Transcript ppt

Virtual Memory Primitives for
User Programs
Andrew W. Appel and Kai Li
Presented by Phil Howard
Virtual Memory
• A brief history
• Programmer Control
• Compiler Control
• System Control
• New Applications
•
•
•
•
•
•
Concurrent Garbage Collection
Shared Virtual Memory
Concurrent Checkpointing
Persistent Heap
Extending Addressing
Data Compression Paging
• Conclusions
Programmer Controlled Memory
16 bit
address
space
17 bit
program
size
Programmer Controlled Memory
foo()
{
}
main()
{
foo();
bar();
}
bar()
{
}
Compiler Controlled Memory
20 bit
physical
memory
16 bit
address
space
Compiler Controlled Memory
Program Segment
Program Counter
Compiler Controlled Memory
Call:
push PC
load PC with effective address
Return:
pop PC
Compiler Controlled Memory
Call:
push PC
push PS
load PS,PC with effective address
push DS
Return:
pop DS
pop PS,PC
System Controlled Memory
32 bit
address
space
1M
physical
memory
System Controlled Memory
Physical
Address
Virtual
Address
CPU
MMU
RAM
System Controlled Memory
• System handles page faults
• Allowed protection
•
•
•
•
You can't see my pages
You can't change my pages
I can't execute my data
I can't change my program
• Made life much easier for programmers
But wait…
Appel and Li want to control memory
themselves
Why?
User access to VM primitives
•
•
•
•
•
•
TRAP - Handle page fault
PROT1 - Protect a single page
PROTN - Protect many pages
UNPROT - Unprotect single page
DIRTY - return list of dirty pages
MAP2 - Map a page to two addresses
Concurrent Garbage Collection
Heap
From
root
To
Concurrent Garbage Collection
From
root
Heap
To
root
Concurrent Garbage Collection
Invariants
• Mutator sees only to-space pointers
• New objects contain to-space pointers only
• Objects in to-space contain to-space
pointers only
• Objects in from-space contain from-space
and to-space pointers
Concurrent Garbage Collection
• Use VM to protect from-space
• Collector handles access violations,
validates objects and updates pointers
• Collector uses aliased addresses to scan in
background
Shared Virtual Memory
CPU
CPU
CPU
Memory
Memory
Memory
Mapping
Manager
Mapping
Manager
Mapping
Manager
Shared Virtual Memory
Shared Virtual Memory
• Coherent across processors - each read gets
the last value written
• Multiple readers/Single writer
• Handled the same as "regular" VM except
for fetching and writing pages
Concurrent Checkpointing
•
•
•
•
Stop all threads
Save all thread states
Save all memory
Restart threads
• Stop all threads
• Save all thread states
• Make all memory
read-only
• Restart threads
• Save pages in the
"background" and
mark as read/write
Persistent Heap
• Heap survives across process invocations
• Read/Write access as fast as conventional
heap
• Use memory mapped disk file
• Page faults fetch from heap file instead of
system page file
Extending Addressability
• Persistent Heap with > 232 objects
• Need translation table to convert from 32 to
64 bit address
• Page fault fetches from Persistent Heap and
sets up translation
• Application limited to 232 objects per
invocation
Data Compression Paging
• Paging is slow - 20 ms seek time on disk
plus transfer time
• Many data pages can be compressed 4:1
• Instead of swapping out a page, compress it
• Page fault to compressed page will
decompress it rather than read from disk
VM Primitive Performance
Garbage collection for 4096 byte page = 500 msec
VM Primitive Performance
VM Primitive Performance
• OS Authors didn't pay much attention to VM
Performance
• Why?
• Seek time ~ 20 msec
• Read time ~ 1 msec
• Page fault happens in parallel with another task
• Why do we care?
• Many of the algorithms in this paper don't involve the
disk
Conclusions
"… page-protection and fault-handling
efficiency must be considered as one of the
parameters of the design space."
"It is important that hardware and operating
system designers make the virtual memory
mechanisms required by these algorithms
robust, and efficient."
Conclusions
"… page-protection and fault-handling
efficiency must be considered as one of the
parameters of the design space."
"It is important that hardware and operating
system designers make the virtual memory
mechanisms required by these algorithms
robust, and efficient."