Garbage collection (by alaric)
So imagine my joy when I read the paper One Pass Real-Time Generational Mark-Sweep Garbage Collection by Joe Armstrong and Robert Virding. They present an algorithm that has all of the advantages - and none of the disadvantages - of the mark-sweep algorithm. It's a continuous process that will release memory as it goes, rather than in one big lump; it's efficient and simple; it doesn't require stopping the system for any long periods; and so on.
So why isn't it more widely used?
Alas, it places a restriction on the programs you can run; the programs must be purely functional. Which basically means that they cannot ever modify the data in memory, merely create new blocks of memory, load things into them, and then only read them thereafter.
The reason it requires this is that it uses the assumption that references to memory blocks can only point towards memory blocks older than the memory block containing the reference - it keeps a list of memory blocks in the order they were created, and scans that list backwards, starting with newer objects and moving towards the old. For each memory block, if the memory block is marked, it marks all blocks referenced by this block. If it's not marked, then it knows it can free the block immediately - since it started at the top of the list and went backwards in time, it must have already looked at every memory block that could possibly reference this one. Memory blocks it has not yet examined cannot reference this one, since they were created before this one existed, and cannot have been modified to reference it since their creation.
But although pure functional programming has many advantages, it's very awkward in situations where you really would like to modify your data. And it clearly cannot (as described, anyway) deal with something like a graphical display, which has to be modified if the contents of the screen are ever to change.
So this is what I was pondering on my train into London this morning.
Wed 16th Nov 2005 11:29 pm 4 Comments
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By Faré, Fri 18th Nov 2005 @ 3:21 am
So as to be able to move your object to the top of the list, yet preserve the invariant that no object points to a newer object, you need to also move up all the objects that point to your born-again object, and so on recursively. In practice, this is only affordable if your object has no one pointing to it except the current context -- in which case what you have is a linearity constraint on objects.
By Alaric Snell-Pym, Fri 18th Nov 2005 @ 10:31 am
Ah, but I do have a linearity constraint on objects 😉 That's why I was amused by the similarity to "copy on write" handling of tree structures on disk, which basically works the same way (to safely update a tree structure like a B-Tree, make copies of the changed leaf nodes into empty space, then work out what intermediate nodes would need changing to reflect the new locations of the leaf nodes and do the same with them, then continue bubbling the change up the tree until you have a new root pointer, etc).
I'm sure there's some Fundamental Truth in the fact that the same underlying technique turns out to be useful both on disk and in memory, yet in very different contexts (ACID properties on disk, fast garbage collection in RAM).
By Alaric Snell-Pym, Sun 20th Nov 2005 @ 2:33 pm
Oooh, while sawing up logs (a great time for thinking about abstract stuff) I was struck by a flaw...
When a memory block is modified and gets brought up to the head of the chain, IF the collector has not yet reached the block in this pass, then the blocks referenced by this block will not get marked since the collector will then not visit the block in question until the next pass. If nothing else refers to the same blocks, they'll be freed. Oops!
So we need to make sure that a block moved to the top of the chain still gets seen. My first thought was that the application could just quickly scan the block and mark all the referenced blocks, but that's wrong - it's the collector's job.
So my next thought was to have a (either shared and lock-free, or per-processor, to stop it from becoming a point of contention in SMP systems) stack of 'touched' objects; when altering an object, the application would merely need to push a reference onto this stack (it wouldn't even need to do the move to the head of the chain). Now, the collector, whenever it's about to examine the next object in the chain, would first look on the stack(s) and go through any memory blocks on them, marking all the referenced blocks. That way, it will never be considering a block for freeing unless it has already 'scanned' all modified objects, so there's no chance of it mistakenly freeing something. Whenever the collector has scanned a block from the stack, it can then also do the chore of moving that block to the head of the chain, moving the task from the application code.
However, there is a problem - an application that just sits there modifying the same large array of pointers over and over again would keep the collector forever rescanning that large array; never getting any real collection done. What we need is to only stack memory blocks for scanning if they've not yet been scanned anyway. This is easily resolved; have a 'scanned' flag in each block, that the collector sets whenever it scans a block, be it due to the block being on the stack or by traversing the chain. Newly allocated objects also have the 'scanned' flag set, since all the objectss they refer to must have been reachable anyway, and thus will be marked - they don't need rescanning until the next pass. When the collector finishes scanning the chain and is about to start again, it has to clear all the 'scanned' flags; but rather than walking the chain doing this, it's easier to just reverse the interpretation of the 'scanned' flag. Then newly created blocks will need to be marked as 'unscanned' for the next scan.
There's a potential race condition in that if the collector changes the global variable that says what newly created blocks should be marked as between the application reading the current setting and the application putting the newly created block at the head of the chain, it could end up with the wrong setting. Therefore, before doing the swap, the collector should take a copy of the pointer to the current head of the chain; then when it starts its walk of the chain, it should force the correct value into all the blocks it examines until it hits the point in the chain it marked, this way ensuring nothing gets missed.