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 4.17.1, “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 22.214.171.124, “Setting RTS options on the command line”)
at compile-time, using
(Section 126.96.36.199, “Setting RTS options at compile time”)
with the environment variable
(Section 188.8.131.52, “Setting RTS options with the
by overriding “hooks” in the runtime system (Section 184.108.40.206, ““Hooks” to change RTS behaviour”)
If you set the
-rtsopts flag appropriately
when linking (see Section 4.12.6, “Options affecting linking”), you can
give RTS options on the command line when running your
When your Haskell program starts up, the RTS extracts
command-line arguments bracketed between
as its own. For example:
$ 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
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
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 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 4.12.6, “Options affecting linking”). 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
-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 certain 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 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.
You can change the messages printed when the runtime system “blows up,” e.g., on stack overflow. The hooks for these are as follows:
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 4.14, “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
-H as a
-A option. It says: I want to
use at least
size bytes, so use
whatever is left over to increase the
This option does not put a limit on the heap size: the heap may grow beyond the given size as usual.
size is omitted, then
the garbage collector will take the size of the heap at
the previous GC as the
This has the effect of allowing for a
-A value but without increasing
the overall memory requirements of the program. It can be
useful when the default small
-A value is
suboptimal, as it can be in programs that create large
amounts of long-lived data.
(default: 0.3) In the threaded and SMP versions of the RTS (see
-threaded, Section 4.12.6, “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. (Note: this flag used to be
-k, but was renamed
-ki in GHC 7.2.1. The old name is
still accepted for backwards compatibility, but that may
be removed in a future version).
Thread stacks (including the main thread's stack) live on the heap. As the stack grows, new stack chunks are added as required; if the stack shrinks again, these extra stack chunks are reclaimed by the garbage collector. The default initial stack size is deliberately small, in order to keep the time and space overhead for thread creation to a minimum, and to make it practical to spawn threads for even tiny pieces of work.
[Default: 32k] Set the size of “stack
chunks”. When a thread's current stack overflows, a
new stack chunk is created and added to the thread's
stack, until the limit set by
The advantage of smaller stack chunks is that the garbage collector can avoid traversing stack chunks if they are known to be unmodified since the last collection, so reducing the chunk size means that the garbage collector can identify more stack as unmodified, and the GC overhead might be reduced. On the other hand, making stack chunks too small adds some overhead as there will be more overflow/underflow between chunks. The default setting of 32k appears to be a reasonable compromise in most cases.
[Default: 1k] Sets the stack chunk buffer size.
When a stack chunk overflows and a new stack chunk is
created, some of the data from the previous stack chunk is
moved into the new chunk, to avoid an immediate underflow
and repeated overflow/underflow at the boundary. The
amount of stack moved is set by the
Note that to avoid wasting space, this value should
typically be less than 10% of the size of a stack
-kc), because in a chain of stack
chunks, each chunk will have a gap of unused space of this
[Default: 8M] Set the maximum stack size for
an individual thread to
bytes. If the thread attempts to exceed this limit, it will
be send the
This option is there mainly 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 collects the data but produces no output
-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, you should
access the statistics using
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 4.17.5, “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 occurred this garbage collection.
How many page faults occurred since the end of the last garbage collection.
Which generation is being garbage collected.
The RTS options related to concurrency are described in Section 4.14, “Using Concurrent Haskell”, and those for parallelism in Section 4.15.2, “RTS options for SMP parallelism”.
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:
(can be shortened to
-h.) 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 4.12.6, “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 specified, this logs a
default set of events, suitable for use with tools like ThreadScope.
For some special use cases you may want more control over which
events are included. The
flags is a
sequence of zero or more characters indicating which classes of
events to log. Currently there are four classes of events that can
For spark events there are two modes: sampled and fully accurate. There are various events in the life cycle of each spark, usually just creating and running, but there are some more exceptional possibilities. In the sampled mode the number of occurrences of each kind of spark event is sampled at frequent intervals. In the fully accurate mode every spark event is logged individually. The latter has a higher runtime overhead and is not enabled by default.
The initial enabled event classes are 's', 'g' and 'p'. In addition you can disable specific classes, or enable/disable all classes at once:
-l-ag would disable all event classes
-a) except for GC 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
ghc-events-show 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 available 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 5.8, “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 a stack trace 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
-fprof-auto and running with
-RTS will tell you exactly the call stack at the
point the error was raised.
The output contains one report for each exception raised in the program (the program might raise and catch several exceptions during its execution), where each report looks something like this:
*** Exception raised (reporting due to +RTS -xc), stack trace: GHC.List.CAF --> evaluated by: Main.polynomial.table_search, called from Main.polynomial.theta_index, called from Main.polynomial, called from Main.zonal_pressure, called from Main.make_pressure.p, called from Main.make_pressure, called from Main.compute_initial_state.p, called from Main.compute_initial_state, called from Main.CAF ...
The stack trace may often begin with something
GHC.List.CAF; this is
an artifact of GHC's optimiser, which lifts out exceptions
to the top-level where the profiling system assigns them to
the cost centre "CAF". However,
doesn't just print the current stack, it looks deeper and
reports the stack at the time the CAF was evaluated, and it
may report further stacks until a non-CAF stack is found. In
the example above, the next stack (after
evaluated by) contains plenty of information about
what the program was doing when it evaluated
Implementation details aside, the function names in the stack should hopefully give you enough clues to track down the bug.
See also the function
traceStack in the
Debug.Trace for another way to
view call stacks.
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.)