Garbage collection (by alaric)
One approach is to leave this to the application programmer. Known as "manual memory management", it requires the programmer to tell the system when a bit of memory is no longer needed. This approach has two main flaws, and one main advantage - some of the time:
- It's error-prone. The programmer might accidentally release a block of memory when it's still being used by his program. Nothing will initially appear to go wrong - the program may still keep storing stuff in that block of memory and referring to it without any problems, but when the system one day allocates that bit of memory to something else, then the two users of the memory collide, probably crashing the application. Invariably, this won't happen in testing - and will only happen when the app is being used for real... Also, if the programmer forgets to free up some memory, it will remain marked "in use" forever. If the program keeps doing this, with time it will use up all of the available memory with rubbish - known as a "memory leak".
- It can be tricky to know when a block of memory won't be needed any more. When you're doing complex graph transformations, for example, perhaps with the same set of nodes being arranged into several different graphs... if you delete a node from a given graph, how do you know if any other graphs still contain that node? Because of this, programmers often end up having to write limited forms of automatic garbage collector anyway.
But the main advantage is that it's often efficient. No extra work is done by the system to automatically manage memory; the programmer tells it when a block is free, and that's all there is to it. The reason this isn't always efficient is that freeing memory blocks inherently takes some time (at the very least, the newly freed block needs to be added to a list of available blocks, and ideally two or more adjacent free blocks should be merged into one big one). Now, a programmer will write the "free this memory" instruction into their program at the precise point where the memory is no longer needed, meaning that memory-freeing work has to be done there and then before the program can continue running.
This is not always the best time for the system to be taking a break to manage its internal affairs. Sometimes, it would be better if the freeing of memory could be done a few seconds later, when the system is idly awaiting the next command from its user.
Because of these concerns, it is becoming increasingly common for applications to use some form of automated garbage collection to locate no-longer-required memory.
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.