Transcript Chapter 13. Virtual Memory

Chapter 13. Virtual Memory
• Introduction
• Demand Paging
• Hardware Requirements
• 4.3 BSD Virtual Memory
• 4.3 BSD Memory Management Operations
Virtual Memory 1
Introduction
• Memory management unit (MMU)
– Responsible for getting data to and from main
memory
• Virtual memory
– The notion of an address space as distinct
from memory locations
– Translation tables
– Page-based
Virtual Memory 2
Introduction (cont)
• Memory management in the Stone Age
– Software overlay
– Swapping
– Demand paging
– Segmentation
Virtual Memory 3
Demand Paging
• Primary goal
– To allow a process to run in a virtual address
space
– To perform translations from virtual to physical
addresses transparently to the process
• Functional requirements
– Address space management
• fork( ), exec( )
– Address translation
• Virtual address  Virtual page number + offset
Virtual Memory 4
Demand Paging (cont)
– Physical memory management
– Memory protection
– Memory sharing
– Monitoring system load
– Other facilities
• Memory-mapped files
• Dynamically linked shared libraries
• etc.
Virtual Memory 5
Demand Paging (cont)
• Page swapping
uninitialized data pages
zero-filled on first access
text and
executable
initialized data
file
main
memory
dirty pages saved
initialized data
subsequent faults on
outswapped pages
swap
area
on disk
stack and heap pages
allocated on first access
Virtual Memory 6
Demand Paging (cont)
• Translation maps
address
space map
backup
store map
page fault
process
+
virtual page
number
hardware
address
translations
Data
physical
page
number
physical
memory map
Virtual Memory 7
Demand Paging (cont)
• Page replacement policies
– Local v.s. global replacement policy
– Locality of reference
– Concept of working set
– Least recently used (LRU) policy
Virtual Memory 8
Hardware Requirements
• MMU
– Translation of virtual addresses
– Using page tables or TLBs
• Page table entry
– Physical page frame number
– Protection information
– a valid bit
– a modified bit
– a referenced bit
Virtual Memory 9
Hardware Requirements (cont)
• Page table
– Hardware-prescribed
– Located in main memory
– MMU uses only the active tables, whose
locations are loaded in h/w page table registers
– Typically, on a uniprocessor, there are two
active page tables - one for kernel and one for
the currently running process
– Paging the page table to reduce the required
memory
Virtual Memory 10
Hardware Requirements (cont)
• Address translation may fail for three
reasons
– Bounds error
– Validation error
– Protection error
• MMU caches
– A high-speed cache that is searched before
each memory access
– Translation lookaside buffer (TLB)
Virtual Memory 11
Hardware Requirements (cont)
• Intel x86
– Unix implementations on the x86 hides the
notion of segmentation from user processes,
who see a flat address space
– Two-level page table
– 4k byte page size
– Control register CR3 stores the physical page
number of the current page directory
– CR3 register needs to be reset on each context
switch
Virtual Memory 12
Hardware Requirements (cont)
– Page table entry
• Physical page number, protection field
• Valid, referenced, and modified bit
– x86 supports 4 privilege levels of which UNIX
uses only two
– The kernel runs in the innermost ring, which is
the most privileged
– User code runs in the outermost, least
privileged ring
Virtual Memory 13
Address Translation on Intel x86
page directory of
current process
CR3
31
11
0
one of the page
tables of current
process
PFN
31
11
0
PFN
31
21
DIR
PAGE
11
0
OFFSET
virtual address
31
11
PFN
0
OFFSET
physical address
Virtual Memory 14
Intel x86 Page Table Entry
31
12
PFN
PFN
D
A
U
W
P
6 5
2 1
D A
U W P
0
Page Frame Number
Dirty
Accessed (Referenced)
User (0) / Supervisor (1)
Read (0) / Write (1)
Present (valid)
Virtual Memory 15
4.3BSD
• Target platform: VAX-11
– Several BSD-based implementations emulate
the VAX memory architecture in software,
including its address space layout and page
table entry format
• Core map
– Describes physical memory
• Page tables
– Describe virtual memory
Virtual Memory 16
4.3BSD (cont)
• Disk maps
– Describe the swap areas
• Resource maps
– Manage allocation of resources such as page
tables and swap space
Virtual Memory 17
4.3BSD Physical Memory
nonpaged
pool
paged
pool
error
buffer
cmap[ ]
(in nonpaged pool)
Virtual Memory 18
4.3BSD Physical Memory (cont)
• Core map: struct cmap
– Is a kernel data structure, allocated at boot
time and resident in the nonpaged pool
– One entry for each frame in the paged pool
• Core map entry
– Name
• <type, owner, virtual page number>
– Text page cache
– Synchronization
Virtual Memory 19
4.3BSD Physical to Virtual address
Translation
proc[ ]
cmap[ ]
type = data
owner
VPN
…
page
table
type = data
owner
VPN
…
page table
text[ ]
Virtual Memory 20
4.3BSD Address Space
• Uses VAX-11 address space model
– 32-bit machine with 512-byte page size
– 4 Giga byte address space is divided into four
regions of equal size
– P0
• Program region
• Text and data section of the process
Virtual Memory 21
4.3BSD Address Space (cont)
– P1
• Control region
• User stack, u area, kernel stack
– S0
• System region
• Kernel text and data
– 4th region
• Reserved and not supported by current VAX h/w
Virtual Memory 22
4.3BSD Page Tables
• Single system page table
– Map the kernel text and data
• Each process has two page tables to map
its P0 and P1
– Mapped by a set of contiguous PTEs in the
Userptmap section of the system page table
• State of a particular page of a process
– Resident
• The page is in physical memory, and the page table
entry contains its physical page frame number
Virtual Memory 23
4.3BSD Page Tables (cont)
– Fill-on-demand
• Fill-from-text: Text and initialized data pages are read
in from the executable file upon first access
• Zero-fill: Uninitialized data, heap, and stack pages
are created and filled with zero when required
– Outswapped
• These pages may be recovered from their swap area
locations
Virtual Memory 24
4.3BSD Page Tables (cont)
• Kernel maintains information about all
nonresident pages
– For swapped out pages
• Kernel must stores their location on the swap device
– For zero-fill pages
• Kernel only needs to recognize them as such
– For fill-from-text pages
• Kernel must determine their location in the
filesystem
Virtual Memory 25
4.3BSD Page Tables (cont)
• Page table entry
– Since all nonresident pages have the valid bit
clear, other fields can be replaced by other
information that tracks these pages
– Kernel maintains separate swap maps to
locate those pages on the swap device
Virtual Memory 26
4.3BSD Page Tables Entry
(a) VAX-11 page table entry format
31
26
V
PROT M
valid
20
unused
0
Page Frame Number (PFN)
modified
(b) Ordinary page table entry
31
26 25
V
PROT M 0
valid
V
31
20
0
Page Frame Number (PFN)
modified
fill-on-demand
fill-from-text (1) or zero-fill (0)
PROT
1 F
26
File System Block Number
23
0
(c) Fill-on-demand page table entry
Virtual Memory 27
4.3BSD Swap Space
• Per region dmap structure
swap space
data region
0
dmmin
dmap
2*dmmin
4*dmmin
Virtual Memory 28
4.3BSD Swap Space (cont)
• Text pages
– Once the page is brought into memory, the fillon-demand PTE is overwritten by the page
frame number
– As a result, retrieving the page from the file
involving recomputing its location and
perhaps, accessing one or more indirect
blocks
– To avoid that, such pages are saved in swap as
well
Virtual Memory 29
4.3BSD Swap Space (cont)
• U area holds the maps for the data and
stack region
• The text region swap map is part of the
text structure
Virtual Memory 30
4.3BSD Memory Management
Operations
• Process creation
– Swap space, u area
– Page tables
• Kernel must allocate contiguous PTEs in Userptmap
to map page tables for their process
– Text region
• The child is added to the list of processes sharing
the text structure used by the parent
– Data and Stack
• Data and stack must be copied one page at a time
• Copy-on-write, vfork( )
Virtual Memory 31
4.3BSD Memory Management
Operations (cont)
• Page table handling
– Two types of page faults
• validation, protection
– Validation
• No PTE for that page (bounds error)
• PTE is marked invalid
– Pagein( ) is called to handle the fault
• The faulting virtual address  PTE
Virtual Memory 32
Pagein( ) (1/2)
start
No
No
Yes
PFN == 0?
in transit
text page?
Yes
No
text page on
hash queue?
set wanted flag
No
on free
list?
sleep on text struct
Yes
take page off
free list
start over when
woken up
Fill on
demand?
Yes
(next page)
Yes
(next page)
No
allocate new page
read page
from swap
set valid bit
Virtual Memory 33
Pagein( ) (2/2)
(previous page)
Yes
Fill on
demand?
zero-fill?
Yes
(previous page)
No
Yes
take page off
free list
text page on
hash queue?
No
Yes
flush cache
copy to disk
allocate new page
fill it with zeros
page in
buffer cache?
No
Read page
from file
mark page
modified
Virtual Memory 34
4.3BSD Memory Management
Operations (cont)
• Free page list
– Ideally, to keep all garbage pages at the head
of the free list, followed by some useful pages
in LRU order
– 4.3BSD replaces the least recently used policy
by a not recently used policy
• Uses referenced bit and two passes over each page
• VAX-11 does not support a referenced bit in the h/w,
so BSD simulates the referenced bit in software
• Pagedaemon process is responsible for page
replacement
Virtual Memory 35
4.3BSD Memory Management
Operations (cont)
• Swapping
– Problem of thrashing
– Swapper process monitors the system load,
and swap processes in and out when needed
– Swapper will swap out a process in the
following cases
• Userptmap fragmentation
• Memory shortfall
• Inactive processes
Virtual Memory 36
4.3BSD Memory Management
Operations (cont)
– Swapper performs the following task when
swapping out a process
• Allocates swap space for the u area, kernel stack,
and page tables
• Detach the process from its text region
• Save the resident data and stack on swap
• Release the system PTEs in Userptmap
• Record the swap location of the u area in the proc
structure
Virtual Memory 37
Analysis of 4.3BSD
• No support for execution of remote
programs
• No support for sharing of memory
• vfork( ) is not a true substitute for fork( )
– The lack of copy-on-write hurts the
performance of Ap that rely extensively on fork
• No support for memory mapped files
Virtual Memory 38
Analysis of 4.3BSD (cont)
• Reserves enough swap space in advance
to page out very single page in the
process address space
• No support for using swap space on
remote nodes
Virtual Memory 39