deltalmg1 deltalmg1

Niner since 2007


  • Patrick Dussud: Garbage Collection - Past, Present and Future

    Great talk. Never really thought of gc as improving performance, always thought of it as a good thing at a price. While I haven't done extensive testing of it, it rings true, that for non-trivial programs trying to keep track of all the references to an object, or worse trying to "get a copy, rather than point to the original", would be hard, and in general poorly done. Having something that overlooks all of the systems memory seems like it would have a better grasp of what needs to be done. That is why we get Microsoft to hire smart people to figure out good ways of doing things and leave it at that Wink

    Also, I've seen spots in the MSDN documentation where it recommends pre-allocating a bunch of objects (ex. the collection documentation) to improve performance. I can see where stuff like that would mess up the garbage collector, now unless I'm adding things to the collection in a simple loop, I think I'll  dynamically resize using the .add or .addrange methods. I know the C++ STL had issues with that because it would use its  copy constructor to move all the old stuff over to  a new object one element bigger, it sounds like  the .Net framework handles it smarter than that.

    For a concurrent gc could you create a set of trees of allocations, essentially each potential scope in the source would have at least a leaf in the tree set, as things get allocated in the scope it adds leaves to the tree (sort of like how Sun's ZFS does its "instanteous snapshots" I think), then when it comes time to do gc you find out which scope your in, and every sub tree on a lower node in the set can be safely pruned. Presumably an app isn't just spinning around doing allocations, it actually is adding data to the objects, running through logic etc, so at the cost of adding a step in the allocation process you gain scope level knowledge of the allocations in the app.

    You could then whenever the gc gets the CPU (or on a spare one Smiley) mark the node where everything lower than it is dead, and start deleting it. If you get interrupted it would be fine because when you get the CPU back you still know everything below that is dead and can continue. I think in such a scheme the only time you'd have to block execution of the app would be if when the gc gets the CPU back, the app has gone back into something it thinks is dead. Then it would have to force the completion of gc on that part of the tree, before it allows the app to go back into that scope.

    Perhaps that is how the gc works now, I'd love there to be another video that goes into the nitty gritties of the gc process.