15 213 Memory Management I Dynamic Storage Allocation March 1 2001 Topics Explicit memory allocation Data structures Mechanisms class14 ppt Harsh Reality 3 Memory Matters Memory is not unbounded It must be allocated and managed Many applications are memory dominated Especially those based on complex graph algorithms Memory referencing bugs especially pernicious Effects are distant in both time and space Memory performance is not uniform Cache and virtual memory effects can greatly affect program performance Adapting program to characteristics of memory system can lead to major speed improvements class14 ppt 2 CS 213 S 01 Dynamic Storage Allocation Application Dynamic Storage Allocator Heap Memory Explicit vs Implicit Storage Allocator Explicit application allocates and frees space E g malloc and free in C Implicit application allocates but does not free space E g garbage collection in Java ML or Lisp Allocation In both cases the storage allocator provides an abstraction of memory as a set of blocks Doles out free memory blocks to application Will discuss explicit storage allocation today class14 ppt 3 CS 213 S 01 Process memory image kernel virtual memory memory invisible to user code stack esp Memory mapped region for shared libraries the brk ptr run time heap via malloc uninitialized data bss initialized data data program text text 0 class14 ppt 4 CS 213 S 01 Malloc package void malloc int size if successful returns a pointer to a memory block of at least size bytes if VL H returns NULL if unsuccessful returns NULL void free void p returns the block pointed at by p to pool of available memory p must come from a previous call to malloc Assumptions made in this lecture memory is word addressed each word can hold a pointer Allocated block 4 words class14 ppt Free block 3 words 5 Free word Allocated word CS 213 S 01 Allocation example p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 2 class14 ppt 6 CS 213 S 01 Constraints Applications Can issue arbitrary sequence of allocation and free requests Free requests must correspond to an allocated block Allocators Can t control number or size of allocated blocks Must respond immediately to all allocation requests i e can t reorder or buffer requests Must allocate blocks from free memory i e can only place allocated blocks in free memory Must align blocks so they satisfy all alignment requirements usually 8 byte alignment Can only manipulate and modify free memory Can t move the allocated blocks once they are allocated i e compaction is not allowed class14 ppt 7 CS 213 S 01 Goals of good malloc free Primary goals Good time performance for malloc and free Ideally should take constant time not always possible Should certainly not take linear time in the number of blocks Good space usage User allocated structures should be large fraction of operating system allocated pages Need to avoid fragmentation Some other goals Good locality properties structures allocated close in time should be close in space similar objects should be allocated close in space Robust can check that free p1 is on a valid allocated object p1 can check that memory references are to allocated space class14 ppt 8 CS 213 S 01 Fragmentation Tendency for free blocks to become smaller over time leading to wasted space p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 6 oops No general solution assuming we cannot move blocks We will consider several heuristics class14 ppt 9 CS 213 S 01 Implementation issues How do we know how much memory to free just given a pointer How do we keep track of the free blocks What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in How do we pick a block to use for allocation many might fit How do we reinsert freed block into the data structure that keeps track of freed blocks p0 free p0 class14 ppt p1 malloc 1 10 CS 213 S 01 Knowing how much to free Standard method keep the length of a structure in the word preceeding the structure This word is often called the header field requires an extra word for every allocated structure p0 p0 malloc 4 5 free p0 class14 ppt Block size data 11 CS 213 S 01 Keeping track of free blocks Method 1 implicit list using lengths links all blocks 5 4 6 2 Method 2 explicit list among the free blocks using pointers within the free blocks 5 4 6 2 Method 3 segregated free lists Different free lists for different size classes Method 4 blocks sorted by size Can use a balanced tree e g Red Black tree with pointers within each free block and the length used as a key class14 ppt 12 CS 213 S 01 Method 1 implicit list Need to identify whether each block is free or allocated Can use extra bit Bit can be put in the same word as the size if block sizes are always multiples of two mask out low order bit when reading size 1 word size Format of allocated and free blocks class14 ppt a a 1 allocated block a 0 free block size block size data data application data allocated blocks only 13 CS 213 S 01 Implicit list finding a free block First fit Search list from beginning choose first free block that fits p start while p end p 1 p len not passed end already allocated too small Can take linear time in total number of blocks allocated and free In practice it can cause splinters at beginning of list Next fit Like first fit but search list from location of end of previous search Does a better job of spreading out the free blocks Best fit Search the list choose the free block with the closest size that fits Keeps fragments small usually helps fragmentation Will typically run slower than first fit class14 ppt 14 CS 213 S 01 Implicit list allocating in a free block Allocating in a free block splitting Since allocated space might be smaller than free space we need to split the block 4 4 6 2 p void addblock ptr p int l int newsize l 1 1 1 int oldsize p 2 p newsize 1 if newsize oldsize p newsize oldsize newsize add 1 and round up mask out low bit set new length set length in remaining part of block addblock p 2 4 class14 ppt 4 4 15 2 2 CS 213 S 01 Implicit list freeing a block Simplest implementation Only need to clear allocated flag void free block ptr p p p 2 But can lead to false fragmentation 4 4 2 2 2 2 p free p 4 malloc 5 4 4 4 Oops There is enough free space but the allocator won t be able to find it class14 ppt 16 CS 213 S 01 Implicit list coalescing Join with next and or previous block if they are free Coalescing with next block void free block ptr p p p 2 clear allocated flag
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