Kĩ thuật lập trình - Chapter 11: Memory management

Programming Languages2nd editionTucker and NoonanChapter 11Memory ManagementC makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do it blows your whole leg off. B. StroustrupContents11.1 The Heap11.2 Implementation of Dynamic Arrays11.3 Garbage Collection11.1 The HeapThe major areas of memory: Static area: fixed size, fixed content allocated at compile time Run-time stack: variable size, variable content center of control for function call and return Heap: fixed size, variable content dynamically allocated objects and data structures The Structure of Run-Time MemoryFig 11.1Allocating Heap BlocksThe function new allocates a block of heap space to the program. E.g., new(5) returns the address of the next block of 5 words available in the heap:Stack and Heap OverflowStack overflow occurs when the top of stack, a, would exceed its (fixed) limit, h.Heap overflow occurs when a call to new occurs and the heap does not have a large enough block available to satisfy the call. 11.2 Implementation of Dynamic ArraysConsider the declaration int A[n];Its meaning (Meaning Rule 11.1) is: 1. Compute addr(A[0]) = new(n). 2. Push addr(A[0]) onto the stack. 3. Push n onto the stack. 4. Push int onto the stack.Step 1 creates a heap block for A. Steps 2-4 create the dope vector for A in the stack.Stack and Heap Allocation for int A[10];Fig 11.3Array ReferencesMeaning Rule 11.2 The meaning of an ArrayRef ar for an array declaration ad is: 1. Compute addr(ad[ar.index]) = addr(ad[0])+ar.index-1 2. If addr(ad[0])addr(ad[ar.index])<addr(ad[0])+ad.size, return the value at addr(ad[ar.index]) 3. Otherwise, signal an index-out-of-range error.E.g., consider the ArrayRef A[5]. The value of A[5] is addressed by addr(A[0])+4.Note: this definition includes run-time range checking.Array AssignmentsMeaning Rule 11.3 The meaning of an Assignment as is: 1. Compute addr(ad[ar.index])=addr(ad[0])+ar.index-1 2. If addr(ad[0])addr(ad[ar.index])<addr(ad[0])+ad.size then assign the value of as.source to addr(ad[ar.index]). 3. Otherwise, signal an index-out-of-range error.E.g., The assignment A[5]=3 changes the value at heap address addr(A[0])+4 to 3, since ar.index=5 and addr(A[5])=addr(A[0])+4. 11.3 Garbage CollectionGarbage is a block of heap memory that cannot be accessed by the program.Garbage can occur when either: 1. An allocated block of heap memory has no reference to it (an “orphan”), or 2. A reference exists to a block of memory that is no longer allocated (a “widow”).Garbage Example (Fig 11.4)class node { int value; node next;}node p, q;p = new node();q = new node();q= p;delete p;Garbage Collection AlgorithmsGarbage collection is any strategy that reclaims unused heap blocks for later use by the program.Three classical garbage collection strategies:Reference Counting - occurs whenever a heap block is allocated, but doesn’t detect all garbage.Mark-Sweep - Occurs only on heap overflow, detects all garbage, but makes two passes on the heap.Copy Collection - Faster than mark-sweep, but reduces the size of the heap space.11.3.1 Reference CountingThe heap is a chain of nodes (the free_list).Each node has a reference count (RC).For an assignment, like q = p, garbage can occur:But not all garbage is collectedSince q’s node has RC=0, the RC for each of its descendents is reduced by 1, it is returned to the free list, and this process repeats for its descendents, leaving:Note the orphan chain on the right.11.3.2 Mark-SweepEach node in the free_list has a mark bit (MB) initially 0.Called only when heap overflow occurs:Pass I: Mark all nodes that are (directly or indirectly) accessible from the stack by setting their MB=1.Pass II: Sweep through the entire heap and return all unmarked (MB=0) nodes to the free list. Note: all orphans are detected and returned to the free list.Heap after Pass I of Mark-SweepTriggered by q=new node() and free_list = null. All accessible nodes are marked 1.Heap after Pass II of Mark-SweepNow free_list is restored and the assignment q=new node() can proceed.11.3.3 Copy CollectionHeap partitioned into two halves; only one is active.Triggered by q=new node() and free_list outside the active half:Accessible nodes copied to other halfNote: The accessible nodes are packed, orphans are returned to the free_list, and the two halves reverse roles.Garbage Collection SummaryModern algorithms are more elaborate.Most are hybrids/refinements of the above three.In Java, garbage collection is built-in.runs as a low-priority thread.Also, System.gc may be called by the program. Functional languages have garbage collection built-in.C/C++ default garbage collection to the programmer.