Operating Systems: Three Easy Pieces Ch. 21

Beyound Physical Memory: Mechanisms

• Memory Hierarchy → additional level, if OS needs to support large address space, it needs to keep every portion of address spaces even if it is not currently using
  • Hard Disk Drive
  • Why do we want to support a single large space? It is convenient to use
• Memory overlays → programmers to manually move pieces of code or data in and out
• Swap space → swap the spaces, pages out of memory to it and swap pages into memory from it
  • Need to remember disk address
  • We should note that swap space is not the only on-disk location for swapping traffic.
  • Safely re-use the memory space for the file that is in the disk storage
• Present Bit
  • Running process generates virtual memory references → the hardware translates them into physical addresses before fetching the desired data from memory
    • TLB hit, TLB miss
    • PTE
  • If we need to allow pages to be swapped to disk → need to indicate the page is present in the physical memory or not
  • it is set to zero, the page is not in memory but rather on disk somewhere. The act of accessing a page that is not in physical memory is commonly referred to as a page fault
• Page fault
  • OS is in charge to handle the page fault → page-fault handler
    • Hardware page-fault handler
    • Software page-fault handler
  • OS needs to swap the page into memory → how?
    • the page table is a natural place to store such information. Thus, the OS could use the bits in the PTE normally used for data such as the PFN of the page for a disk address.
    • Disk I/O → update the page table → TLB Miss → Bring up the page to TLB
    • The process will be blocked state so OS will be free to run
    • Overlap of I/O of one process and execution of another way to the effective usage of its hardware
• Page-replacement policy
  • Page in, Page out
  • Replacing, cost a lot
• Three cases
  • Page was both present and valid → TLB miss handler grap PFN from PTE, retry the instruction
  • Page is valid but not present, Page fault handler → when the page is legitimate
  • Invalid page access → bug, trap the invalid access, terminating
• OS to keep a small portion of memory free more proactively
  • High watermark
  • Low watermark
    • OS notice fewer than LW page available, the background thread is responsible for freeing memory runs
    • evicts the page until there are HW pages available
  • Swap daemon
    • Many systems cluster or group a number of pages and write them out at once to the swap partition
• these actions all take place transparently to the process
• Replacement occurs
  • high watermark (HW ) and low watermark (LW ) to help decide when to start evicting pages from memory
  • systems will cluster or group a number of pages and write them out at once to the swap partition