I have an unusual application with some unusual performance problems and I'm trying to understand how I might use unsafeFreezeArray to help me, as well as understand in detail what's going on with boxed mutable arrays and GC. I'm using the interface from 'primitive' below. First some basic questions, then a bit more background 1) What happens when I do `newArray s x >>= \a-> unsafeFreezeArray a
return a` and then use `a`? What problems could that cause?
2) And what if a do a `cloneMutableArray` on `a` and likewise use the resulting array? Background: I've been looking into an issue [1] in a library in which as more mutable arrays are allocated, GC dominates (I think I verified this?) and all code gets slower in proportion to the number of mutable arrays that are hanging around. I've been trying to understand how this is working internally. I don't quite understand how the "remembered set" works with respect to MutableArray. As best I understand: the remembered set in generation G points to certain objects in older generations, which objects hold references to objects in G. Then for MutableArrays specifically, card-marking is used to mark regions of the array with garbage in some way. So my hypothesis is the slowdown is associated with the size of the remembered set, and whatever the GC has to do on it. And in my tests, freezing the array seems to make that overhead (at least the overhead proportional to number of arrays) disappear. Now I'm really lost in the woods though. My hope is that I might be able to safely use unsafeFreezeArray to help me here [3]. Here are the particulars of how I use MutableArray in my algorithm, which are somewhat unusual: - keep around a small template `MutableArray Nothing` - use cloneMutableArray for fast allocation of new arrays - for each array only a *single* write (CAS actually) happens at each position In fact as far as I can reason, there ought to be no garbage to collect at all until the entire array becomes garbage (the initial value is surely shared, especially since I'm keeping this template array around to clone from, right?). In fact I was even playing with the idea of rolling a new CAS that skips the card-marking stuff. Any guidance is appreciated. Thanks, Brandon [1]: http://stackoverflow.com/questions/23462004/code-becomes-slower-as-more-boxe... [2]: http://www.haskell.org/pipermail/glasgow-haskell-users/2012-March/022142.htm... [3]: https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC/EagerPromoti...