Memory Management Chapter 4

Memory hierarchy Programmers want a lot of fast, non- volatile memory But, here is what we have:

Memory manager Tracks which parts of memory are in use Allocates memory to processes –Long-term scheduling De-allocates memory from processes Swaps between main memory and disk when main memory is too small to hold all of the processes –Intermediate scheduling

Simple memory management An operating system with one user process

Multiprogramming Use fixed-sized partitions (MFT) –Each partition contained only one process –Very simple More flexibility is achieved with MVT –OS knows which parts of memory are available –When a process is to be loaded, it needs a large enough block of contiguous memory

Multiprogramming with fixed size partitions (MFT) single input queue for all memory separate input queue for each partition

MVT Example job queue at time 0 operating system 0 400K 2160K 2560K How to schedule the job queue in a FCFS fashion? process memory time in system P1 600K10 P2 1000K 5 P3 300K20 P4 700K 8 P5 500K15

MVT Example – at time 5 job queue process memory time in system P4 700K 8 P5 500K15 operating system 0 400K 260K 2560K Now P2 is done, so replace with P4… P1 P2 P3 2000K 2300K 1000 K

MVT Example – at time 8 job queue process memory time in system P5 500K15 operating system 0 400K 260K 2560K Now P4 is done, so replace with P5… P1 P4 P3 2000K 2300K 1000 K 1700 K 300K

MVT Example – at time 8+ Empty job queue operating system 0 400K 260K 2560K This worked well with our configuration of jobs, but what can go wrong? P1 P5 P3 2000K 2300K 1000 K 1500 K 500K

CPU utilization as a function of number of processes in memory Degree of multiprogramming

Relocatability Processes are loaded into main memory –They may be loaded into any unoccupied user space. –Successive executions of the same process may be loaded into different main memory locations. This is the concept of relocatability –Any memory address references within a process (i.e., variables, instructions) are relative

Address mapping CPUmemory logical address 346 physical address MMU relocatio n register

Protecting processes from each other Use base and limit values –Each location within a process is added to base value to map it to a physical address –Any address locations larger than the limit value are flagged as errors

Base-limit registers One base-limit pair and two base-limit pairs

Swapping What to do if there is not enough memory to store all active processes? –Swap certain processes out and then back in An executing process must be in main memory, but can be temporarily swapped out & then back into memory –Consider a preemptive CPU scheduling algorithm such as RR

About swapping... When the CPU is ready for the next process, it selects it from the ready queue. –If it has been swapped out, the memory manager brings it back in. –If there is no room for it, another process is swapped out first. –Context-switch time is high. If a process is to be swapped out of main memory, it must be completely idle –If it is waiting for I/O, then another candidate is found, if possible

Swapping entire processes Swapping can create holes or fragments in memory

Fragmentation As processes are loaded and removed from memory, available memory space is broken into pieces When there is enough memory space to satisfy a request, but it is not contiguous, we say there is external fragmentation When there is wasted space within a process’ address space, we have internal fragmentation (later on this)

Compaction A solution to external fragmentation. Partitions are rearranged to collect all the fragments into one large block. –Requires all internal addresses to be remapped to new physical addresses –All partitions are moved to one end of memory and all base and limit registers are altered. Very costly

Swapping with room for growth Space for growing data segment Space for growing data & stack segments

Implementation How is dynamic memory allocation actually implemented? Two approaches –Using bitmaps –Using linked lists Strategies for assignment of processes to memory spaces

Using bit maps Part of memory with 5 processes, 3 holes –tick marks show allocation units – may be a few words up to several KB –shaded regions are free –Searching bitmaps can be slow

Using linked lists Part of memory with 5 processes, 3 holes This example uses a singly linked list – a doubly linked list would make it easier to merge available slots See next slide We have several strategies for assigning memory

Memory management with linked lists Four neighbor combinations for the terminating process X

Allocation strategies First-fit: Starting at the head of the list, allocate the first hole that is found to be big enough. –Next fit: pick up where it left the list last time Best-fit: Search entire list to find the optimal fit. –this allocates the smallest hole that is big enough. Worst-fit: Search entire list to find the largest available hole. Quick fit: Uses separate lists for more common process sizes

Assignment In-class: –p #5 HW: –Given memory partitions of 100K, 500K, 200K, 300K, 600K (in order), how would each of first-fit, best-fit, worst-fit, and next-fit algorithms place processes of 212K, 417K, 112K, 426K (in this order)? –Read Sections 4.3 & 4.4