In order to easily differentiate these objects a concept called tricolour abstraction emerged (authored by Dijkstra). The process described above implies, that there will be objects in the graph that were not visited by GC, the ones marked but with not yet visited children, and objects that were visited and their children a swell. This topic will be discussed in the future posts too. We must be sure about two things – that the heap won’t become so fragmented in time, that it won’t be possible for GC to actually allocate new objects on it (or just running GC too often to achieve that), and also to make sure that all the garbage is reachable (due to memory padding constraints, etc.). Last thing that authors mention is memory/heap layout. Second issue is – when to reset the mark bit (the information if the object is marked/unmarked) – during sweeping or maybe at the beginning of the next mark phase. First – traversing the heap freeing the memory can be time-consuming, depending on its size. It may look simple, but it raises a couple of issues that we must be aware of. As we skip the objects that are already marked or do not contain pointers, worklist won’t grow forever, and eventually will become empty. In general the algorithm runs as long, as the worklist is not empty. Ref ← remove(worklist) /∗ ref is marked ∗/ MarkFromRoots() // It traverses the whole graph starting from Roots Very simplistic pseudocode of mark can be found below (it comes from the second book):Ītomic collect(): // Pay attention to the 'atomic' - it means 'stop-the-world' For the time being we assume, that at this stage, all the mutator threads are paused, and stop-the-world happens (no mutator threads running).
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What must be said, is that every time a mutator (so our running application) tries to allocate a memory, and there’s none, a collector is called to free some of it. It rather finds all live objects, and treat everything else as garbage. In other words – it does not find garbage. As the authors say – this type of algorithm is indirect collection algorithm. It consists of two separate parts – marking – where all the objects are identified as used/unused, and sweep – when they’re just removed from the memory. To remind us what we’re discussing here – mark&sweep GC was first introduced in the 1960 by McCarthy.
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