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 4.11, “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 -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.

[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: 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 4.10.7, “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.

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.

Write a one-line GC stats summary after running the program. This output is in the same format as that produced by the -Rghc-timing option.

As with -s, the default file is program.stat. The file stderr is treated specially, with the output really being sent to stderr.

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 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 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 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 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.