Memory Virtualization: The UNIX Memory API Prof. Patrick G. Bridges - - PowerPoint PPT Presentation

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Memory Virtualization: The UNIX Memory API

• Prof. Patrick G. Bridges

University of New Mexico

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Memory API: malloc()

 Main programmer-level API in C.  Language-level interface, not the actual OS interface  Allocate a memory region on the heap.

▪ Argument

▪ size_t size : size of the memory block(in bytes) ▪ size_t is an unsigned integer type.

▪ Return

▪ Success : a void type pointer to the memory block allocated by

malloc

▪ Fail : a null pointer

 free/calloc/realloc also exist in the same vein

#include <stdlib.h> void* malloc(size_t size)

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Lots of ways to go wrong with memory

 Some sample things we’ve all done

▪ Not copying enough data (e.g. terminating nulls) ▪ Not allocating space for the data copied ▪ Not freeing data (memory leaks) ▪ Accessing data after its freed ▪ Freeing an area multiple times

 What does each of these actually do?  Requires understanding how the language API is built on

top of the OS memory API

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Memory Management System Calls

 malloc  malloc library call use brk and/or mmap system calls.

▪ brk is called to expand the program’s break.

▪ break: The location of the end of the heap in address space

▪ sbrk is an additional call similar with brk. ▪ Programmers should never directly call either brk or sbrk.

 What does this actually do?

#include <unistd.h> int brk(void *addr) void *sbrk(intptr_t increment);

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How do brk and sbrk work?

 The greyed-out area of an address

space is not actually allocated.

 To use it, the OS has to make it

available.

 Brk/sbrk sets the location of the

boundary between allocated heap memory and unallocated memory!

(free) Code (Text) Stack stack heap Address Space Data Heap

0x400000 0xcf2000 0x7fff9ca49000 0x401000 0xd13000 0x7fff9ca28000

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System Calls(Cont.)

▪ mmap system call can create an anonymous memory region.

#include <sys/mman.h> void *mmap(void *ptr, size_t length, int port, int flags, int fd, off_t offset)

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What about mmap?

 mmap lets the program request finer-

grain allocation of parts of its address space

 More than just moving the program

break

 Note that the address space is now

disjoint!

 mmap can also do implicit file I/O –

that’s a later topic…

(free) Code (Text) Stack stack heap Address Space Data Heap

0x400000 0xcf2000 0x7fff9ca49000 0x401000 0xd13000 0x7fff9ca28000

mmap region

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How are malloc/new implemented?

 The language runtime (libc) gets memory in chunks from

the operating system

▪ Using mmap or sbrk – 4k or more at a time ▪ (Why not in smaller pieces?)

 The language runtime divides these big blocks up to

satisfy malloc/new requests

▪ Basic data structure is a “free list”, a linked list of free chunks of

memory

▪ Malloc/new searches list to find a chunk to satisfy an allocation

request

▪ Free returns things to this list

 Important questions

▪ Where are the pointers for the linked lists stored? ▪ What block do you use to satisfy an allocation request?

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Fragmentation: Storage Virtualization Enemy #1

 We generally have to divide big blocks into smaller blocks,

and there’s rarely an exact fit

▪ Variable-size allocations can result in with small, hard-to-allocate

blocks

▪ Sometimes have to allocate space bigger than we want to

 Wasted space from storage allocation is called

fragmentation

▪ Internal fragmentation – wasted space that’s allocated but not

• used. (You want 5 bytes but we have to allocate 8)

▪ External fragmentation – small bits we can’t allocate

 How do we allocate storage to handle fragmentation?