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The GC Heap

Introduction to the GC Heap

CLR's GC is a generational, mark and sweep GC. There are 3 generations in the process, gen0, gen1, and gen2. Objects are allocated in gen0, and whenever an object survives a GC they are promoted to the next generation (thus a gen0 object becomes a gen2 object and a gen1 object becomes a gen2 object). The only exception to this rule are large objects. Large objects are allocated directly into gen2 (on a special segment) and are never relocated. An object is considered a "large object" if it takes up more than 85,000 bytes.

There are also two types of GC's: the workstation GC and the server GC. There are two primary differences between a workstation GC and a server GC from a diagnostics standpoint: First, the workstation GC runs on only one thread whereas the server GC runs on multiple threads. Second (and related to the first point), a workstation GC has only one logical heap whereas the server GC has as many logical heaps as there are cores on the machine.

When you make an allocation with the server GC running, the newly allocated object goes into the logical heap associated with the core that thread happened to be running on. In general, the distinction between the two types of GCs does not matter for diagnostics. The only real case where you need to care about the logical heaps in the process is when they become unbalanced...when one heap has had much more allocated on it than other heaps. This can lead to a performance issue where GCs pause the process for longer than it should. (An example of displaying heap balance is shown later.)

Each logical heap in the process has a number of heap Segments associated with them. A segment is a region of memory which contains managed objects. There are three kinds of GC segments:

Ephemeral segments, gen2 segments, and large object segments. There is exactly 1 ephemeral segment per logical heap, and the ephemeral segment contains gen0, gen1, and gen2 objects. When we run out of space on the ephemeral segment, we allocate a gen2 segment and move some (or all) gen2 objects from the ephermal segment to the new segment. If there's already a gen2 segment for that logical heap, we will continue to move gen2 objects out of the ephemeral segment onto the gen2 segment until we run out of room. There can be any number of gen2 segments per logical heap.

Finally, there are large object segments. When a large object (85,000 bytes or more) are allocated, we allocate this object directly into gen2. We do not, however, use the gen2 segments to do this. Instead we allocate these objects in "large object segments" directly. Large object segments have two main properties: all objects in them are considered gen2 and no object in them may be relocated (we treat objects in these segments as pinned).

Getting the Heap and Walking Segments

The CLRRuntime object has a function called GetHeap, which returns a GCHeap object. The GCHeap object, among other things, allows you to walk each segment in the process. Here's a simple example of walking each segment and printing out data for each segment:

CLRRuntime runtime = ...;
Console.WriteLine("{0,12} {1,12} {2,12} {3,12} {4,4} {5}", "Start", "End", "Committed", "Reserved", "Heap", "Type");
foreach (GCHeapSegment segment in heap.Segments)
{
    string type;
    if (segment.Ephemeral)
        type = "Ephemeral";
    else if (segment.Large)
        type = "Large";
    else
        type = "Gen2";

    Console.WriteLine("{0,12:X} {1,12:X} {2,12:X} {3,12:X} {4,4} {5}", segment.Start, segment.End, segment.Committed, segment.Reserved, segment.ProcessorAffinity, type);
}

As you can see, each GCSegment object gives you a Start address for the beginning of the GC segment and the End address (which is the address after the end of the last object). You also have access to the Committed line, that is, the address which is the limit of what we have committed (so Committed - End is the amount of memory we have committed but not filled). Similarly we also give you the reserved line: the limit of the memory we have reserved for the segment.

Note that the GCSegment.ProcessorAffinity is actually the logical GC Heap to which the segment belongs. Here is a simple linq query which will print out a table showing logical heap balance:

foreach (var item in (from seg in heap.Segments
                      group seg by seg.ProcessorAffinity into g
                      orderby g.Key
                      select new
                      {
                          Heap = g.Key,
                          Size = g.Sum(p=>(uint)p.Length)
                      }))
{
    Console.WriteLine("Heap {0,2}: {1:n0} bytes", item.Heap, item.Size);
}

As mentioned before, logical heap imbalance in server GC can cause perf issues.

Walking Managed Objects in the Process

As mentioned before, GC segments contain managed objects. You can walk all objects on a segment by starting at GCHeapSegment.FirstObject and repeatedly calling GCHeapSegment.NextObject until it returns 0. To get the type of an object, you can call GCHeap.GetObjectType. This returns a GCHeapType object, which we will cover in more detail in a later tutorial.

Here is an example of walking each object on each segment in the process and printing the address, size, generation, and type of the object:

CLRRuntime runtime = ...;
GCHeap heap = runtime.GetHeap();

if (!heap.CanWalkHeap)
{
    Console.WriteLine("Cannot walk the heap!");
}
else
{
  foreach (GCHeapSegment seg in heap.Segments)
  {
      for (ulong obj = seg.FirstObject; obj != 0; obj = seg.NextObject(obj))
      {
          GCHeapType type = heap.GetObjectType(obj);

          // If heap corruption, continue past this object.
          if (type == null)
              continue;

          ulong size = type.GetSize(obj);
          Console.WriteLine("{0,12:X} {1,8:n0} {2,1:n0} {3}", obj, size, seg.GetGeneration(obj), type.Name);
      }
  }

}

There are two parts in this example you should pay attention to. First is checking the GCHeap.CanWalkHeap property. This property specifies whether the process is in a state where you can reliably walk the heap. If the crashdump was taken during the middle of a GC, the GC could have been relocating objects. At which point a linear walk of the GC heap is not possible. If this is the case, CanWalkHeap will return false.

Second, you need to check the return value of GetObjectType to make sure it's non-null. GCHeapSegment.NextObject does not attempt to detect heap corruption, so it is possible GetObjectType will return null if the address that NextObject returns is a corrupt object.

Walking objects without walking the segments

There is another way to walk the heap, one which takes far less code than walking each segment: GCHeap.EnumerateObjects. Here is an example:

CLRRuntime runtime = ...;
GCHeap heap = runtime.GetHeap();

if (!heap.CanWalkHeap)
{
    Console.WriteLine("Cannot walk the heap!");
}
else
{
  foreach (ulong obj in heap.EnumerateObjects())
  {
      GCHeapType type = heap.GetObjectType(obj);

      // If heap corruption, continue past this object.
      if (type == null)
          continue;

      ulong size = type.GetSize(obj);
      Console.WriteLine("{0,12:X} {1,8:n0} {2,1:n0} {3}", obj, size, heap.GetObjectGeneration(obj), type.Name);
  }

}

The above code's results are equivalent to the one above it. In general, you should choose the heap walking approach that best fits your scenario. In general, if you need to walk only portions of the heap (such as only gen0 objects) or if you need segment data (such as what generation an object is), you should use the first code. If you simply need to walk all objects on the heap, use the second code.

A non-linear heap walk

The approach above is a good way to to walk every object on the heap. But what if you want to only walk a subset of objects? For example, let's say you have an object and you want to know all of the objects it points to, and all the objects those point to, and so on. This is what we call the !objsize algorithm.

(If you are not familiar with !objsize in SOS, this command takes an object as (a parameter and counts the number of objects it keeps alive as well as reports (the total size of all objects the given object keeps alive.)

Given an object, you can enumerate all objects it points to using GCHeapType.Enumerate object references. We will use that function to implement objsize:

private static void ObjSize(GCHeap heap, ulong obj, out uint count, out ulong size)
{
    // Evaluation stack
    Stack<ulong> eval = new Stack<ulong>();

    // To make sure we don't count the same object twice, we'll keep a set of all objects
    // we've seen before.  Note the ObjectSet here is basically just "HashSet<ulong>".
    // However, HashSet<ulong> is *extremely* memory inefficient.  So we use our own to
    // avoid OOMs.
    ObjectSet considered = new ObjectSet(heap);

    count = 0;
    size = 0;
    eval.Push(obj);

    while (eval.Count > 0)
    {
        // Pop an object, ignore it if we've seen it before.
        obj = eval.Pop();
        if (considered.Contains(obj))
            continue;

        considered.Add(obj);

        // Grab the type. We will only get null here in the case of heap corruption.
        GCHeapType type = heap.GetObjectType(obj);
        if (type == null)
            continue;

        count++;
        size += type.GetSize(obj);

        // Now enumerate all objects that this object points to, add them to the
        // evaluation stack if we haven't seen them before.
        type.EnumerateRefsOfObject(obj, delegate(ulong child, int offset)
        {
            if (child != 0 && !considered.Contains(child))
                eval.Push(child);
        });
    }
}

Why do we need EnumerateRefsOfObject?

You might be wondering why we need EnumerateRefsOfObject at all. As you will see in the next tutorial, you can walk each field in the object and get its value. You could implement this algorithm by walking fields instead. However, EnumerateRefsOfObject is much, much faster. It uses the same algorithm the GC does to get object references out of the object, which is far more efficient than walking fields to look for objects.

Conclusion

As you can see, it doesn't take much work to walk the heap. To do anything useful with the objects you get, though, you will need to work with the GCHeapType for that object. In the next tutorial we will fully explore types in CLRMD.

Next Tutorial: Types and Fields in CLRMD