Basic Memory Management 1

Readings r Silbershatz et al: chapters

Outline r Memory Hierarchy r Memory management requirements r Basic memory management r Fixed Partitions r Dynamic Partitions 3

Introduction r Our machines today have 10,000 times more memory than the IBM 7094 – leading edge machine of the 1960’s r Cost of memory has dropped dramatically r Bill Gates (former chair of Microsoft) once said “640K should be enough” 4

Introduction r Software tends to expand to fill the memory available r Technology does not allow for each program to have infinitely large and fast memory r Operating systems must manage memory 5

Instruction-Execution Cycle r You can think of memory as a large array of bytes m Each byte has its own address r Fetch an instruction from memory r Instruction is decoded m May cause operands to be fetched from memory r After instruction execution m Results may be stored back in memory r Each of these operations require memory addresses 6

Registers On-chip Cache Main Memory Magnetic (Hard) Disk Memory Hierarchy r Registers built into the CPU are generally accessible within one cycle of the CPU clock r Completing a memory access may take many cycles of the CPU clock 7

Memory Hierarchy r A processor waiting for data from main memory is not desired r Remedy: Add fast memory between the CPU and main memory called a cache 8

Address Binding r Program execution requires that a program be brought into memory from the disk r The process can reside in any part of the physical memory r Address space starts at but the first address of a user need not be

Address Binding r Addresses in source program are generally symbolic e.g., count r Compiler will bind these symbolic addresses to relocatable addresses m Example: “14 bytes from the beginning of this module” r Binding to physical addresses is typically delayed until run-time 10

Address Binding r Programmer does not know where the program will be placed in memory when it is executed r While the program is executing, it may be placed in disk and returned to main memory at a different location (swapping) r Memory references in the code (virtual or logical) must be translated to actual physical memory addresses r Run-time Mapping from virtual to physical addresses is done by a hardware device called the memory-management unit (MMU) 11

Simple MMU r Simple mapping r Relocation register: Value is added to every address generated by a user process 12

Contiguous Memory Allocation r We will start out with the most basic method used that allows multiple processes to reside in memory r With contiguous memory allocation each process is contained in a single section of memory that is contiguous 13

Fixed Partitioning r Main memory use is inefficient. r Any program, no matter how small, occupies an entire partition. m This is internal fragmentation. 8 M Operating System 14

Fixed Partitioning r Unequal-size partitions m Lessens the problem with equal-size partitions Operating System 8 M 12 M 8 M 6 M 4 M 2 M 15

Placement Algorithm with Partitions r Equal-size partitions m Since all partitions are of equal size, it does not matter which partition is used r Unequal-size partitions m Each process can be assigned to the smallest partition within which it will fit 16

One Process Queue per Partition New Processes Operating System 17

One Common Process Queue r When its time to load a process into main memory the smallest available partition that will hold the process is selected Operating System New Processes 18

Questions r Single Queue: m What is the problem with taking the first job that fits into the partition? m What is the problem with taking the largest job that fits into the partition? r Multiple Queues: m What is the disadvantage? 19

Multiprogramming with Fixed Partitions r Was used by OS/360 on large IBM mainframes for many years r Incoming jobs were queued until a suitable partition was available r Today no modern OS uses fixed partitions 20

Dynamic Partitioning r Partitions are of variable length and number r Process is allocated exactly as much memory as required r Eventually get holes in the memory. This is called external fragmentation r Compaction is required to obtain a large block at the end of memory m Shift processes so they are contiguous and all free memory is in one block 21

Example Dynamic Partitioning Operating System Process K Process 2 Process K 288 K 64 K Operating System Process K Process K 288 K 64 K Operating System Process K Process 3288 K 64 K Process K 96 K 22

Example Dynamic Partitioning Operating System 320 K Process 3288 K 64 K Process K 96 K Operating System Process 3288 K 64 K Process K 96 K Process 2224 k 96 K 23

Dynamic Partitioning Placement Algorithm r Operating system must decide which free block to allocate to a process r Best-fit algorithm m Choose the block that is closest in size to the request m This has the worst overall performance m The smallest block is found for a process The smallest amount of fragmentation is left; Memory compaction must be done more often 24

Dynamic Partitioning Placement Algorithm r First-fit algorithm m Starts scanning memory from the beginning and chooses the first available block that is large enough. r Next-fit m Starts scanning memory from the location of the last placement and chooses the next available block that is large enough 25

Summary r This section studied basic memory allocation techniques 26