VM Design Issues

1
VM Design Issues

• Vivek Pai / Kai Li
• Princeton University

2
Mini-Gedankenexperimenten

• Whats the refresh rate of your monitor?
• What is the access time of a hard drive?
• What response time determines sluggishness or
  speediness? Whats the relation?
• What determines the running speed of a program
  thats paging heavily?
• If you have a program that pages heavily, what
  are your options to improve the situation?

3
Mechanics

• Lets finish off last lecture
• Memory mapping, Unified VM next time
• No assigned reading yet, may not exist
• Mid-term on track
• Covers everything before it
• Open QA session?
• Is there interest?
• If so, when?

4
Where We Left Off Last Time

• Various approaches to evicting pages
• Some discussion about why doing even well is
  hard to implement
• Beladys algorithm for off-line analysis
• We just finished variations on FIFO
• In particular, enhanced FIFO with 2nd chance

5
Lessons From Enhanced FIFO

• Observation its easier to evict a clean page
  than a dirty page
• 2nd observation sometimes the disk and CPU are
  idle
• Optimization when systems free, write dirty
  pages back to disk, but dont evict
• Called flushing often falls to pager daemon

6
Least Recently Used (LRU)

• Algorithm
• Replace page that hasnt been used for the
  longest time
• Question
• What hardware mechanisms required to implement
  LRU?

7
Implementing LRU
Least recently used
Mostly recently used
5
3
4
7
9
11
2
1
15

• Perfect
• Use a timestamp on each reference
• Keep a list of pages ordered by time of reference

8
Approximate LRU
Most recently used
Least recently used
LRU
N categories
pages in order of last reference
Crude LRU
2 categories
pages referenced since the last page fault
pages not referenced since the last page fault
8-bit count
. . .
256 categories
255
254
0
1
2
3
9
Aging Not Frequently Used (NFU)
00000000
00000000
10000000
01000000
10100000
00000000
10000000
01000000
10100000
01010000
10000000
11000000
11100000
01110000
00111000
00000000
00000000
00000000
10000000
01000000

• Algorithm
• Shift reference bits into counters
• Pick the page with the smallest counter
• Main difference between NFU and LRU?
• NFU has a short history (counter length)
• How many bits are enough?
• In practice 8 bits are quite good
• Pros Require one reference bit
• Cons Require looking at all counters

10
Where Do We Get Storage?

• 32 bit VA to 32 bit PA no space, right?
• Offset within page is the same
• No need to store offset
• 4KB page 12 bits of offset
• Those 12 bits are free in PTE
• Page other info lt 32 bits
• Makes storing info easy

11
x86 Page Table Entry
Page frame number
D
L
Gl
Cw
P
U
A
Cd
Wt
O
W
V
12
31

• Valid
• Writable
• Owner (user/kernel)
• Write-through
• Cache disabled
• Accessed (referenced)
• Dirty
• PDE maps 4MB
• Global

Reserved
12
What Happens on Diagonal Lines

• My screen is 1024768 pixels
• 256 colors 1 byte per pixel .75MB
• 64K colors 2 bytes/pixel 1.5MB
• Page size is 4KB
• Screen is 192 or 384 pages
• 1 page several horizontal lines
• Diagonal/vertical lines TLB badness
• Superpages to the rescue

13
The Big Picture

• Weve talked about single evictions
• Most computers are multiprogrammed
• Single eviction decision still needed
• New concern allocating resources
• How to be fair enough and achieve good overall
  throughput
• This is a competitive world local and global
  resource allocation decisions

14
Program Behaviors

• 80/20 rule
• gt 80 memory references are made by lt 20 of code
• Locality
• Spatial and temporal
• Working set
• Keep a set of pages in memory would avoid a lot
  of page faults

Working set
page faults
pages in memory
15
Observations re Working Set

• Working set isnt static
• There often isnt a single working set
• Multiple plateaus in previous curve
• Program coding style affects working set
• Working set is hard to gauge
• Whats the working set of an interactive program?

16
Working Set

• Main idea
• Keep the working set in memory
• An algorithm
• On a page fault, scan through all pages of the
  process
• If the reference bit is 1, record the current
  time for the page
• If the reference bit is 0, check the last use
  time
• If the page has not been used within d, replace
  the page
• Otherwise, go to the next
• Add the faulting page to the working set

17
WSClock Paging Algorithm

• Follow the clock hand
• If the reference bit is 1, set reference bit to
  0, set the current time for the page and go to
  the next
• If the reference bit is 0, check last use time
• If page has been used within d, go to the next
• If page hasnt been used within d and modify bit
  is 1
• Schedule the page for page out and go to the next
• If page hasnt been used within d and modified
  bit is 0
• Replace this page

18
Simulating Modify Bit with Access Bits

• Set pages read-only if they are read-write
• Use a reserved bit to remember if the page is
  really read-only
• On a read fault
• If it is not really read-only, then record a
  modify in the data structure and change it to
  read-write
• Restart the instruction

19
Implementing LRU without Reference Bit

• Some machines have no reference bit
• VAX, for example
• Use the valid bit or access bit to simulate
• Invalidate all valid bits (even they are valid)
• Use a reserved bit to remember if a page is
  really valid
• On a page fault
• If it is a valid reference, set the valid bit and
  place the page in the LRU list
• If it is a invalid reference, do the page
  replacement
• Restart the faulting instruction

20
Demand Paging

• Pure demand paging relies only on faults to bring
  in pages
• Problems?
• Possibly lots of faults at startup
• Ignores spatial locality
• Remedies
• Loading groups of pages per fault
• Prefetching/preloading

21
Speed and Sluggishness

• Slow is gt .1 seconds (100 ms)
• Speedy is ltlt .1 seconds
• Monitors tend to be 60 Hz
• lt16.7ms between screen paints
• Disks have seek rotational delay
• Seek is somewhere between 7-16 ms
• At 7200rpm, one rotation 1/120 sec 8ms.
  Half-rotation is 4ms
• Conclusion? One disk access OK, six are bad

22
Disk Address

• Use physical memory as a cache for disk
• Where to find a page on a page fault?
• PPage field is a disk address

Virtual address space
Physical memory
invalid
23
Imagine a Global LRU

• Global across all processes
• Idea when a page is needed, pick the oldest
  page in the system
• Problems? Process mixes?
• Interactive processes
• Active large-memory sweep processes
• Mitigating damage?

24
Amdahls Law

• Gene Amdahl (IBM, then Amdahl)
• Noticed the bottlenecks to speedup
• Assume speedup affects one component
• New time
• (1-not affected) affected/speedup
• In other words, diminishing returns

25
NT x86 Virtual Address Space Layouts
00000000
Application code Globals Per-thread stacks DLL
code
3-GB user space
7FFFFFFF 80000000
Kernel exec HAL Boot drivers
C0000000 C0800000
Process page tables Hyperspace
BFFFFFFF C0000000
System cache Paged pool Nonpaged pool
1-GB system space
FFFFFFFF
FFFFFFFF
26
Virtual Address Space in Win95 and Win98
00000000
User accessible
Unique per process (per application), user mode
7FFFFFFF 80000000
Shared, process-writable (DLLs, shared
memory, Win16 applications)
Systemwide user mode
C0000000
Win95 and Win98
Systemwide kernel mode
Operating system (Ring 0 components)
FFFFFFFF
27
Details with VM Management

• Create a processs virtual address space
• Allocate page table entries (reserve in NT)
• Allocate backing store space (commit in NT)
• Put related info into PCB
• Destroy a virtual address space
• Deallocate all disk pages (decommit in NT)
• Deallocate all page table entries (release in NT)
• Deallocate all page frames

28
Page States (NT)

• Active Part of a working set and a PTE points to
  it
• Transition I/O in progress (not in any working
  sets)
• Standby Was in a working set, but removed. A
  PTE points to it, not modified and invalid.
• Modified Was in a working set, but removed. A
  PTE points to it, modified and invalid.
• Modified no write Same as modified but no write
  back
• Free Free with non-zero content
• Zeroed Free with zero content
• Bad hardware errors

29
Dynamics in NT VM
Demand zero fault
Page in or allocation
Standby list
Free list
Zero list
Bad list
Process working set
Modified writer
Zero thread
Soft faults
Modified list
Working set replacement
30
Shared Memory

• How to destroy a virtual address space?
• Link all PTEs
• Reference count
• How to swap out/in?
• Link all PTEs
• Operation on all entries
• How to pin/unpin?
• Link all PTEs
• Reference count

w
. . .
. . .
Page table
. . .
Process 1
w
Physical pages
. . .
. . .
Page table
Process 2
31
Copy-On-Write

• Childs virtual address space uses the same page
  mapping as parents
• Make all pages read-only
• Make child process ready
• On a read, nothing happens
• On a write, generates an access fault
• map to a new page frame
• copy the page over
• restart the instruction

r
r
. . .
. . .
Page table
. . .
Parent process
r
r
Physical pages
. . .
. . .
Page table
Child process
32
Issues of Copy-On-Write

• How to destroy an address space
• Same as shared memory case?
• How to swap in/out?
• Same as shared memory
• How to pin/unpin
• Same as shared memory