Point of memory management algorithms - support sharing
of main memory. We will focus on having multiple processes sharing
the same physical memory. Key issues:
Protection. Must allow one process to protect its
memory from access by other processes.
Naming. How do processes identify shared pieces
Transparency. How transparent is sharing. Does
user program have to manage anything explicitly?
Efficiency. Any memory management strategy should
not impose too much of a performance burden.
Why share memory between processes? Because want to
multiprogram the processor. To time share system, to
overlap computation and I/O. So, must provide for multiple
processes to be resident in physical memory at the same time.
Processes must share the physical memory.
For first computers, loaded one program onto
machine and it executed to completion. No sharing
required. OS was just a subroutine library, and
there was no protection. What addresses does
Desire to increase processor utilization in the
face of long I/O delays drove the adoptation of multiprogramming.
So, one process runs until it does I/O, then OS lets another
process run. How do processes share memory? Alternatives:
Load both processes into memory, then switch between
them under OS control. Must relocate program when load
it. Big Problem: Protection. A bug in one process can
kill the other process. MS-DOS, MS-Windows use this strategy.
Copy entire memory of process to disk when it does
I/O, then copy back when it restarts. No need to
relocate when load. Obvious performance problems.
Early version of Unix did this.
Do access checking on each memory reference. Give
each program a piece of memory that it can access, and
on every memory reference check that it stays within
its address space. Typical mechanism: base and bounds
registers. Where is check done? Answer: in hardware for speed.
When OS runs process, loads the base and bounds registers
for that process.
Cray-1 did this. Note: there is now a translation process.
Program generates virtual addresses that get translated
into physical addresses. But, no longer have a protection
problem: one process cannot access another's memory,
because it is outside its address space. If it tries to
access it, the hardware will generate an exception.
End up with a model where physical memory of machine
is dynamically allocated to processes as they enter and
exit the system. Variety of allocation strategies: best
fit, first fit, etc. All suffer from external fragmentation.
In worst case, may have enough memory free to
load a process, but can't use it because it is fragmented
into little pieces.
What if cannot find a space big enough to run a process?
Either because of fragmentation or
because physical memory is too small to hold all address
spaces. Can compact and relocate processes (easy with
base and bounds hardware, not so easy for direct physical
address machines). Or, can swap a process out to disk then
restore when space becomes available. In both cases incur
copying overhead. When move process within memory, must
copy between memory locations. When move to disk, must
copy back and forth to disk.
One way to avoid external fragmentation: allocate
physical memory to processes in
fixed size chunks called page frames. Present abstraction to application of
a single linear address space. Inside machine, break address space
of application up into fixed size chunks called pages. Pages and
page frames are same size. Store pages in page frames. When process
generates an address, dynamically translate to the physical page
frame which holds data for that page.
So, a virtual address now consists of two pieces: a page
number and an offset within that page. Page sizes are typically
powers of 2; this simplifies extraction of page numbers and
offsets. To access a piece of data at a given address, system
automatically does the following:
Extracts page number.
Translate page number to physical page frame id.
Accesses data at offset in physical page frame.
How does system perform translation? Simplest solution:
use a page table. Page table is a linear array indexed by
virtual page number that gives the physical page frame
that contains that page. What is lookup process?
Extract page number.
Check that page number is within address space of process.
Look up page number in page table.
Add offset to resulting physical page number
Access memory location.
With paging, still have protection. One process cannot
access a piece of physical memory unless its page table
points to that physical page. So, if the page tables of
two processes point to different physical pages, the processes
cannot access each other's physical memory.
Fixed size allocation of physical memory in page
frames dramatically simplifies allocation algorithm.
OS can just keep track of free and used pages and allocate
free pages when a process needs memory.
There is no fragmentation of physical memory into smaller
and smaller allocatable chunks.
But, are still pieces of memory that are unused. What
happens if a program's address space does not end on a
page boundary? Rest of page goes unused. This kind of
memory loss is called internal fragmentation.