Paging
Andrew Whitaker
CSE451
Review: Process (Virtual) Address Space
Each process has its own address spaceThe OS and the hardware translate virtual
addresses to physical frames
kernel spaceuser space
Multiple Processes
Each process has its own address space And, its own set of page tables
Kernel mappings are the same for all
kernel spaceuser space
proc1
proc2
Linux Physical Memory Layout
Paging Issues
Memory scarcity Virtual memory, stay tuned…
Making Paging FastReducing the Overhead of Page Tables
Review: Mechanics of address translation
pageframe 0
pageframe 1
pageframe 2
pageframe Y
…
pageframe 3
physical memory
offset
physical address
page frame #page frame #
page table
offset
virtual address
virtual page #
Problem: page tables live in memory
Making Paging Fast
We must avoid a page table lookup for every memory reference This would double memory access time
Solution: Translation Lookaside Buffer Fancy name for a cache
TLB stores a subset of PTEs (page table translation entries)
TLBs are small and fast (16-48 entries) Can be accessed “for free”
TLB Details
In practice, most (> 99%) of memory translations handled by the TLB
Each processor has its own TLB TLB is fully associative
Any TLB slot can hold any PTE entry Who fills the TLB? Two options:
Hardware (x86) walks the page table on a TLB miss Software (MIPS, Alpha) routine fills the TLB on a miss
TLB itself needs a replacement policy Usually implemented in hardware (LRU)
What Happens on a Context Switch?
Each process has its own address spaceSo, each process has its own page tableSo, page-table entries are only relevant for
a particular process Thus, the TLB must be flushed on a
context switch This is why context switches are so expensive
Alternative to flushing: Address Space IDsWe can avoid flushing the TLB if entries
are associated with an address space
When would this work well? When would this not work well?
page frame numberprotMRV
202111
ASID
4
TLBs with Multiprocessors
Each TLB stores a subset of page table state Must keep state consistent on a multiprocessor
page frame #
page tableTLB 1
TLB 2
Today’s Topics
Page Replacement StrategiesMaking Paging FastReducing the Overhead of Page Tables
Page Table Overhead
For large address space, page table sizes can become enormous
Example: IA64 architecture
64 bit address space, 8KB pages
Num PTEs = 2^64 / 2^13 = 2^51
Assuming 8 bytes per PTE:Num Bytes = 2^54 = 16 Petabytes
And, this is per-process!
Optimizing for Sparse Address Spaces Observation: very little of the address space is in
use at a given time
Basic idea: only allocate page tables where we need to And, fill in new page tables lazily (on demand)
virtualaddressspace
Implementing Sparse Address SpacesWe need a data structure to keep track of
the page tables we have allocatedAnd, this structure must be small
Otherwise, we’ve defeated our original goal
Solution: multi-level page tables Page tables of page tables “Any problem in CS can be solved with a layer
of indirection”
Two level page tables
pageframe 0
pageframe 1
pageframe 2
pageframe Y
…
pageframe 3
physical memory
offset
physical address
page frame #
masterpage table
secondary page#
virtual address
master page # offset
secondarypage table
empty
empty
secondarypage table
page framenumber
Key point: not all secondary page tables must be allocated
Generalizing
Early architectures used 1-level page tablesVAX, x86 used 2-level page tablesSPARC uses 3-level page tablesAlpha 68030 uses 4-level page tables
Key thing is that the outer level must be wired down (pinned in physical memory) in order to break the recursion
Cool Paging Tricks
Basic Idea: exploit the layer of indirection between virtual and physical memory
Trick #1: Shared Libraries
Q: How can we avoid 1000 copies of printf?
A: Shared libraries Linux: /usr/lib/*.so
libc libc
Firefox Open Office
libc
Physical memory
Shared Memory Segments
Virt Address space 2Virt Address space 1
Physical memory
Trick #2: Copy-on-write
Copy-on-write allows for a fast “copy” by using shared pages Especially useful for “fork” operations
Implementation: pages are shared “read-only” OS intercepts write operations, makes a real copy
page frame numberprotMRV
Trick #3: Memory-mapped Files
Normally, files are accessed with system calls Open, read, write, close
Memory mapping allows a program to access a file with load/store operations
Virt Address space
Foo.txt