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, thread scheduling, profiling, and so on.
The RTS has a lot of options to control its behaviour. For example, you can change the context-switch interval, the default size of the heap, and enable heap profiling. These options can be passed to the runtime system in a variety of different ways; the next section (Section 4.16.1, “Setting RTS options”) describes the various methods, and the following sections describe the RTS options themselves.
There are four ways to set RTS options:
on the command line between
+RTS ... -RTS, when running the program
(Section 18.104.22.168, “Setting RTS options on the command line”)
at compile-time, using
(Section 22.214.171.124, “Setting RTS options at compile time”)
with the environment variable
(Section 126.96.36.199, “Setting RTS options with the
by overriding “hooks” in the runtime system (Section 188.8.131.52, ““Hooks” to change RTS behaviour”)
If you set the
-rtsopts flag appropriately
when linking (see Section 4.11.6, “Options affecting linking”), you can
give RTS options on the command line when running your
$ ghc prog.hs -rtsopts [1 of 1] Compiling Main ( prog.hs, prog.o ) Linking prog ... $ ./prog -f +RTS -H32m -S -RTS -h foo bar
The RTS will
-S for itself,
and the remaining arguments
-f -h foo bar
will be available to your program if/when it calls
-RTS option is required if the
runtime-system options extend to the end of the command line, as in
% hls -ltr /usr/etc +RTS -A5m
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
NOTE: since GHC is itself compiled by GHC, you can change RTS
options in the compiler using the normal
+RTS ... -RTS
combination. eg. to set the maximum heap
size for a compilation to 128M, you would add
+RTS -M128m -RTS
to the command line.
GHC lets you change the default RTS options for a program at
compile time, using the
flag (Section 4.11.6, “Options affecting linking”). For example, to
-H128m -K64m, link
-rtsopts flag is set to
something other than
none when linking,
RTS options are also taken from the environment variable
GHCRTS. For example, to set the maximum heap size
to 2G for all GHC-compiled programs (using an
GHCRTS='-M2G' export GHCRTS
RTS options taken from the
variable can be overridden by options given on the command
Tip: setting something like
in your environment is a handy way to avoid Haskell programs
growing beyond the real memory in your machine, which is
easy to do by accident and can cause the machine to slow to
a crawl until the OS decides to kill the process (and you
hope it kills the right one).
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 when the program is built dynamically.
ghc_rts_optslets you set RTS
options permanently for a given program, in the same way as the
-with-rtsopts linker option does. 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
-K1m, place the following definition in a C source
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
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:
For examples of the use of these hooks, see GHC's own
versions in the file
ghc/compiler/parser/hschooks.c in a GHC
Sets the interval that the RTS clock ticks at. The runtime uses a single timer signal to count ticks; this timer signal is used to control the context switch timer (Section 4.13, “Using Concurrent Haskell”) and the heap profiling timer Section 5.4.1, “RTS options for heap profiling”. Also, the time profiler uses the RTS timer signal directly to record time profiling samples.
Normally, setting the
directly is not necessary: the resolution of the RTS timer is
adjusted automatically if a short interval is requested with
-V is required in order to
increase the resolution of the time profiler.
Using a value of zero disables the RTS clock completely, and has the effect of disabling timers that depend on it: the context switch timer and the heap profiling timer. Context switches will still happen, but deterministically and at a rate much faster than normal. Disabling the interval timer is useful for debugging, because it eliminates a source of non-determinism at runtime.
If yes (the default), the RTS installs signal handlers to catch things like ctrl-C. This option is primarily useful for when you are using the Haskell code as a DLL, and want to set your own signal handlers.
Note that even
--install-signal-handlers=no, the RTS
interval timer signal is still enabled. The timer signal
is either SIGVTALRM or SIGALRM, depending on the RTS
configuration and OS capabilities. To disable the timer
signal, use the
-V0 RTS option (see
WARNING: this option is for working around memory
allocation problems only. Do not use unless GHCi fails
with a message like “
failed to mmap() memory below 2Gb”. If you need to use this option to get GHCi working
on your machine, please file a bug.
On 64-bit machines, the RTS needs to allocate memory in the
low 2Gb of the address space. Support for this across
different operating systems is patchy, and sometimes fails.
This option is there to give the RTS a hint about where it
should be able to allocate memory in the low 2Gb of the
address space. For example,
-RTS would hint that the RTS should allocate
starting at the 0.5Gb mark. The default is to use the OS's
built-in support for allocating memory in the low 2Gb if
MAP_32BIT on Linux), or
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.
[Default: 512k] 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
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 (
-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
Use 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
NOTE: compaction doesn't currently work when a single
generation is requested using the
[Default: 30] Automatically enable
compacting collection when the live data exceeds
n% of the maximum heap size
-M 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
[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
-F setting will be automatically
reduced by the garbage collector when the maximum heap size
setting) is approaching.
[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
-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
-H option, see below).
[New in GHC 6.12.1] [Default: 0]
Use parallel GC in
gen and higher.
gen turns off the
parallel GC completely, reverting to sequential GC.
The default parallel GC settings are usually suitable
for parallel programs (i.e. those
par, Strategies, or with multiple
threads). However, it is sometimes beneficial to enable
the parallel GC for a single-threaded sequential program
too, especially if the program has a large amount of heap
data and GC is a significant fraction of runtime. To use
the parallel GC in a sequential program, enable the
parallel runtime with a suitable
option, and additionally it might be beneficial to
restrict parallel GC to the old generation
[New in GHC 6.12.1] [Default: 1] Use
load-balancing in the parallel GC in
gen and higher.
Load-balancing shares out the work of GC between the
available cores. This is a good idea when the heap is
large and we need to parallelise the GC work, however it
is also pessimal for the short young-generation
collections in a parallel program, because it can harm
locality by moving data from the cache of the CPU where is
it being used to the cache of another CPU. Hence the
default is to do load-balancing only in the
old-generation. In fact, for a parallel program it is
sometimes beneficial to disable load-balancing entirely
[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
improving GC performance, using
usually a better bet than
(default: 0.3) In the threaded and SMP versions of the RTS (see
-threaded, Section 4.11.6, “Options affecting linking”), a
major GC is automatically performed if the runtime has been idle
(no Haskell computation has been running) for a period of time.
The amount of idle time which must pass before a GC is performed is
set by the
-I0 disables the idle GC.
For an interactive application, it is probably a good idea to use the idle GC, because this will allow finalizers to run and deadlocked threads to be detected in the idle time when no Haskell computation is happening. Also, it will mean that a GC is less likely to happen when the application is busy, and so responsiveness may be improved. However, if the amount of live data in the heap is particularly large, then the idle GC can cause a significant delay, and too small an interval could adversely affect interactive responsiveness.
This is an experimental feature, please let us know if it causes problems and/or could benefit from further tuning.
[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.
[Default: 8M] Set the maximum stack size for
an individual thread to
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.
n of heap
which must be available for allocation. The default is
[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
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
will be reduced in order to avoid exceeding the maximum heap
These options produce runtime-system statistics, such
as the amount of time spent executing the program and in the
garbage collector, the amount of memory allocated, the
maximum size of the heap, and so on. The three
variants give different levels of detail:
-t produces a single line of output in the
same format as GHC's
-s produces a more detailed summary at the
end of the program, and
produces information about each and every garbage
The output is placed in
file is omitted, then the output
is sent to
If you use the
-t flag then, when your
program finishes, you will see something like this:
<<ghc: 36169392 bytes, 69 GCs, 603392/1065272 avg/max bytes residency (2 samples), 3M in use, 0.00 INIT (0.00 elapsed), 0.02 MUT (0.02 elapsed), 0.07 GC (0.07 elapsed) :ghc>>
This tells you:
The total number of bytes allocated by the program over the whole run.
The total number of garbage collections performed.
The average and maximum "residency", which is the amount of
live data in bytes. The runtime can only determine the
amount of live data during a major GC, which is why the
number of samples corresponds to the number of major GCs
(and is usually relatively small). To get a better picture
of the heap profile of your program, use
-hT RTS option
(Section 4.16.5, “RTS options for profiling”).
The peak memory the RTS has allocated from the OS.
The amount of CPU time and elapsed wall clock time while initialising the runtime system (INIT), running the program itself (MUT, the mutator), and garbage collecting (GC).
You can also get this in a more future-proof, machine readable
[("bytes allocated", "36169392") ,("num_GCs", "69") ,("average_bytes_used", "603392") ,("max_bytes_used", "1065272") ,("num_byte_usage_samples", "2") ,("peak_megabytes_allocated", "3") ,("init_cpu_seconds", "0.00") ,("init_wall_seconds", "0.00") ,("mutator_cpu_seconds", "0.02") ,("mutator_wall_seconds", "0.02") ,("GC_cpu_seconds", "0.07") ,("GC_wall_seconds", "0.07") ]
If you use the
-s flag then, when your
program finishes, you will see something like this (the exact
details will vary depending on what sort of RTS you have, e.g.
you will only see profiling data if your RTS is compiled for
36,169,392 bytes allocated in the heap 4,057,632 bytes copied during GC 1,065,272 bytes maximum residency (2 sample(s)) 54,312 bytes maximum slop 3 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 67 collections, 0 parallel, 0.04s, 0.03s elapsed Generation 1: 2 collections, 0 parallel, 0.03s, 0.04s elapsed SPARKS: 359207 (557 converted, 149591 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 0.01s ( 0.02s elapsed) GC time 0.07s ( 0.07s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.08s ( 0.09s elapsed) %GC time 89.5% (75.3% elapsed) Alloc rate 4,520,608,923 bytes per MUT second Productivity 10.5% of total user, 9.1% of total elapsed
The "bytes allocated in the heap" is the total bytes allocated by the program over the whole run.
GHC uses a copying garbage collector by default. "bytes copied during GC" tells you how many bytes it had to copy during garbage collection.
The maximum space actually used by your program is the "bytes maximum residency" figure. This is only checked during major garbage collections, so it is only an approximation; the number of samples tells you how many times it is checked.
The "bytes maximum slop" tells you the most space that is ever wasted due to the way GHC allocates memory in blocks. Slop is memory at the end of a block that was wasted. There's no way to control this; we just like to see how much memory is being lost this way.
The "total memory in use" tells you the peak memory the RTS has allocated from the OS.
Next there is information about the garbage collections done. For each generation it says how many garbage collections were done, how many of those collections were done in parallel, the total CPU time used for garbage collecting that generation, and the total wall clock time elapsed while garbage collecting that generation.
SPARKS statistic refers to the
Control.Parallel.par and related
functionality in the program. Each spark represents a call
par; a spark is "converted" when it is
executed in parallel; and a spark is "pruned" when it is
found to be already evaluated and is discarded from the pool
by the garbage collector. Any remaining sparks are
discarded at the end of execution, so "converted" plus
"pruned" does not necessarily add up to the total.
Next there is the CPU time and wall clock time elapsed broken down by what the runtime system was doing at the time. INIT is the runtime system initialisation. MUT is the mutator time, i.e. the time spent actually running your code. GC is the time spent doing garbage collection. RP is the time spent doing retainer profiling. PROF is the time spent doing other profiling. EXIT is the runtime system shutdown time. And finally, Total is, of course, the total.
%GC time tells you what percentage GC is of Total. "Alloc rate" tells you the "bytes allocated in the heap" divided by the MUT CPU time. "Productivity" tells you what percentage of the Total CPU and wall clock elapsed times are spent in the mutator (MUT).
-S flag, as well as giving the same
output as the
-s flag, prints information
about each GC as it happens:
Alloc Copied Live GC GC TOT TOT Page Flts bytes bytes bytes user elap user elap 528496 47728 141512 0.01 0.02 0.02 0.02 0 0 (Gen: 1) [...] 524944 175944 1726384 0.00 0.00 0.08 0.11 0 0 (Gen: 0)
For each garbage collection, we print:
How many bytes we allocated this garbage collection.
How many bytes we copied this garbage collection.
How many bytes are currently live.
How long this garbage collection took (CPU time and elapsed wall clock time).
How long the program has been running (CPU time and elapsed wall clock time).
How many page faults occured this garbage collection.
How many page faults occured since the end of the last garbage collection.
Which generation is being garbage collected.
Most profiling runtime options are only available when you compile your program for profiling (see Section 5.2, “Compiler options for profiling”, and Section 5.4.1, “RTS options for heap profiling” for the runtime options). However, there is one profiling option that is available for ordinary non-profiled executables:
Generates a basic heap profile, in the
To produce the heap profile graph,
use hp2ps (see Section 5.5, “hp2ps––heap profile to PostScript”). The basic heap profile is broken down by data
constructor, with other types of closures (functions, thunks,
etc.) grouped into broad categories
get a more detailed profile, use the full profiling
support (Chapter 5, Profiling).
When the program is linked with the
option (Section 4.11.6, “Options affecting linking”), runtime events can
be logged in two ways:
In binary format to a file for later analysis by a variety of tools. One such tool is ThreadScope, which interprets the event log to produce a visual parallel execution profile of the program.
As text to standard output, for debugging purposes.
Log events in binary format to the
flags is a sequence of
zero or more characters indicating which kinds of events
to log. Currently there is only one type
-ls, for scheduler events.
The format of the log file is described by the header
EventLogFormat.h that comes with
GHC, and it can be parsed in Haskell using
library. To dump the contents of
.eventlog file as text, use the
show-ghc-events that comes with
Log events as text to standard output, instead of to
flags are the same as
-l, with the additional
t which indicates that the
each event printed should be preceded by a timestamp value
(in the binary
.eventlog file, all
events are automatically associated with a timestamp).
generate events which are logged using the tracing framework.
By default those events are dumped as text to stdout
-v), but they may instead be stored in
the binary eventlog file by using the
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!
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…”
An RTS debugging flag; only availble if the program was
linked with the
-debug option. Various
x are provided to
enable debug messages and additional runtime sanity checks
in different subsystems in the RTS, for
+RTS -Ds -RTS enables debug
messages from the scheduler.
+RTS -? to find out which
debug flags are supported.
Debug messages will be sent to the binary event log file
instead of stdout if the
-l option is
added. This might be useful for reducing the overhead of
Produce “ticky-ticky” statistics at the
end of the program run (only available if the program was
file business works just like
-S RTS option, above.
For more information on ticky-ticky profiling, see Section 5.7, “Using “ticky-ticky” profiling (for implementors)”.
(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
This can be particularly useful for debugging: if your
program is complaining about a
error and you haven't got a clue which bit of code is
causing it, compiling with
-auto-all and running with
-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 >
a cost centre in the program (see Section 5.1, “Cost centres and cost-centre stacks”), 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
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.)
It is possible to ask the RTS to give some information about
itself. To do this, use the
--info flag, e.g.
$ ./a.out +RTS --info [("GHC RTS", "YES") ,("GHC version", "6.7") ,("RTS way", "rts_p") ,("Host platform", "x86_64-unknown-linux") ,("Host architecture", "x86_64") ,("Host OS", "linux") ,("Host vendor", "unknown") ,("Build platform", "x86_64-unknown-linux") ,("Build architecture", "x86_64") ,("Build OS", "linux") ,("Build vendor", "unknown") ,("Target platform", "x86_64-unknown-linux") ,("Target architecture", "x86_64") ,("Target OS", "linux") ,("Target vendor", "unknown") ,("Word size", "64") ,("Compiler unregisterised", "NO") ,("Tables next to code", "YES") ]
The information is formatted such that it can be read as a
[(String, String)]. Currently the following
fields are present:
Is this program linked against the GHC RTS? (always "YES").
The version of GHC used to compile this program.
The variant (“way”) of the runtime. The
most common values are
rts_thr (threaded runtime, i.e. linked using the
-threaded option) and
(profiling runtime, i.e. linked using the
option). Other variants include
t (ticky-ticky profiling) and
dyn (the RTS is
linked in dynamically, i.e. a shared library, rather than statically
linked into the executable itself). These can be combined,
e.g. you might have
These are the platform the program is compiled to run on.
These are the platform where the program was built on. (That is, the target platform of GHC itself.) Ordinarily this is identical to the target platform. (It could potentially be different if cross-compiling.)
These are the platform where GHC itself was compiled. Again, this would normally be identical to the build and target platforms.
reflecting the word size of the target platform.
Was this program compiled with an “unregistered” version of GHC? (I.e., a version of GHC that has no platform-specific optimisations compiled in, usually because this is a currently unsupported platform.) This value will usually be no, unless you're using an experimental build of GHC.
Tables next to code
Putting info tables directly next to entry code is a useful performance optimisation that is not available on all platforms. This field tells you whether the program has been compiled with this optimisation. (Usually yes, except on unusual platforms.)