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"Resources and Memory" lecture @ ELEKS Summer School 2012 (Jul 2012)
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.NET uses managed storage…So why do I need do know this
stuff?
Summer School 2012
Process memory layout (typical)
Summer School 2012
Storage management is hard
Summer School 2012
Manual• Fast• Precise• Deterministic
BUT
• Error-prone• Distracting• Hard to debug
Summer School 2012
Automaticvs.• Easy to program• Keeps you focused• Heuristically
optimized
BUT
• Unpredictable• Causes noticeable
delays• Sometimes leaks as
well
Common GC strategies
Summer School 2012
Mark & Sweep
Stop & Copy
Reference Counting
Mark & Sweep
Summer School 2012
Advantages
• Keeps objects in their places• Will work well with pointer arithmetics
Summer School 2012
Disadvantages
• Can cause heap fragmentation• Must process every object in the heap• May require O(n) space to execute
Summer School 2012
Stop & Copy
Summer School 2012
Advantages
• Fast allocation and collection• Operates only on live objects• Does not cause fragmentation
Summer School 2012
Disadvantages
• May require some time and space to rearrange pointers on live objects
• Unsuitable for C / C++
Summer School 2012
Reference Counting
Summer School 2012
• Remember the number of references to each object• Adjust it accordingly on every assignment
Advantages
• Easy to implement• Collects garbage incrementally without
significant delays
Summer School 2012
Disadvantages
• Unable to clean up circular structures• Requires atomic operations (expensive)• Slows down assignment operations
Summer School 2012
Which approach does .NET use?
Summer School 2012
???
.NET CLR Managed Heap
Summer School 2012
• Items are allocated consecutively.
• The only allocation overhead is about incrementing the pointer!
• When objects are destroyed, the managed heap gets automatically condensed (except the Large Object Heap!).
Object A Object B Object C Object D Object E Object F Managed heap
NextObjPtr
CLR Object Lifecycle
Summer School 2012
Allocation (IL newobj)
Construction (.ctor)
Member usage (your code)
Cleanup (finalizer)
Deallocation (GC)
Increase NextObjPtr and return the address of newly created object.
Fill the block with zero bytes, call the constructor (pass NextObjPtr as this).
Make sure there is enough free storage space available in the managed heap.
Append two system fields of type int.Type pointer SyncBlockIndex
Calculate the number of bytes required.
Object construction algorithm
Garbage Collection Triggers
Summer School 2012
• System is running low on memory1• A threshold of acceptable memory
usage has been exceeded on the managed heap.2
• GC.Collect() has been called3
Garbage Collection Algorithm
Summer School 2012
Heap Defragmentation (relocate objects).
Mark & Sweep
Define application roots (live objects)Stack roots GC handles Static
objectsCPU
registers
Suspend all threads except the one that triggered GCThread 1 [GC’ing] Thread 2
[suspended]Thread 3
[suspended]
GC Generations
Summer School 2012
Deterministic Cleanup: IDisposable
Summer School 2012
[ComVisibleAttribute(true)]public interface IDisposable{ void Dispose();}
Dispose() — Performs application-defined tasks associated with freeing, releasing, or resetting resources.
‘using’: syntactic sugar
Summer School 2012
using (Bitmap bitmap = new Bitmap(100, 100)){ Console.WriteLine(bitmap.Height);}
…is equivalent to:Bitmap bitmap = null;try{ bitmap = new Bitmap(100, 100); Console.WriteLine(bitmap.Height);}finally{ if (bitmap != null) { (IDisposable)bitmap.Dispose(); }}
Deterministic Cleanup: Finalization
Summer School 2012
Do NOT confuse with C++ destructor!!!
class UnmanagedResourceWrapper{ // Declare and use an unmanaged resource...
~UnmanagedResourceWrapper() { // Clean up... }}
Summer School 2012
1 2
3 4
Some Best Practices
Summer School 2012
• DON’T use IDisposable and/or Finalize unless you really have to (see below).
• DON’T use Finalize if you do not have unmanaged resources (such as handles).
1
• Use IDisposable if your type works with managed resources or contains an IDisposable member.
• Make sure you call base.Dispose() if base class is IDisposable.2• Use IDisposable AND Finalize if your type works with
unmanaged resources.• Finalize() should always release the resource and not throw any
exceptions.3
GCHandle Tricks
Summer School 2012
Lifetime monitoring & control
Pinning objects in memory
Weak references
Thank you!
Summer School 2012
