15 213 The course that gives CMU its Zip Writing You Own malloc March 29 2003 Adapted From Fall 2003 Lectures Topics Explicit Allocation Data structures Mechanisms Policies class19 ppt Process Memory Image kernel virtual memory stack esp memory invisible to user code Memory mapped region for shared libraries Allocators request additional heap memory from the operating system using the sbrk function the brk ptr run time heap via malloc uninitialized data bss initialized data data program text text 2 0 15 213 F 03 Malloc Package include stdlib h void malloc size t size If successful Returns a pointer to a memory block of at least size bytes typically aligned to 8 byte boundary If size 0 returns NULL If unsuccessful returns NULL 0 and sets errno 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 or realloc void realloc void p size t size Changes size of block p and returns pointer to new block Contents of new block unchanged up to min of old and new size 3 15 213 F 03 Allocation Examples p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 2 4 15 213 F 03 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 utilization User allocated structures should be large fraction of the heap Want to minimize 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 5 15 213 F 03 Performance Goals Throughput Given some sequence of malloc and free requests R0 R1 Rk Rn 1 Want to maximize throughput and peak memory utilization These goals are often conflicting Throughput Number of completed requests per unit time Example 5 000 malloc calls and 5 000 free calls in 10 seconds Throughput is 1 000 operations second 6 15 213 F 03 Performance Goals Peak Memory Utilization Given some sequence of malloc and free requests R0 R1 Rk Rn 1 Def Aggregate payload Pk malloc p results in a block with a payload of p bytes After request Rk has completed the aggregate payload Pk is the sum of currently allocated payloads Def Current heap size is denoted by Hk Assume that Hk is monotonically nondecreasing Def Peak memory utilization After k requests peak memory utilization is Uk maxi k Pi Hk 7 15 213 F 03 Internal Fragmentation Poor memory utilization caused by fragmentation Comes in two forms internal and external fragmentation Internal fragmentation For some block internal fragmentation is the difference between the block size and the payload size block Internal fragmentation 8 payload Internal fragmentation Caused by overhead of maintaining heap data structures padding for alignment purposes or explicit policy decisions e g not to split the block Depends only on the pattern of previous requests and thus is easy to measure 15 213 F 03 External Fragmentation Occurs when there is enough aggregate heap memory but no single free block is large enough p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 6 oops External fragmentation depends on the pattern of future requests and thus is difficult to measure 9 15 213 F 03 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 p0 free p0 10 p1 malloc 1 15 213 F 03 Knowing How Much to Free Standard method Keep the length of a block in the word preceding the block This word is often called the header field or header Requires an extra word for every allocated block p0 malloc 4 p0 5 free p0 11 Block size data 15 213 F 03 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 list Different free lists for different size classes Method 4 Blocks sorted by size 12 Can use a balanced tree e g Red Black tree with pointers within each free block and the length used as a key 15 213 F 03 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 payload a a 1 allocated block a 0 free block size block size payload application data allocated blocks only optional padding 13 15 213 F 03 Implicit List Finding a Free Block First fit Search list from beginning choose first free block that fits p start while p end not passed end p 1 already allocated p len too small p p p 2 goto next block 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 Research suggests that fragmentation is worse Best fit 14 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 15 213 F 03 Bitfields How to represent the Header Masks and bitwise operators define PACK size alloc size alloc define getSize x x size SIZEMASK bitfields struct unsigned allocated 1 unsigned size 31 Header 15 15 213 F 03 Implicit List Allocating in Free Block Allocating in a free block splitting Since allocated space might be smaller than free space we might want to split the block 4 4 6 2 p void addblock ptr p int len int newsize len 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 16 4 4 2 2 15 213 F 03 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 17 15 213 F 03 Implicit List Coalescing Join coalesce with next and …
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