Mark&Sweep garbage collector for C

Modern C++ provides many new features which moves this language from "object-oriented assembler" to safe high-level programming language. Unfortunately one of the main problems of C/C++ is still unresolved: memory deallocation. Thousands man/years of programmers were spent for debugging and fixing various bugs related with incorrect memory deallocation: memory leaks and dangling references. If you forget to deallocate some object, you are losing memory and sooner or later your program will get not-enough memory error. If you remove object which is still referenced by other objects, then situation is even worse: application can try to access deleted objects. Such attempt usually cause unpredictable behavior (which depends on content of memory of deallocated object - actually garbage). Such bugs are non-deterministic and very difficult to reproduce and debug.

C++ address the problem with memory deallocation using "smart pointers", such as "std::shared_ptr" or "boost::shared_ptr". In both case reference counters are used to determine a moment when an object is not needed any more. Programmer has to use shared_ptr instead of normal C++ pointers or references. Copying reference cause increment of reference counter. And when variable is deleted or assigned other value, reference counter is decremented. When reference counter becomes equal to zero, object is deleted. This schema seems to be quite simple. But unfortunately there are several disadvantages of this approach:

1. Noticeable performance penalty. Each manipulation with pointers requires update of counter. In case of multithreaded application we have to use atomic operations or synchronization primitives to avoid race conditions.

2. Reference counters are not able to deal with cyclic data structures. For example L2-List (double linked list) elements never can be deallocated using reference counters. Programmer should somehow explicitly break the loop.

3. You should always use smart pointers instead of normal pointers: method parameters should be declared as smart pointers, method return type should be also smart pointer, local variable should have smart pointer type... If you try to use normal C++ pointer instead of smart pointer, then it can happen that object will be deallocated and you will access garbage.

4. Deleting last reference to huge tree of objects can cause large number of cascade (recursive) deletes which can cause stack overflow.

5. Extra space overhead. boost::shared_ptr requires twice more space than normal pointer. It means that size of object consisting mostly of pointers (for example tree node) can be almost doubled.

Now let's return a little bit to theory. Implicit memory deallocation or garbage collection algorithms can be splitted in two main classes: reference-counter based and and reference-tracing based. Reference counters approach was described earlier. Tracing-based algorithms are essentially modifications or combinations of two classic algorithms: Mark&Sweep and copying.

Copying collectors have good performance characteristics but require objects to be moved in memory so they need tight cooperation with language runtime and break common assumptions of C++ programmer.

Mark&sweep algorithms consist of two phases. At the first phase (mark) algorithm recursively traverses all accessible objects and marks them. At the second phase (sweep) it deallocates all non-marked objects. Mark&sweep algorithms can handle cyclic data structures and that is why them are used in most of modern high-level object-oriented languages, like Java or C#.

To be able to mark live (accessible) objects, make&sweep algorithm needs to know some set of root references. Usually it includes local and global variables. It is not a problem to detect such variables for example in Java virtual machine. But C++ program is compiled to native code and here detection of such variables is problematic. One of possible solutions of the problem is conservative garbage collector. "Conservative" here means that algorithm treats as a pointer everything which looks like a pointer. So if you wrote:

int x = 0x605018;

then this "x" can be treated as a pointer, if it's content occasionally corresponds to address of some object, although it is actually integer. It is one of the main problem of conservative garbage collectors which can cause false detection of live object and so lead to memory leak.

Another big problem of conservative garbage collector for C++ is that we need to somehow detect location of global and local variables. Them can be located in static memory, in stack, in registers... So conservative garbage collector has to contain a lot of system and machine dependent code which allows him to scan heap, stack and hardware registers. As a result, conservative garbage collectors are slow, very unportable and that is why them are rarely used.

Is it possible to implement non-conservative mark&sweep garbage collector for C++? We have to address two main challenges:

1. How to locate variables?
2. How to locate references inside objects?

For portability and maintainability reasons we prefer to avoid access to C++ runtime internals and C++ doesn't support necessary level of reflection. So we still have to use something like smart pointers instead of normal C++ pointers. But their usage will be quite different comparing with shared_ptr using reference counter. And we really need to have two different kind of pointers. One will be used inside class definitions - their goal is to mark referenced objects. Another one will be used instead of local variables - them should provide set of root references for garbage collector.

Let's call first kind of smart pointer as Ref and second kind as Var. The role of Var is more simple: it should somehow registers pointer in garbage collector when variable is created and unregisters pointer at the end of variable scope.

For simplicity and efficiency reasons we assume that each thread will have its own local memory allocator. It is possible to have objects shared by all threads, but them should be created and controlled by global allocator. Registering variables is trivial in this case: we can just link them in L1 list. So constructor of Var links itself in the root list of memory allocator and destructor of Var unlinks itself from this list.

The goal of Ref class is more complex. It should somehow mark referenced object. C++ can help as in two ways:

1. It implicitly invokes constructors of all object components when object is constructed.
2. If implicitly invokes destructors of all object components when object is destructed.
3. It invokes copy constructors of all object components when object is copied

So if class Ref has constructor and destructor, then C++ compiler will generate code which invokes them. Been invoked them can somehow propagate to the garbage collector reference to the object. It is the second task. Let's first concentrate on the question what to prefer: constructors or destructors?

C++ doesn't allow to call object constructor or destructor directly. But we can redefine new/delete operators so that it will be possible to invoke constructor/destructor multiple times for the same object:

class Object 
{
    virtual ~Object() {}
    virtual void mark() = 0;    
    void* operator new(size_t size, void* addr) { return addr; } // return self address
    void operator delete(void* ptr) {} // do nothing
};

class TreeNode : public Object 
{ 
    Ref<TreeNode> left;
    Ref<TreeNode> right;

    virtual void mark() { (void)new (this) TreeNode(); } // it will invoke constructors of Ref for left and right
};

Virtual method TreeNode::mark will invoke constructor of Ref class for left and right components. And destructor of this class (impolitely generated by compiler) will invoke destructor of Ref class for left and right components.

So we can force invocation of both constructors and destructors for this object. But in both cases we are faced with the problems.

The problem with redefined operator new and constructors is that compiler (at least GCC does it) initializes memory returned by operator new with zeros before calling constructor when the class has virtual functions. So we are loosing all content of the object.

The problem with destructor is that it updates pointer to virtual table. When destructor of some class is completed, it replaces pointer to self virtual table with pointer to virtual table of base class and then invokes destructor of the base class. As a result, after returning from destructor, virtual table pointer contains wrong value.

Classes without multiple inheritance contains only one pointer to virtual table which most of C++ compiler places at the beginning of the object. So we can implement some hack - save first pointer of the object before calling destructor and then restore it:

 void* vtable = *(void**)obj;
 delete obj; // invoke destructors to mark referenced
 *(void**)obj = vtable; // restore virtual table pointer replaced by destructor

Certainly it is non-portable solution and it will not work in case of multiple inheritance. But my first implementation of mark&sweep collector for C++ really uses this approach. Garbage collection is done by destructors! Very strange approach, isn't it?

Unfortunately if we invoke constructors/destructors for the self object, we should prohibit use constructors/destructors for normal purposes: for initialization/destruction of object components. So we can not use for example std::string as class component. This is why I finally reject both 1 and 2 approaches.

Approach with copy constructors do not have some limitation. Certainly there is also some elements of hacking here: to mark object references we need to create copy of the object. But it allows object to have std::string components and normally use default constructors/destructors. Copying such components as std::string will require to perform useless allocation and copying of data. But at least it doesn't change object content and doesn't cause any memory leaks. Garbage collector is intended to be invoked not so frequently, so this extra overhead is not so critical.

Now return back to the problem of notification of garbage collector about referenced object from destructor of Ref object. We can not pass some parameter to destructor and we can not use some global variable because application can be multithreaded. So the only choice is to use thread local (or thread specific) memory. Fortunately this mechanism is provided at most systems and is fast enough.

So we can grab destructor's mechanism for the purposes of garbage collection!

What is good with mark&sweep garbage collector is that we can easily control when garbage collection should be initiated. It can start automatically after reaching some threshold value for total size for allocated object since last GC. But we can also invoke it explicitly (may be also checking amount of allocated space since last GC). In the last case we can temporally use normal C++ pointer variables instead of Var smart pointers, because objects can not be deleted before we start garbage collection. We should only save roots of all needed object trees in Var variables before initiation of GC.

Let's now provide some implementation details. The main class is MemoryAllocator - it is responsible for allocation and implicit deallocation of objects:

class MemoryAllocator 
{
    /**
     * Get allocator for the current thread. Each thread should have its own allocator.
     */
    static MemoryAllocator* getCurrent();
   
    /**
     * Allocate object 
     * @param object size
     */
    static void* allocate(size_t size);

    /**
     * Mark object as reachable and recursively mark all references objects
     * @param obj marked objects (may be NULL)
     */
    static void mark(Object* obj);

    /**
     * Register root object. Registering root protects it and all referenced objects from GC.
     */
    static void registerRoot(Root* root);
        
    /**
     * Unregister root object. Make this object tree available for GC.
     */
    static void unregisterRoot(Root* root);

    /**
     * Explicitly starts garbage collection.
     */
    static void gc();

    /**
     * Start garbage collection if number of allocated objects since last GC exceeds StartThreshold 
     */
    static void allowGC();
    
    /**
     * Create instance of memory allocator 
     * @param gcStartThreshold total size of objects allocated since last GC after which allowGC() method initiates garbage collection
     * @param gcAutoStartThreshold  total size of objects allocated since last GC after which garbage collection is automatically started. 
     * Please notice that all used objects should be protected from GC in this case by registering their roots.
     */
    MemoryAllocator(size_t gcStartThreshold = 1024*1024, size_t gcAutoStartThreshold = (size_t)-1);

    /**
     * Deallocate all objects create by GC.
     */
    ~MemoryAllocator();
};

Each thread (connection, session,...) should construct and use its own allocator. Current allocator is located using thread local memory mechanism (Posix pthread_getspecific or Windows TlsGetValue). All static methods of MemoryAllocator class refer to the current allocator.

As it was mentioned above, there are two ways of starting garbage collection: automatic and manual. So we provide two threshold values in constructor of MemoryAllocator. By default automatic start of GC is disabled, because in this case programmer should always protect all objects from GC using Var variables. Manual start of GC can be performed either by gc() method either by allowGC() method. Last one compares total size of objects allocated since last GC with threshold value and if it exceeds threshold, then starts garbage collection.

Base class for all garbage collectable objects is defined in this way:

class Object 
{
  public:
    /**
     * Redefined operator new for all derived classes
     */
    void* operator new(size_t size) 
    { 
        return MemoryAllocator::allocate(size);
    }

    /**
     * Objects should not be explicitly deleted.
     * Unreachable object is deleted by garbage collector.
     */
    void operator delete(void* obj) {
        free((ObjectHeader*)obj - 1); 
    } 

  protected:
    friend class MemoryAllocator;
    
    /**
     * Mark referenced objects
     */
    virtual void mark() {}

    /**
     * Virtual destructor used by memory allocator to finilize object
     */
    virtual~Object() {}
};

It redefines new/delete operators and declares virtual method mark() which should be redefined in derived classes to make object copy and mark referenced objects. It can be done using GC_MARK macro:

#define GC_MARK(Class) Class(*this)

class TreeNode : public Object 
{ 
    Ref<TreeNode> left;
    Ref<TreeNode> right;

    virtual void mark() { GC_MARK(TreeNode); } 
};

References in garbage collected classes should be represented using Ref class:

template<class T>
class Ref 
{
    T* obj;

  public:
    T* operator->() { 
        return obj; 
    }
    // ... many other methods emulating normal pointer operations
   
    /**
     * Copy constructor marks referenced objects using current allocator (if any)
     */         
    Ref(Ref<T> const& ref) : obj(ref.obj) {
        MemoryAllocator::mark(obj);
    }
};

Here the most interesting thing is destructor. It marks referenced objects.

The last puzzle in the picture is variable protecting object from garbage collection. It pins root of object tree making all objects accessible from this root reachable for marking.

class Root
{
    friend class MemoryAllocator;

  protected:
    Root* next; // roots are linked in L1 list

    /**
     * Mark root object
     */
    virtual void mark() = 0;

    /**
     * Register objects tree root in the current memory allocator
     */
    Root() 
    { 
        MemoryAllocator::registerRoot(this);
    }
    
    /**
     * Unregister objects tree root in the current memory allocator
     */
    ~Root() 
    { 
        MemoryAllocator::unregisterRoot(this);
    }        
};

No surprises here: constructor is linking object in the list, destructor unlinks it from the list.

There are may be several implementation of Root interface. The simplest is Var representing variable with single reference:

template<class T>
class Var : Root
{
    T* obj;

  public:
    T* operator->() { 
        return obj; 
    }
    // ... many other methods emulating normal pointer operations

    virtual void mark() { 
        MemoryAllocator::mark(obj);
    }
};

There are also implementations of Root interface representing fixed and varying size array of references. At mark phase memory allocator traverses list of roots and invokes mark() method for each root.

So we are using copy constructors for marking references by doing fake copy of the object (which is immediately deleted). But if we are in any case doing copies, why not to use them? Copying garbage collector is one of the possible ways of implementing GC and it is used in many languages. Actually it is the most efficient form of GC, because it is processing only live objects. Idea of copying garbage collector is simple: let's split heap in two parts. Objects are allocated in one of this parts. When GC is started, it copies live (reachable) objects to another part. After completion of GC, parts are changing their roles: old one is not needed any more (waiting for the next GC) and new one is used for object allocation.

Copying garbage collector makes memory allocator extremely simple and fast: we just allocate space for new objects sequentially. We do not need some extra per-object overhead, do not need to link objects in some lists, ... There is completely no fragmentation... So copying garbage collector has a lot of benefits. And what is the price for them? Well, the price is high... at least for C++. Copying object means that it's address is changed. Garbage collector will certainly update all references to this object, which it knows. But it means that you can not use normal C++ pointers to access this objects. Including "this" pointer. Please look at the following "init" method of SomeClass:

class SomeClass : public GC::OBject
{
     GC::String name;
     GC::String ident;
     
   public:
     void init(char const* nm, char const* id) { 
         name = String::create(nm);
         ident = String::create(id);
     }
 };

It seems to be trivial and obvious. But it will not work with copying garbage collector. At least if it is invoked automatically. Initialization of "name" may start garbage collection and it will copy all live objects, including this object. After it, "this" pointer is not pointing to correct object any more. So we have to protect "this" pointer:

class SomeClass : public GC::OBject
{
     GC::String name;
     GC::String ident;
     
   public:
     void init(char const* nm, char const* id) { 
         Pin ptr(this);
         name = String::create(nm);
         ident = String::create(id);
     }
 };

Copying garbage collector has one more significant disadvantage: lack of finalization. It is consequence of its advantage: processing only live objects. So dead (unreachable) object are not processed and so not destructed. You can not use in them some components which destructors are used to perform some cleanup, for example std::string.

I have implemented copying garbage collector (it can be found in copygc directory) mostly to compare its performance with mark&sweep garbage collector. Its implementation is very similar with mark&sweep. The difference is that Object interface contains virtual clone () method instead of mark and MemoryAllocator::mark() method now returns copy of cloned object:

class MemoryAllocator
{ 
    ...
    /**
     * Deep copy
     * @param obj cloned object
     * @return cloned object 
     */
    static Object* copy(Object* obj);
    ...
};

class Object 
{
    ...
    /**
     * Recursively clone object
     * @param allocator used allocator
     */
    virtual Object* clone() = 0;
};

template<class T>
class Ref 
{
    ...
    Ref(Ref<T> const& ref) 
    {
        obj = (T*)MemoryAllocator::copy(ref.obj);
    }
};

template<class T>
class Var : Root
{
    ...
    virtual void copy() { 
        obj = (T*)MemoryAllocator::copy(obj);
    }
};

class TreeNode : public GC::Object
{
  public:
    GC::Ref<GC::String> label;
    GC::Ref<Tree> left;    
    GC::Ref<Tree> right;

  protected:
    virtual GC::Object* clone() { 
        return new Tree(*this); 
    }
};

Copying garbage collector is really fast, because it is not using standard malloc/free. Testgc example shows about 8 times better results with copying garbage collector than with mark&sweep garbage collector. But lack of finalization and necessity to pin object are also significant drawbacks.

Let's now compare performance. I have implemented simple memory allocator benchmark which just creates in a loop larger number of simple objects and store pointers to them in cyclic buffer. So relatively small subset of objects is alive at each moment of time. I have tested standard C++ memory allocator and several specialized allocator: stack allocator and fixed size allocator. This is implementation using standard C++ new/delete mechanism:

Object* objects[liveObjects];
memset(objects, 0, sizeof(objects));
for (size_t i = 0; i < totalObjects; i++) { 
    delete objects[i % liveObjects];
    objects[i % liveObjects] = new Object();
}

Implementation for memory allocator with garbage collection is even simpler. This is one for boost::shared_ptr:

boost::shared_ptr<Object> objectRefs[liveObjects];
for (size_t i = 0; i < totalObjects; i++) { 
    objectRefs[i % liveObjects] = boost::shared_ptr<Object>(new Object());
}

And this is for mark&sweep allocator:

GC::MemoryAllocator mem(1*Mb, 1*Mb);
GC::ArrayVar<GCObject,liveObjects> objectRefs;
for (size_t i = 0; i < totalObjects; i++) { 
    objectRefs[i % liveObjects] = new GCObject();
}

Results at Linux are the following (in seconds - smaller is better):

Standard new/delete     35
Fixed size allocator    11
Stack allocator         12
boost::shared_ptr:      68
std::shared_ptr:        68
Mark&Sweep:             50
Copying GC:             31

Results at Windows (Visual Studio 2012) at the same system but under VmWare:

Standard new/delete:    215
Fixed size allocator:   11
Stack allocator:        12
std::shared_ptr:        447
Mark&Sweep:             235
Copying GC:             28

Please notice that both shared_ptr and mark&sweep use standard C runtime memory allocation functions. So their time is sum of memory allocation/deallocation itself and smart pointer overhead. In case of reference counters (shared_ptr) it is twice large than for mark&sweep algorithm.

Finally I want to summarize pros and contras of mark&sweep allocator:

• It is able to handle cyclic data structures.
• Has less overhead for manipulation with pointers.
• Allows to control time of initiating GC so makes it possible to temporarily use normal C++ pointers.
• Less space overhead: sizeof(Ref) == sizeof(T*)

• Performs extra copying of objects during mark phase.
• Unpredictable object destruction time in case of automatic start of garbage collection. So destruction of unreferenced objects (and releasing all resource used by the object) can be delayed for a long time. This is why it is better to explicitly release all objects resources, for example add close() method to object interface which will close used file.
• Requires special base class for garbage collectable objects.
• Garbage collection can take significant amount of time (if number of objects is large) and so it may delay normal activity of application for noticeable amount of time. It may cause increase of application response time. Many modern garbage collector are able to work in background. But it introduces a lot of challenges which can not be solved at C++ level.
• Recursive traversal of huge object clusters at mark stage may cause stack overflow.

So mark&sweep collector for C++ definitely can not be considered as "silver bullet" which solves the problem with memory deallocation in C++ or as better alternative to standard C++ new/delete and smart pointers based on reference counters. But for some use cases it can provide better results than alternative approaches.