CS 201
Virtual Memory
Gerson Robboy
Portland State University
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Virtual Memory - Computer Systems Programming - Lecture Notes | CS 201, Study notes of Computer Science
Material Type: Notes; Class: COMPUTER SYSTEMS PROG; Subject: Computer Science; University: Portland State University; Term: Unknown 1989;
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CS 201
Virtual Memory
Gerson Robboy
Portland State University
15-213, F’ 02
Motivations for Virtual Memory
Use Physical DRAM as a Cache for the Disk
Use Physical DRAM as a Cache for the Disk
Simplify Memory Management
Simplify Memory Management
Provide Protection
Provide Protection
15-213, F’ 02
Levels in Memory Hierarchy
CPU
regs
C
a
c
h
e
Memory
disk
size:
speed:
$/Mbyte:
line size:
32 B
1 ns
8 B
Register Cache Memory Disk Memory
32 KB-4MB
2 ns
$125/MB
32 B
1024 MB
30 ns
$0.20/MB
4 KB
100 GB
8 ms
$0.001/MB
larger, slower, cheaper
8 B 32 B 4 KB
cache virtual memory
15-213, F’ 02
DRAM as a “Cache”
DRAM vs. disk is more extreme than SRAM vs. DRAM
DRAM vs. disk is more extreme than SRAM vs. DRAM
Access latencies:
DRAM ~10X slower than SRAM
Disk ~100,000X slower than DRAM
Importance of exploiting spatial locality:
First byte is ~100,000X slower than successive bytes on disk
Bottom line:
Design decisions driven by enormous cost of misses
DRAM
SRAM
Disk
15-213, F’ 02
Locating an Object in “Cache” (cont.)
Data
N-1:
X
Object Name
Location
D:
J:
X: 1
On Disk
Page Table “Cache”
DRAM Cache
DRAM Cache
Each allocated page of virtual memory has entry in page table
Mapping from virtual pages to physical pages
From uncached form to cached form
If the page is not in memory
“Present” bit is not set
Page table entry gives disk address
OS retrieves information
15-213, F’ 02
Page Faults (like “Cache Misses”)
What if an object is not in memory?
What if an object is not in memory?
Page table entry indicates virtual address not present
Page fault
OS exception handler imoves data from disk into memory
current process suspends, others can resume
OS has full control over placement, etc.
CPU
Memory
Page Table
Disk
Virtual
Addresses
Physical
Addresses
CPU
Memory
Page Table
Disk
Virtual
Addresses
Physical
Addresses
Before fault
After fault
15-213, F’ 02
Virtual
Address
Space for
Process 1:
Physical
Address
Space
(DRAM)
VP 1
VP 2
PP 2
Address Translation 0
0
N-
0
N-
M-
VP 1
VP 2
PP 7
PP 10
(e.g., read/only
library code)
Solution: Separate Virt. Addr. Spaces
Virtual and physical address spaces divided into equal-sized
blocks
blocks are called “pages” (both virtual and physical)
Each process has its own virtual address space
operating system controls how virtual pages as assigned to
physical memory
Virtual
Address
Space for
Process 2:
15-213, F’ 02
What this means for linking/loading
The linker binds programs to absolute addresses.
The linker binds programs to absolute addresses.
Nothing is left relocatable.
No relocation at load time.
No allocation of memory segments at load time.
kernel virtual memory
Memory mapped region
forshared libraries
runtime heap (via malloc)
program text (.text)
initialized data (.data)
uninitialized data (.bss)
stack
forbidden
0
%esp
memory invisible to
user code
the “brk” ptr
All
processes
look just
like this
15-213, F’ 02
Motivation #3: Protection
Page table entry contains access rights information
Page table entry contains access rights information
hardware enforces this protection (trap into OS if violation
occurs)
Page Tables
Process i:
Read? Write? Physical Addr
Yes No PP 9
Yes Yes PP 4
No No XXXXXXX
VP 0:
VP 1:
VP 2:
Process j:
N-1:
Memory
Read? Write? Physical Addr
Yes Yes PP 6
Yes No PP 9
No No XXXXXXX
VP 0:
VP 1:
VP 2:
15-213, F’ 02
Protection
The O. S. kernel gives each process a virtual memory
The O. S. kernel gives each process a virtual memory
space
space
Each process has its own set of page tables
Page tables for other processes are not visible
Process B ’ s memory is not merely protected from Process A
by permissions
It doesn ’ t even exist in Process A ’ s memory space
Physical memory belonging to another process is
Physical memory belonging to another process is
completely outside the memory space.
completely outside the memory space.
Note: sharing is also possible
In Linux, threads are a kind of process with shared memory.
15-213, F’ 02
What happens when a new process
starts running?
No pages are in memory.
No pages are in memory.
The first access to an instruction causes a page fault.
The first access to an instruction causes a page fault.
Pages are pulled in as needed (
Pages are pulled in as needed (
demand paged
demand paged
15-213, F’ 02
What happens when you say
malloc(32000000)?
What exactly does the O. S. allocate at that time?
What exactly does the O. S. allocate at that time?
Is it necessary to allocate 32 MB of physical memory?
Is it necessary to allocate 32 MB of physical memory?
15-213, F’ 02
Page Replacement algorithms
Analogous to cache line replacement.
Analogous to cache line replacement.
A complex topic.
A complex topic.
Beyond the scope of this class
A popular topic with computer scientists because it lends
itself to research.
15-213, F’ 02