Garbage collection (by )

One of the interesting little problems in converting a CPU, some RAM, and a bunch of I/O devices into a useful system is memory management.

Your everyday PC with 512MB of RAM contains about 512 million little memory cells, each of which contains an eight-digit binary number. This is where all running applications, and all the information they are dealing with, must be stored; programs, text, images, and whatnot are all encoded as sequences of these eight-digit numbers (known as bytes) and stuffed into memory.

The problem is - where to put everything? After all, when you double click on a program, the computer looks at the size of the program on disk, and needs that many bytes of memory to load the program into. How does it know which bits are free and which are in use? Different computers have different amounts of memory, and may be running any combination of apps already, so there's no way to reserve a given bit of memory for a particular program; the system has to keep track of what's in use and what isn't in real time. And when you load your app, it will start asking for bits of memory to keep track of the windows it has open, to store your document in, and so on.

Now, the first problem to be solved is how to keep track of which bits of memory are in use and which aren't, in such a way that the computer can efficiently find a block of free memory of a specified size - that problem is harder than it may seem, especially when you consider that multiple threads of execution will be requesting memory at once. But that's not the problem I was pondering as I sat on the train today.

My problem is how to figure out when a block of memory isn't used any more, so that it can be handed back to the system that keeps track of free blocks and reused.

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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.

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  1. Snell-Pym » Garbage collection — Tue 31st Jul 2007 @ 5:56 pm

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