CSE 221 - Operating Systems - Notes on "Virtual Memory Management in the Vax/VMS Operating System"

Q: The paper states, “VAX/VMS, then, is a collection of procedures that exist in the address space of each process.” Explain in your own words what this statement means.

Answer:

VAX-11 Hardware

• published March 1982
• basic entity in VAX is the process
  • each process has a byte addressable virtual address space, divided into 512 byte pages.
    • 21 bit virtual page number
    • 9 byte offset.
  • page is the basic unit of mapping and protection
• upper two bits divide virtual address space into a number of regions (4)
• low half of the address space (last bit = 0) is known as the system space, shared by all processes in the system
  • only half system space is used in current arch
  • other half is simply reserved
• the upper address of the first half address space is process-specific control region, the lower portion is program-specific.
  • program region (p0) grows upwards toward control region
  • control region (p1) grows downwards toward program region.
• each region defined by page table entries (PTEs)
  • PTE contains
    • valid bit (31)
    • protection field (30:27) indicating required privilege
    • modify bit (26) indicating write has occurred to the page
    • field use by OS (25:21)
    • physical page frame number (20:0)
• P0 and P1 page tables for every process are located in system space section of the address space.
  • base registers for p0 and p1 can be paged because they are in virtual memory
• Hardware provides a translation buffer for caching virtual to physical translations.
  • two sections
    • system translations
    • process translations
  • only process translations need to be flushed.
• VMS OS is shared implicitly via TLB
• user calls OS routines like any procedure
• prevent access to 1st page to catch program errors executing on null pointers.

Memory Management Implementation

• concerns:
  • effect one a heavily paging programs in the system
  • high cost of program startup and restart time in VM environments
  • increased disk workload due to paging
  • processor time consumed by searching page lists in the OS
• divide into pager and swapper
• pager is OS construct that runs as a consequence of a page fault
• swapper responsible for loading/storing pages
• page replacement on global placement bad for other processes
  • process local page replacement
  • limit on physical memory of a process
  • FIFO replacement for selecting page from resident set.
  • set reference bit whenever referencing a page
• pages when being removed will look at modify bit to determine whether page can be added to the free list or modified list (written later)
• page fault time is ~200us
• free list helps reduce fault rates]
• delays modified page writes
  • modified page list acts as a cache of recently removed pages
  • if a page on the list is referenced, it can be returned at low cost
  • when a write request is performed, many pages written at once
  • pages can be arranged on paging file so that they are clustered
  • because writes are delayed, many page writes are entirely avoided because either the pages are faulted and/or program terminates.
• paging in system space:
  • in system space, paged and non-paged code and data.
    • once the system is full, system page faults cause pages to be removed from system RSS and placed on a free or modified page list.
      • exception is the process page tables
      • process page table is added to process private resident set and not eligible for removal from resident set as long as it contains valid PTEs
• swaps can move entire processes, not just pages.
  • keep highest priority processes resident
  • avoid typically high page rates that occur when resuming a process.
• entire process written to swap when removed from memory
• process not in memory becomes able to execute, read in pages from the swapper
• will not load a process to execute unless there is sufficient physical memory for the entire resident set.
• page membership of swap and memory is identical after process is read back in.

Lecture Notes

• VAX: Virtual Address Extension
• VMS: Virtual Memory System
• VAX and VMS were very influential to today’s systems
  • powerful minicomputer and affordable by universities.
  • UNIX ported to VAX
• VMS
  • de-facto virtual memory system for unix
  • cutler one of VMS project leaders
  • Bill Gates hired cutler to design and develop NT kernel
  • many aspects of NT kernel have shades of VMS
• Main point:
  • mechanisms and policies for VM system of minicomputers that supports a wide range of application requirements
    • real-time
    • batch
• Address Space
  • four regions of VAX/VMS address space
    • P0, P1, Sys, Res (see notes from above section for additional info)
• OS in address space of every process
• OS can directly access user code and data
• page out OS structures
• is the OS actually copied though?
  • no, physical page just mapped to multiple virtual addresses
• VM Hardware
  • page table
    • page tables don’t have holes
    • implications for address space usage:
      • cannot efficiently support sparse address spaces
      • multiple stacks for threads, shared mem, shared libs
• TLB
  • 1 or 2 levels in some modern systems
• user vs kernel
  • user and kernel page tables are similar
  • how do they differ?
    • user page tables in kernel address space
    • kernel page tables exist in physical memory space
  • mapping for kernel data structure cannot be paged out.
  • user page tables may be paged
  • kernel page tables cannot be paged out
• command interpreter of VAX/VMS
  • command interpreter is a part of each user’s addr space
  • windows/unix command interpreter is not like this because we use fork
• 3 techniques for VM performance
  • local page replacement
  • page caching (free list mechanism)
  • clustering
• local page replacement - why?
  • each process gets a max RSS
  • when reaching max, replace its own set of pages
    • isolation, prevent single process from using too much mem
• Page replacement policy
  • FIFO
  • suffers from Belady’s Anomaly (more physical mem results in more page faults)
• what is page caching?
  • page caching
    • similar to hardware’s victim cache for L1/L2
      • keep tiny tiny cache of recently evicted cache lines in case needed again
  • two lists
    • free list: clean pages evicted from a process resident set
    • modified list: dirt pages
  • a fault to a page in either list brings is back into resident set w/o disk op
  • 50% of faults were to pages on the list
• swapper swaps entire process and data structures to disk
  • maintains highest priority processes in memory
  • need enough physical memory for resident set to be swapped in
• page clustering
  • small pages: 512 bytes
    • PDP-11 compatibility..
  • inefficient for IO
  • want to do IO in large chunks, clustering emulates large pages
  • read: demand paging from executable file, reads many pages at same time
  • write:
    • write chunks of modified pages at ~100 at a time
    • try to write contiguous
    • cluster reads on subsequent faults