- FFI - Haskell.org

Re: Proposal: Pooled memory management
by Sven Panne 21 Jan '03

21 Jan '03

Manuel wrote: > [...] > * I want to get v1.0 of the spec fixed. We are really only > in bug fix mode for quite a while and only the finalizer > problems held us back from finishing the spec. That's OK and I understand your motivation. Let's finish v1.0 first. > * I am sure there are plenty more useful FFI-related > libraries. However, the initial plan was to define basic > functionality on top of which more elaborate schemes can > be implemented. We need to draw the line somewhere. [...] But I strongly disagree here: The initial plan was to make a very small, but sufficient addition to the language (=> foreign import/export), which can be implemented easily on existing systems. In this respect, we have reached our goal quite elegantly IMHO. The related libraries are a totally different beast: Minimality is *not* a design goal here. Otherwise we might be happy with e.g. Foreign.Marshal.Alloc.mallocBytes Foreign.Marshal.Alloc.allocaBytes Foreign.Marshal.Alloc.reallocBytes Foreign.Marshal.Alloc.free Foreign.Marshal.Util.copyBytes Foreign.Marshal.Util.moveBytes alone. Or another example: Why should we include such trivialities like Foreign.Marshal.Error.void or Foreign.Marshal.Util.toBool in the FFI spec? Minimality? Possibility of a lightning-fast special implementation? Definitely not. The basic task of a library (or a collection of related libraries) is to establish a common "language" and make common notions (like scoped or pooled allocations) explicit. Looking at the OO world, that's the reason for all this pattern hype. In a nutshell: Let's include as many useful "patterns" in the next FFI spec versions as possible! Otherwise we might talk the same language without recognizing it... Cheers, S.

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RE: Proposal: Pooled memory management
by Simon Marlow 20 Jan '03

20 Jan '03

> Manuel wrote: > > [...] > > * I want to get v1.0 of the spec fixed. We are really only > > in bug fix mode for quite a while and only the finalizer > > problems held us back from finishing the spec. > > That's OK and I understand your motivation. Let's finish v1.0 first. I agree, but I don't have any problem with forking the spec at this point. The changes to the current 1.0 RC are going to be very small from now on. > In a nutshell: Let's include as many useful "patterns" in the > next FFI spec > versions as possible! Otherwise we might talk the same > language without recognizing it... So we might well ask what useful new functionality is provided by a pool-style memory manager. One feature is performance: compared to using malloc/free, a pool is going to be much cheaper. But I'd be interested to compare the performance you get from using pools to GHC's implementation of mallocForeignPtr, which also sidesteps malloc/free to get decent performance. Compared to mallocForeignPtr, there are some qualitative differences which might make one or the other more convenient in some situations: - mallocForeignPtr provides garbage-collected storage. No need to free the storage later. - However, you do need to use withForeignPtr to get access to the object. This can be quite annoying. So, assuming the performance is roughly the same (I'm guessing that using pools may be slightly faster than mallocForeignPtr, but not significantly), do people see any other compelling reasons to be using pools? GHC's runtime has an Arena abstraction which is an implementation of pool-style memory management, and could be used to implement Pools, BTW. Cheers, Simon

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Proposal: Pooled memory management
by Sven Panne 20 Jan '03

20 Jan '03

The FFI libraries currently contain support for explicit allocation and deallocation via the malloc/free family and support for implicit allocation and deallocation via alloca and friends. But there is a very useful level between these extremes: Pooled memory management. Under this scheme, (re-)allocations belong to a given pool, and everything in a pool is deallocated when the pool itself is deallocated. You can find the implementation and the (lean) docs at: http://haskell.org/HOpenGL/PoolRFC/Pool.hs http://haskell.org/HOpenGL/PoolRFC/html/Foreign.Marshal.Pool.html I propose adding this module to FFI addendum and the exports of module Foreign.Marshal. Although the module is definitely not a candidate for the next ACM award, it's a useful and common abstraction which can easily be implemented on the existing FFI modules. Comments? Cheers, S.

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alignment
by Ross Paterson 14 Jan '03

14 Jan '03

Two additions I think are required: 1) The spec should state that mallocBytes and allocaBytes return a block of memory sufficiently aligned for any of the primitive types supported by the architecture. 2) The description of Storable should require sizeOf x `mod` alignment x = 0 The library implementation of mallocArray implicitly assumes this, and C also requires that elements of an array are allocated contiguously.

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Foreign proxy
by Manuel M T Chakravarty 13 Jan '03

13 Jan '03

An interface like the following has been repeatedly proposed (in slightly varying versions): data ForeignObj a -- abstract instance Eq (ForeignObj a) type Finalizer a = FunPtr (a -> IO ()) newForeignObj :: a -> Finalizer a -> IO (ForeignObj a) addFinalizer :: ForeignObj a -> Finalizer a -> IO () withForeignObj :: ForeignObj a -> (a -> IO b) -> IO b touchForeignObj :: ForeignObj a -> IO () If we adopt it, we might define `ForeignPtr' in terms of the new interface: type ForeignPtr a = ForeignObj (Ptr a) newForeignPtr :: a -> Finalizer (Ptr a) -> IO (ForeignPtr a) <etc> We, btw, also need foreignObjToObj :: ForeignObj a -> a to implement `foreignPtrToPtr'. Unfortunately, there doesn't seem to be any easy way to implement `castForeignPtr', which is valid for pointers, but can't be generalised to `ForeignObj'. We could assume the position that such casts need always to be made on the vanilla `Ptr' after it has been obtained from the `ForeignPtr'. However, this might be a nuisance in some situations. Open Questions ~~~~~~~~~~~~~~ (1) Shall we adopt an interface like `ForeignObj'? (Seems like a nice idea to me.) (2) What shall we do about `castForeignPtr'? (3) Shall we call the new objects `ForeignObj'? (It's a good name, but conflicts with some of GHC's legacy libraries. `ForeignProxy' is one of the more appealing possible alternatives that was suggested ealier. `ForeignFinalized' is also nice, but maybe a bit long.) What do you think? Manuel

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05 Jan '03

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lesson about ffi
by Po Popo 14 Dec '02

14 Dec '02

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