Post on 31-Dec-2015
description
transcript
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 bitaddressspace
17 bitprogramsize
Programmer Controlled Memory
foo(){}
bar(){
}
main(){ foo(); bar();}
Compiler Controlled Memory
20 bitphysicalmemory
16 bitaddressspace
Compiler Controlled Memory
Program Counter
Program Segment
Compiler Controlled Memory
Call:
push PCload PC with effective address
Return:pop PC
Compiler Controlled MemoryCall:
push PCpush PSload PS,PC with effective address
push DS
Return:pop DSpop PS,PC
System Controlled Memory
32 bit address space
1Mphysicalmemory
System Controlled Memory
CPU MMU RAM
VirtualAddress
PhysicalAddress
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
HeapFrom To
root
Concurrent Garbage Collection
HeapFrom To
root root
Concurrent Garbage Collection
• 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
Invariants
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
Memory
MappingManager
Shared Virtual Memory
CPU
Memory
MappingManager
CPU
Memory
MappingManager
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 sec
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."