Advice on: sooner, faster, smaller, thriftier
Please advise us of other “helpful hints” that should go here!
Sooner: producing a program more quickly
compiling fasterfaster compilingDon't use or (especially) :
By using them, you are telling GHC that you are willing to suffer
longer compilation times for better-quality code.
GHC is surprisingly zippy for normal compilations without !
Use more memory:
Within reason, more memory for heap space means less garbage
collection for GHC, which means less compilation time. If you use the
option, you'll get a garbage-collector
report. (Again, you can use the cheap-and-nasty option to send the GC stats straight to standard error.)
If it says you're using more than 20% of total time in garbage
collecting, then more memory would help.
If the heap size is approaching the maximum (64M by default), and you
have lots of memory, try increasing the maximum with the
-M<size> option option, e.g.: ghc -c -O
-M1024m Foo.hs.
Increasing the default allocation area size used by the compiler's RTS
might also help: use the -A<size> option
option.
If GHC persists in being a bad memory citizen, please report it as a
bug.
Don't use too much memory!
As soon as GHC plus its “fellow citizens” (other processes on your
machine) start using more than the real memory on your
machine, and the machine starts “thrashing,” the party is
over. Compile times will be worse than terrible! Use something
like the csh-builtin time command to get a report on how many page
faults you're getting.
If you don't know what virtual memory, thrashing, and page faults are,
or you don't know the memory configuration of your machine,
don't try to be clever about memory use: you'll just make
your life a misery (and for other people, too, probably).
Try to use local disks when linking:
Because Haskell objects and libraries tend to be large, it can take
many real seconds to slurp the bits to/from a remote filesystem.
It would be quite sensible to compile on a fast machine using
remotely-mounted disks; then link on a slow machine that had
your disks directly mounted.
Don't derive/use Read unnecessarily:
It's ugly and slow.
GHC compiles some program constructs slowly:
Deeply-nested list comprehensions seem to be one such; in the past,
very large constant tables were bad, too.
We'd rather you reported such behaviour as a bug, so that we can try
to correct it.
The part of the compiler that is occasionally prone to wandering off
for a long time is the strictness analyser. You can turn this off
individually with .
-fno-strictness anti-option
To figure out which part of the compiler is badly behaved, the
option is your friend.
If your module has big wads of constant data, GHC may produce a huge
basic block that will cause the native-code generator's register
allocator to founder. Bring on -fvia-C option
(not that GCC will be that quick about it, either).
Explicit import declarations:
Instead of saying import Foo, say import
Foo (...stuff I want...) You can get GHC to tell you the
minimal set of required imports by using the
option (see ).
Truthfully, the reduction on compilation time will be very small.
However, judicious use of import declarations can make a
program easier to understand, so it may be a good idea anyway.
Faster: producing a program that runs quicker
faster programs, how to produce
The key tool to use in making your Haskell program run faster are
GHC's profiling facilities, described separately in . There is no substitute for
finding where your program's time/space is really going, as
opposed to where you imagine it is going.
Another point to bear in mind: By far the best way to improve a
program's performance dramatically is to use better
algorithms. Once profiling has thrown the spotlight on the guilty
time-consumer(s), it may be better to re-think your program than to
try all the tweaks listed below.
Another extremely efficient way to make your program snappy is to use
library code that has been Seriously Tuned By Someone Else. You
might be able to write a better quicksort than the one in the
HBC library, but it will take you much longer than typing import
QSort. (Incidentally, it doesn't hurt if the Someone Else is Lennart
Augustsson.)
Please report any overly-slow GHC-compiled programs. The current
definition of “overly-slow” is “the HBC-compiled version ran
faster”…
Optimise, using or :
This is the most basic way
to make your program go faster. Compilation time will be slower,
especially with .
At present, is nearly indistinguishable from .
Compile via C and crank up GCC:
The native code-generator is designed to be quick, not mind-bogglingly
clever. Better to let GCC have a go, as it tries much harder on
register allocation, etc.At the moment, if you turn on you get GCC
instead. This may change in the future.
So, when we want very fast code, we use: .
Overloaded functions are not your friend:
Haskell's overloading (using type classes) is elegant, neat, etc.,
etc., but it is death to performance if left to linger in an inner
loop. How can you squash it?
Give explicit type signatures:
Signatures are the basic trick; putting them on exported, top-level
functions is good software-engineering practice, anyway. (Tip: using
-fwarn-missing-signatures
option can help enforce good signature-practice).
The automatic specialisation of overloaded functions (with )
should take care of overloaded local and/or unexported functions.
Use SPECIALIZE pragmas:SPECIALIZE pragmaoverloading, death to
Specialize the overloading on key functions in your program. See
and
.
“But how do I know where overloading is creeping in?”:
A low-tech way: grep (search) your interface files for overloaded
type signatures; e.g.,:
% egrep '^[a-z].*::.*=>' *.hi
Strict functions are your dear friends:
and, among other things, lazy pattern-matching is your enemy.
(If you don't know what a “strict function” is, please consult a
functional-programming textbook. A sentence or two of
explanation here probably would not do much good.)
Consider these two code fragments:
f (Wibble x y) = ... # strict
f arg = let { (Wibble x y) = arg } in ... # lazy
The former will result in far better code.
A less contrived example shows the use of cases instead
of lets to get stricter code (a good thing):
f (Wibble x y) # beautiful but slow
= let
(a1, b1, c1) = unpackFoo x
(a2, b2, c2) = unpackFoo y
in ...
f (Wibble x y) # ugly, and proud of it
= case (unpackFoo x) of { (a1, b1, c1) ->
case (unpackFoo y) of { (a2, b2, c2) ->
...
}}
GHC loves single-constructor data-types:
It's all the better if a function is strict in a single-constructor
type (a type with only one data-constructor; for example, tuples are
single-constructor types).
Newtypes are better than datatypes:
If your datatype has a single constructor with a single field, use a
newtype declaration instead of a data declaration. The newtype
will be optimised away in most cases.
“How do I find out a function's strictness?”
Don't guess—look it up.
Look for your function in the interface file, then for the third field
in the pragma; it should say __S
<string>. The <string> gives
the strictness of the function's arguments. L is
lazy (bad), S and E are
strict (good), P is “primitive”
(good), U(...) is strict and
“unpackable” (very good), and A is
absent (very good).
For an “unpackable” U(...) argument, the info inside
tells the strictness of its components. So, if the argument is a
pair, and it says U(AU(LSS)), that means “the first component of the
pair isn't used; the second component is itself unpackable, with three
components (lazy in the first, strict in the second \& third).”
If the function isn't exported, just compile with the extra flag ;
next to the signature for any binder, it will print the self-same
pragmatic information as would be put in an interface file.
(Besides, Core syntax is fun to look at!)
Force key functions to be INLINEd (esp. monads):
Placing INLINE pragmas on certain functions that are used a lot can
have a dramatic effect. See .
Explicit export list:
If you do not have an explicit export list in a module, GHC must
assume that everything in that module will be exported. This has
various pessimising effects. For example, if a bit of code is actually
unused (perhaps because of unfolding effects), GHC will not be
able to throw it away, because it is exported and some other module
may be relying on its existence.
GHC can be quite a bit more aggressive with pieces of code if it knows
they are not exported.
Look at the Core syntax!
(The form in which GHC manipulates your code.) Just run your
compilation with (don't forget the ).
If profiling has pointed the finger at particular functions, look at
their Core code. lets are bad, cases are good, dictionaries
(d.<Class>.<Unique>) [or anything overloading-ish] are bad,
nested lambdas are bad, explicit data constructors are good, primitive
operations (e.g., eqInt#) are good,…
Use unboxed types (a GHC extension):
When you are really desperate for speed, and you want to get
right down to the “raw bits.” Please see for some information about using unboxed
types.
Use foreign import (a GHC extension) to plug into fast libraries:
This may take real work, but… There exist piles of
massively-tuned library code, and the best thing is not
to compete with it, but link with it.
describes the foreign function interface.
Don't use Floats:
If you're using Complex, definitely use
Complex Double rather than Complex
Float (the former is specialised heavily, but the latter
isn't).Floats (probably 32-bits) are almost always a bad idea, anyway,
unless you Really Know What You Are Doing. Use Doubles. There's
rarely a speed disadvantage—modern machines will use the same
floating-point unit for both. With Doubles, you are much less
likely to hang yourself with numerical errors.
One time when Float might be a good idea is if you have a
lot of them, say a giant array of Floats. They take up
half the space in the heap compared to Doubles. However, this isn't
true on a 64-bit machine.
Use a bigger heap!
If your program's GC stats (-S RTS option RTS option)
indicate that it's doing lots of garbage-collection (say, more than
20% of execution time), more memory might help—with the
-M<size> RTS option or
-A<size> RTS option RTS options (see
).
Smaller: producing a program that is smaller
smaller programs, how to produce
Decrease the “go-for-it” threshold for unfolding smallish
expressions. Give a
-funfolding-use-threshold0
option option for the extreme case. (“Only unfoldings with
zero cost should proceed.”) Warning: except in certain specialised
cases (like Happy parsers) this is likely to actually
increase the size of your program, because unfolding
generally enables extra simplifying optimisations to be performed.
Avoid Read.
Use strip on your executables.
Thriftier: producing a program that gobbles less heap space
memory, using less heapspace-leaks, avoidingheap space, using less
“I think I have a space leak…” Re-run your program
with , and remove all doubt! (You'll
see the heap usage get bigger and bigger…)
[Hmmm…this might be even easier with the
RTS option; so… ./a.out +RTS
-Sstderr -G1...]
-G RTS option-Sstderr RTS option
Once again, the profiling facilities () are
the basic tool for demystifying the space behaviour of your program.
Strict functions are good for space usage, as they are for time, as
discussed in the previous section. Strict functions get right down to
business, rather than filling up the heap with closures (the system's
notes to itself about how to evaluate something, should it eventually
be required).