdrHookWhat's printed out before the message from
error.void OutOfHeapHook (unsigned long, unsigned long)OutOfHeapHookThe heap-overflow message.void StackOverflowHook (long int)StackOverflowHookThe stack-overflow message.void MallocFailHook (long int)MallocFailHookThe message printed if malloc
fails.void PatErrorHdrHook (FILE *)PatErrorHdrHookThe message printed if a pattern-match fails (the
failures that were not handled by the Haskell
programmer).void PreTraceHook (FILE *)PreTraceHookWhat's printed out before a trace
message.void PostTraceHook (FILE *)PostTraceHookWhat's printed out after a trace
message.For examples of the use of these hooks, see GHC's own
versions in the file
ghc/compiler/parser/hschooks.c in a GHC
source tree.