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Today Basic concepts Dynamic Memory Allocation: Performance concerns Approach 1: implicit free lists Basic Concepts CSci 2021: Machine Architecture and Organization April 24th-27th, 2020 Your instructor: Stephen McCamant Based on

  1. Today  Basic concepts Dynamic Memory Allocation:  Performance concerns  Approach 1: implicit free lists Basic Concepts CSci 2021: Machine Architecture and Organization April 24th-27th, 2020 Your instructor: Stephen McCamant Based on slides originally by: Randy Bryant, Dave O’Hallaron 1 2 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Dynamic Memory Allocation Dynamic Memory Allocation  Programmers use Application  Allocator maintains heap as collection of variable sized dynamic memory Dynamic Memory Allocator blocks , which are either allocated or free allocators (such as Heap malloc ) to acquire VM  Types of allocators at run time.  Explicit allocator : application allocates and frees space  For data structures whose  E.g., malloc and free in C User stack size is only known at  Implicit allocator: application allocates, but does not free space runtime.  E.g. garbage collection in Java, ML, and Lisp Top of heap  Dynamic memory ( brk ptr) Heap (via malloc ) allocators manage an  Will discuss simple explicit memory allocation today Uninitialized data (. bss ) area of process virtual memory known as the Initialized data ( .data ) heap . Program text ( .text ) 0 3 4 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition malloc Example The malloc Package #include <stdlib.h> #include <stdio.h> #include <stdlib.h> void *malloc(size_t size)  Successful: void foo(int n) { int i, *p;  Returns a pointer to a memory block of at least size bytes aligned to an 8-byte (x86) or 16-byte (x86-64) boundary /* Allocate a block of n ints */ p = (int *) malloc(n * sizeof(int));  If size == 0 , returns NULL if (p == NULL) {  Unsuccessful: returns NULL (0) and sets errno perror("malloc"); exit(0); void free(void *p) }  Returns the block pointed at by p to pool of available memory /* Initialize allocated block */  p must come from a previous call to malloc or realloc for (i=0; i<n; i++) p[i] = i; Other functions  calloc : Version of malloc that initializes allocated block to zero. /* Return allocated block to the heap */  realloc: Changes the size of a previously allocated block. free(p);  sbrk : Used internally by allocators to grow or shrink the heap } 5 6 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 1

  2. Allocation Example Assumptions Made in These Slides  Memory is word addressed. p1 = malloc(4)  Words are int-sized. p2 = malloc(5) p3 = malloc(6) Allocated block Free block (4 words) (3 words) Free word Allocated word free(p2) p4 = malloc(2) 7 8 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Constraints Today  Applications  Basic concepts  Can issue arbitrary sequence of malloc and free requests  free request must be to a malloc ’d block  Performance concerns  Approach 1: implicit free lists  Allocators  Can’t control number or size of allocated blocks  Must respond immediately to malloc 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  8-byte (x86) or 16-byte (x86-64) alignment on Linux boxes  Can manipulate and modify only free memory  Can’t move the allocated blocks once they are malloc ’d  i.e ., compaction is not allowed 9 13 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Performance Goal: Peak Memory Utilization Performance Goal: Throughput  Given some sequence of malloc and free requests:  Given some sequence of malloc and free requests:  R 0 , R 1 , ..., R k , ... , R n-1  R 0 , R 1 , ..., R k , ... , R n-1  Def: Aggregate payload P k  malloc(p) results in a block with a payload of p bytes  Goals: maximize throughput and peak memory utilization  After request R k has completed, the aggregate payload P k is the sum of  These goals are often conflicting currently allocated payloads  Def: Current heap size H k  Throughput:  Assume H k is monotonically nondecreasing  Number of completed requests per unit time  i.e., heap only grows when allocator uses sbrk  Example:  Def: Peak memory utilization after k+1 requests  5,000 malloc calls and 5,000 free calls in 10 seconds  U k = ( max i<=k P i ) / H k  Throughput is 1,000 operations/second 14 15 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 2

  3. Internal Fragmentation Fragmentation  For a given block, internal fragmentation occurs if payload is  Poor memory utilization caused by fragmentation smaller than block size  internal fragmentation: inside a block  external fragmentation: between blocks Block Internal Internal Payload fragmentation fragmentation  Caused by  Overhead of maintaining heap data structures  Padding for alignment purposes  Explicit policy decisions (e.g., to return a big block to satisfy a small request)  Depends only on the pattern of previous requests  Thus, easy to measure 16 17 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition External Fragmentation Implementation Issues  Occurs when there is enough aggregate heap memory,  How do we know how much memory to free given just a but no single free block is large enough pointer? p1 = malloc(4)  How do we keep track of the free blocks? p2 = malloc(5)  What do we do with the extra space when allocating a p3 = malloc(6) structure that is smaller than the free block it is placed in? free(p2)  How do we pick a block to use for allocation -- many Oops! (what would happen now?) p4 = malloc(6) might fit?  Depends on the pattern of future requests  Thus, difficult to measure  How do we reinsert freed block? 18 19 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Knowing How Much to Free Keeping Track of Free Blocks  Method 1: Implicit list using length — links all blocks  Standard method  Keep the length of a block in the word preceding the block.  This word is often called the header field or header 5 4 6 2  Requires an extra word for every allocated block  Method 2: Explicit list among the free blocks using pointers p0 5 4 6 2 p0 = malloc(4) 5  Method 3: Segregated free list  Different free lists for different size classes block size payload  Method 4: Blocks sorted by size free(p0)  Can use a balanced tree (e.g. Red-Black tree) with pointers within each free block, and the length used as a key 20 21 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 3

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