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tanzanite7

Testing a GC implementation

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Just finished implementing a "cooperative" incremental conservative mark and sweep garbage collector. Basic test coverage is also present and passes, BUT ... having trouble coming up with bigger tests and stress tests to better understand its behavior and theoretical limits.

How to generate a mess of classes and modify the relations without accidentally killing off too many branches or having it infinitely glow? Ideas?

(minor note: in the end i want to record the modifications/additions as replay for tests, so - generating it does not have to be super fast, just reasonable).

Some details about the GC implementation (most of it likely fairly irrelevant to the question, but anyway):

User view (slightly relevant to the question):

struct GcObject {
  USE_GC(100); // adds GC trait to this class (not inheritable), 100 = hint of how many objects you expect to have (not a limit in any sense)
  GC<GcObject> var_next; // behaves as a pointer to GcObject
  // GC<NonGcObject> var_error; // this is compile time error (all GC pointers, a'la GCRoot etc, accept only objects with GC trait)
  Array<GcObject> var_array; // there are wrappers for a few things i use the most: array/vector and hashmap - the wrapper detects on its own that it is for GC-trait objects and does what it needs to do (most notably, the array will be for "GcObject*"). + version for dynamic root and static
};

struct NonGcObject {
  GCRoot<GcObject> var_root; // dynamic root for GC - behaves as a pointer to GcObject
  // GC<GcObject> var_error; // this is runtime error (assert) - so, debug mode only
};

GCStatic<GcObject> var_static; // static root for GC - behaves as a pointer to GcObject
//GC<GcObject> var_error; // assert

// currently there is no way to detect misplaced GCRoot/GCStatic

Features/assumptions/intended-usage:
* It is meant for class-objects and not data blobs.
* Discards pointers to already encountered objects very cache friendly (32MB of objects are covered by 2-16 pages of GC internal structures needed for identification / discard) - does not reference objects memory at any point.
* Per object type: discovers and records object structure for faster processing in future (however: never assumes that it identified the structure correctly - except locations for non pointers to GC memory). If it detects that an object can not have any pointers to GC memory then it will skip processing of all objects of the relevant type in future.
* Processes in patches per type (for cache locality of the target objects and GC internals).
* GC trait adds a memory pooling scheme for the class - cache locality for GC and whatever is using the class.
* Pool: maintains an ordered list (inc pointer address) of free items - to eliminate degradation of locality as much as possible.
* GC does not touch the objects memory on delete if the object does not have a destructor. Same for constructor - but that is hardly useful.
* Does not examine stack and hence does not allow invocation at random points.
* Garbage collection invocation: MemGC::tickMain(timeslice_in_seconds).
* It is possible to offload the majority of the work to a worker thread - but i doubt it is useful and i probably will never implement it.
* Allocation of GC-trait objects is not thread safe - simply never needed, which makes justifying the overhead rather impossible.
* probably something more

Edited by tanzanite7
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You have basically three axes to worry about:

- Number of allocations
- Depth of reference tree
- Lifetime of objects


Things get interesting at basically any extreme combination of two or more of these axes. It should be pretty straightforward to invent a test that can stress each axis individually, and then you can combine them to stress multiple areas simultaneously.
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