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 5.11, “Using Concurrent Haskell”) and the heap profiling timer Section 6.4.1, “RTS options for heap profiling”. Also, the time profiler uses the RTS timer signal directly to record time profiling samples.

Normally, setting the -V option directly is not necessary: the resolution of the RTS timer is adjusted automatically if a short interval is requested with the -C or -i options. However, setting -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.

[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).

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 -H 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 -G1 option.

[Default: 30] Automatically enable compacting collection when the live data exceeds n% of the maximum heap size (see the -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 -Msize.

[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 -Hsize than to increase -Ffactor.

The -F setting will be automatically reduced by the garbage collector when the maximum heap size (the -Msize 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 +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).

[Default: 1] [new in GHC 6.10] Set the number of threads to use for garbage collection. This option is only accepted when the program was linked with the -threaded option; see Section 5.10.7, “Options affecting linking”.

The garbage collector is able to work in parallel when given more than one OS thread. Experiments have shown that this usually results in a performance improvement given 3 cores or more; with 2 cores it may or may not be beneficial, depending on the workload. Bigger heaps work better with parallel GC, so set your -H value high (3 or more times the maximum residency). Look at the timing stats with +RTS -s to see whether you're getting any benefit from parallel GC or not. If you find parallel GC is significantly slower (in elapsed time) than sequential GC, please report it as a bug.

This value is set automatically when the -N option is used, so the only reason to use -g would be if you wanted to use a different number of threads for GC than for execution. For example, if your program is strictly single-threaded but you still want to benefit from parallel GC, then it might make sense to use -g rather than -N.

[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 -Hsize. For improving GC performance, using -Hsize is usually a better bet than -Asize.

(default: 0.3) In the threaded and SMP versions of the RTS (see -threaded, Section 5.10.7, “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 -Iseconds option. Specifying -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 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.

Minimum % n of heap which must be available for allocation. The default is 3%.

[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 -F parameter will be reduced in order to avoid exceeding the maximum heap size.

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 -Rghc-timing option, -s produces a more detailed summary at the end of the program, and -S additionally produces information about each and every garbage collection.

The output is placed in file. If file is omitted, then the output is sent to stderr.

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:

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 profiling):

      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

  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 -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:

Generates a basic heap profile, in the file prog.hp. To produce the heap profile graph, use hp2ps (see Section 6.5, “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 (e.g. FUN, THUNK). To get a more detailed profile, use the full profiling support (Chapter 6, Profiling).

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; varying quantities of output depending on which bits are set in num. Only works if the RTS was compiled with the DEBUG option.

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 (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 Section 6.7, “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 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 cc_i is a cost centre in the program (see Section 6.1, “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 function.

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.)

The heap-overflow message.

The stack-overflow message.

The message printed if malloc fails.