Sunday, December 25, 2011

memory management

The memory is managed in Triceps using the reference counters. Each Triceps object has a reference counter in it. There is an Autoref template that produces reference objects. As the references are copied around between these objects, the reference counts in the target objects are adjusted. When the reference count drops to 0, the target object gets destroyed. While there are live references, the object can't get destroyed from under them. All nice and well and simple, however still possible to get wrong.

The major problem with the reference counters is the reference loops. If object A has a reference to object B, and object B has a reference (possibly, indirect) to object A, then neither of them will ever be destroyed. Many of these cases can be resolved by keeping a reference in one direction and a plain pointer in the other.  This of course introduces the problem of hanging pointers, so extra care has to be taken to not reference them. There also are the unpleasant situations when there is absolutely no way around the reference loops. For example, the Triceps label's method may keep a reference to the next label, where to send its processed results. If the labels are connected into a loop (a perfectly normal occurrence), this would cause a reference loop. Here the way around is to know when all the labels are no longer used (before the thread exit), and explicitly tell them to clear their references to the other labels. This breaks up the loop, and then bits and pieces can be collected by the reference count logic.

The reference counting maybe single-threaded or multi-threaded. If an object may only be used inside one thread, the references to it use the faster single-threaded counting.In C++ it's real important to not access and not reference the single-threaded objects from multiple threads. In Perl, when a new thread is created, only the multithreaded objects from the parent thread become accessible to it, the rest become undefined, so the issue gets handled automatically (as of version 1.0 even the potentially multithreaded objects are still exported to Perl as single-threaded, with no connection between threads yet).

The C++ objects are exported into Perl through wrappers. The wrappers perform the adaptation between Perl reference counting and Triceps reference counting, and sometimes more helper functions. Perl sees them as blessed objects, from which you can inherit and otherwise treat like normal objects.

When the Perl references are copied between the variables, this increases the Perl reference count to the same wrapper object. However if an object goes into the C++ land, and then is extracted back (such as, create a Rowop from a Row, and then extract the Row from that Rowop), a brand new wrapper gets created. It's the same underlying C++ object but with multiple wrappers. You can't tell that it's the same object by comparing the Perl references, because they may be pointing to the different wrappers. However Triceps provides the method same() that compares the data inside the wrappers. It can be used as


and if it returns true, then both $row1 and $row2 point to the same underlying row. Note also that if you inherit from the Triceps objects and add some extra data to them, none of that data nor even your derived class identity will be preserved when a anew wrapper is created from the underlying C++ object.

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