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VM Design Issues Vivek Pai / Kai Li Princeton University

VM Design Issues

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VM Design Issues. Vivek Pai / Kai Li Princeton University. Mini-Gedankenexperimenten. What’s the refresh rate of your monitor? What is the access time of a hard drive? What response time determines sluggishness or speediness? What’s the relation? - PowerPoint PPT Presentation

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Page 1: VM Design Issues

VM Design Issues

Vivek Pai / Kai LiPrinceton University

Page 2: VM Design Issues

2

Mini-Gedankenexperimenten

What’s the refresh rate of your monitor? What is the access time of a hard drive? What response time determines

sluggishness or speediness? What’s the relation?

What determines the running speed of a program that’s paging heavily?

If you have a program that pages heavily, what are your options to improve the situation?

Page 3: VM Design Issues

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Mechanics

The midterm was probably too long Haven’t looked closely at them Grading based on mean, std deviation

Almost lost a lecture – today’s Already had some post-midterm slack I’ll adjust the web page appropriately

Project client updated – maybe use it

Page 4: VM Design Issues

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Where We Left Off Last Time

Various approaches to evicting pages

Some discussion about why doing even “well” is hard to implement

Belady’s algorithm for off-line analysis

Page 5: VM Design Issues

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The Big Picture

We’ve 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

Page 6: VM Design Issues

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x86 Page Table Entry

Valid

Writable

Owner (user/kernel)

Write-through

Cache disabled

Accessed (referenced)

Dirty

PDE maps 4MB

Global

Page frame number DLGlCwPU A Cd Wt O W V

Reserved31 12

Page 7: VM Design Issues

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Program Behaviors

80/20 rule > 80% memory

references are made by < 20% of code

Locality Spatial and temporal

Working set Keep a set of pages in

memory would avoid a lot of page faults

# pages in memory#

page

fau

lts

Working set

Page 8: VM Design Issues

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Observations re Working Set

Working set isn’t static There often isn’t a single “working

set” Multiple plateaus in previous curve Program coding style affects working set

Working set is hard to gauge What’s the working set of an interactive

program?

Page 9: VM Design Issues

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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 , replace the page Otherwise, go to the next

Add the faulting page to the working set

Page 10: VM Design Issues

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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 , go to the next If page hasn’t been used within and modify bit is 1

Schedule the page for page out and go to the next If page hasn’t been used within and modified bit is 0

Replace this page

Page 11: VM Design Issues

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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

Page 12: VM Design Issues

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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

Page 13: VM Design Issues

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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

Page 14: VM Design Issues

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Speed and Sluggishness

Slow is > .1 seconds (100 ms) Speedy is << .1 seconds Monitors tend to be 60+ Hz =

<16.7ms 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

Page 15: VM Design Issues

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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

Virtualaddress

spaceinvalid

Physicalmemory

Page 16: VM Design Issues

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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?

Page 17: VM Design Issues

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Amdahl’s 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

Page 18: VM Design Issues

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NT x86 Virtual Address Space Layouts

00000000

7FFFFFFF80000000

System cachePaged pool

Nonpaged pool

Kernel & execHAL

Boot drivers

Process page tablesHyperspace

Application codeGlobals

Per-thread stacksDLL code

3-GB user space

1-GB system space

BFFFFFFFC0000000

FFFFFFFF FFFFFFFF

C0000000

C0800000

Page 19: VM Design Issues

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Virtual Address Space in Win95 and Win98

00000000

7FFFFFFF80000000

Operating system(Ring 0 components)

Shared, process-writable(DLLs, shared memory,

Win16 applications)

Win95 and Win98

User accessible

FFFFFFFF

C0000000

Unique per process(per application),user mode

Systemwideuser mode

Systemwidekernel mode

Page 20: VM Design Issues

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Details with VM Management

Create a process’s 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

Page 21: VM Design Issues

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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

Page 22: VM Design Issues

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Working setreplacement

Page in or allocationDemandzero fault

Dynamics in NT VM

Processworking

set

Standbylist

Modifiedlist

Modifiedwriter

“Soft”faults

Freelist

Zerothread

Zerolist

Badlist

Page 23: VM Design Issues

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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

.

.

.

.

.

.

...

.

.

.

Process 1

Process 2

w

...

w

Page table

Page table

Physicalpages

Page 24: VM Design Issues

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.

.

.

.

.

.

...

.

.

.

Copy-On-Write

Child’s virtual address space uses the same page mapping as parent’s

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

Parent process

Child process

rr

...

rr

Page table

Page table

Physicalpages

Page 25: VM Design Issues

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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