On Fri, May 9, 2014 at 2:06 AM, Edward Z. Yang
Hello Brandon,
Excerpts from Brandon Simmons's message of 2014-05-08 16:18:48 -0700:
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?
Your code as written wouldn't compile, but assuming you're talking about the primops newArray# and unsafeFreezeArray#, what this operation does is allocate a new array of pointers (initially recorded as mutable), and then freezes it in-place (by changing the info-table associated with it), but while maintaining a pointer to the original mutable array. Nothing bad will happen immediately, but if you use this mutable reference to mutate the pointer array, you can cause a crash (in particular, if the array makes it to the old generation, it will not be on the mutable list and so if you mutate it, you may be missing roots.)
2) And what if a do a `cloneMutableArray` on `a` and likewise use the resulting array?
If you do the clone before freezing, that's fine for all use-cases; if you do the clone after, you will end up with the same result as (1).
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.
You're basically correct. In the current GC design, mutable arrays of pointers are always placed on the mutable list. The mutable list of generations which are not being collected are always traversed; thus, the number of pointer arrays corresponds to a linear overhead for minor GCs.
Here is a feature request tracking many of the infelicities that our current GC design has: https://ghc.haskell.org/trac/ghc/ticket/7662 The upshot is that the Haskell GC is very nicely tuned for mostly immutable workloads, but there are some bad asymptotics when your heap has lots of mutable objects. This is generally a hard problem: tuned GC implementations for mutable languages are a lot of work! (Just ask the JVM implementors.)
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.
I don't understand your full workload, but if you have a workload that involves creating an array, mutating it over a short period of time, and then never mutating it afterwards, you should simply freeze it after you are done writing it. Once frozen, the array will no longer be kept on the mutable list and you won't pay for it when doing GC. However, the fact that you are doing a CAS makes it seem to me that your workflow may be more complicated than that...
Cheers, Edward
Another silly question: when card-marking happens after a write or CAS, does that indicate "this segment maybe contains old-to-new generation references, so be sure to preserve (scavenge?) them from collection "? In my initial question I was thinking of the cards as indicating "here be garbage" (e.g. a previous overwritten array value), but I think I had the wrong idea about how copying GC works generally (shouldn't it really be called Non-Garbage Preservation?). Thanks again, Brandon