9 Matching Annotations
  1. Jan 2014
    1. There are three kinds of storage locations: stack locations, heap locations, and registers.
    2. There are three kinds of values: (1) instances of value types, (2) instances of reference types, and (3) references. (Code in C# cannot manipulate instances of reference types directly; it always does so via a reference. In unsafe code, pointer types are treated like value types for the purposes of determining the storage requirements of their values.)
    3. Having made these points many times in the last few years, I've realized that the fundamental problem is in the mistaken belief that the type system has anything whatsoever to do with the storage allocation strategy. It is simply false that the choice of whether to use the stack or the heap has anything fundamentally to do with the type of the thing being stored. The truth is: the choice of allocation mechanism has to do only with the known required lifetime of the storage.

      The type system has nothing to do with the storage allocation strategy; the choice of allocation mechanism has to do only with the known required lifetime of the storage.

    1. A lot of people seem to think that heap allocation is expensive and stack allocation is cheap. They are actually about the same, typically. It’s the deallocation costs – the marking and sweeping and compacting and moving memory from generation to generation – that are massive for heap memory compared to stack memory.
    2. Now compare this to the stack. The stack is like the heap in that it is a big block of memory with a “high water mark”. But what makes it a “stack” is that the memory on the bottom of the stack always lives longer than the memory on the top of the stack; the stack is strictly ordered. The objects that are going to die first are on the top, the objects that are going to die last are on the bottom. And with that guarantee, we know that the stack will never have holes, and therefore will not need compacting. We know that the stack memory will always be “freed” from the top, and therefore do not need a free list. We know that anything low-down on the stack is guaranteed alive, and so we do not need to mark or sweep.
    3. This sketch is complicated by the fact that there are actually three such arenas; the CLR collector is generational. Objects start off in the “short lived” heap. If they survive they eventually move to the “medium lived” heap, and if they survive there long enough, they move to the “long lived” heap. The GC runs very often on the short lived heap and very seldom on the long lived heap; the idea is that we do not want to have the expense of constantly re-checking a long-lived object to see if it is still alive. But we also want short-lived objects to be reclaimed swiftly.
    4. When a garbage collection is performed there are three phases: mark, sweep and compact. In the “mark” phase, we assume that everything in the heap is “dead”. The CLR knows what objects were “guaranteed alive” when the collection started, so those guys are marked as alive. Everything they refer to is marked as alive, and so on, until the transitive closure of live objects are all marked. In the “sweep” phase, all the dead objects are turned into holes. In the “compact” phase, the block is reorganized so that it is one contiguous block of live memory, free of holes.
    5. If we’re in that situation when new memory is allocated then the “high water mark” is bumped up, eating up some of the previously “free” portion of the block. The newly-reserved memory is then usable for the reference type instance that has just been allocated. That is extremely cheap; just a single pointer move, plus zeroing out the newly reserved memory if necessary.
    6. The idea is that there is a large block of memory reserved for instances of reference types. This block of memory can have “holes” – some of the memory is associated with “live” objects, and some of the memory is free for use by newly created objects. Ideally though we want to have all the allocated memory bunched together and a large section of “free” memory at the top.