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Garbage Collection

Garbage Collection. Student: Jack Chang. Introduction. Manual memory management Memory bugs Automatic memory management We know... A program can only use objects that can be found . Reference Counting. Reference Count Store reference count in each object

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Garbage Collection

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  1. Garbage Collection Student: Jack Chang

  2. Introduction • Manual memory management • Memory bugs • Automatic memory management • We know... • A program can only use objects that can be found.

  3. Reference Counting • Reference Count • Store reference count in each object • For each assignment operation, update reference count • If reference count equals 0, we de-allocate the object. Figure 1: Example objects Y Object i Object j 1 2 X Z

  4. Reference Counting Step 0: Start Object i 1 X Object j 1 Y Step 1: Y = Object i; Object i 1+1 X Object j 1-1 Y

  5. Reference Counting Step 3: Check right hand side Object i 2 X Object j 0 Y Step 4: Finish assignment Object i 2 X Y

  6. Reference Counting • Advantages • Easy to implement • Does not wait till memory is exhausted • Disadvantages • ...

  7. Reference Counting • Assignment operation gets slower 1 1 1 1 X 1 1 1

  8. Reference Counting • Assignment operation gets slower 1 1 1 0 X 1 1 1

  9. Reference Counting • Does not work with circular structure Object i 2 X Object j 1

  10. Reference Counting • Does not work with circular structure Object i 1 X Object j 1

  11. Stop and Copy Allocation Pointer Old Space New Space Allocation Pointer A B New Space C D E

  12. Stop and Copy Allocation Pointer Root set A B New Space C D E Scan Pointer

  13. Stop and Copy Allocation Pointer Scan Pointer Root set A B C D E A Allocation Pointer Scan Pointer Root set B A B C D E A

  14. Stop and Copy Root set A B E Old Space New Space

  15. Stop and Copy • Advantages • Defragment memory during garbage collection • The more garbage the faster it will be • Disadvantages • Need to copy all the reachable objects to new space • Need twice as much memory as the program actually uses

  16. At the end • The garbage collection is convenient, but... • Pauses • There are more advanced garbage collection algorithms • Concurrent • Parallel • Thank you.

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