Forking, w Unified VM Wrapup

1
Forking, w/ Unified VM Wrapup

• Vivek Pai / Kai Li
• Princeton University

2
Gedankenverification

• Why is perfect LRU reasonable for the filesystem
  disk cache but not for the VM pages?
• How does the filesystem know what pages it has?
• What are the steps involved prior to a page being
  demand loaded in a VM system?
• What are the possible results of a TLB miss?

3
Mechanics

• Last quiz graded
• Midterm on Thursday
• In class
• Closed everything (no book, no notes, etc)
• Apply Occams razor when answering questions
• Some short answer, some quiz-like questions,
  maybe some multiple choice or true/false
• Dont panic

4
Fear Is The Mind-killer

• I must not fear.
• Fear is the mind-killer
• Fear is the little death that brings total
  obliteration
• I will face my fear
• I will permit it to pass over me and through me
• And when it has gone past I will turn the inner
  eye to see its path
• Where the fear has gone there will be nothing
• Only I will remain Dune, Frank Herbert,
  specifically from the Bene Gesserit Litany
  Against Fear

5
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

6
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?

7
Source of Disk Access

• VM System
• Main memory caches - full image on disk
• Filesystem
• Even here, caching very useful
• New competitive pressure/decisions
• How do we allocate memory to these two?
• How do we know were right?

8
Partitioning Memory

• Originally, specified by administrator
• 20 used as filesystem cache by default
• On fileservers, admin would set to 80
• Each subsystem owned pages, replaced them
• Observation theyre all basically pages
• Why not let them compete?
• Result unified memory systems file/VM

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File Access Efficiency

• read(fd, buf, size)
• Buffer in processs memory
• Data exists in two places filesystem cache
  processs memory
• Known as double buffering
• Various scenarios
• Many processes read same file
• Process wants only parts of a file, but doesnt
  know which parts in advance

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Result Memory-Mapped Files
Process A
Process B
Process C
Process A
Process B
Process C
File
Map
File
Map
File
Map
File
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Lazy Versus Eager

• Eager do things right away
• read(fd, buf, size) returns bytes read
• Bytes must be read before read completes
• What happens if size is big?
• Lazy do them as theyre needed
• mmap() returns pointer to mapping
• Mapping must exist before mmap completes
• When/how are bytes read?
• What happens if size is big?

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Semantics How Things Behave

• What happens when
• Two process obtain data (read or mmap)
• One process modifies data
• Two processes obtain data (read or mmap)
• A third process modifies data
• The two processes access the data

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Being Too Smart

• Assume a unified VM/File scheme
• Youve implemented perfect Global LRU
• What happens on a filesystem dump?

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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

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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
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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
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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

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More Lazy Versus Eager Issues

• Assume 1GB of swap space
• Assume 6 processes each do
• buf malloc(102410241024)
• Should these operations proceed?
• What if they memset(buf, 0, 10241024)?
• What if they memset(buf, 0, 102410241024)?
• This happened in reality IBMs AIX OS

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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

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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
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How To Launch a New Process?

• Obvious choice start process system call
• But not all processes start the same
• testprogram versus testprogram gt outfile
  versus testprogram arg1 arg2 gt outfile
• The parent process wants to specify various
  aspects of the childs environment
• Next step add more parameters to specify
  environment

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Can We Generalize?

• What happens as more information gets added to
  the processs environment more parameters?
  New system calls? This gets ugly
• Whats the most general way of setting up all of
  the environment?
• So, why not allow process setup at any point?
• This is the exec( ) system call (and its variants)

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But We Want a Parent and a Child

• The exec call destroys the current process
• So, instead, destroy a copy of the process
• The fork( ) call duplicates the current process
• Better yet, dont tightly couple fork and exec
• This way, you can customize the childs
  environment
• So what does fork( ) entail?
• Making a copy of everything about the process
• Ouch!

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What Gets Copied

• So far, weve covered the following
• VM system
• File system
• Signals
• How do we go about copying this information?
• What parts are easy to copy, and whats hard?
• Whats the common case with fork/exec?
• What needs to get preserved in this scenario?

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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

w
. . .
. . .
Page table
. . .
Process 1
w
Physical pages
. . .
. . .
Page table
Process 2
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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
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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