Virtual Memory with Linux

1. Looking at Memory with Linux

(Specifics of Linux are not part of learning objectives. However, the following illustrates shared memory, and the pseudo-filesystem /proc will be revisited in other presentations.)

1.1. Linux Kernel: /proc/<pid>/

• /proc is a pseudo-filesystem
  • See https://man7.org/linux/man-pages/man5/proc.5.html
    • (Specific to Linux kernel; incomplete or missing elsewhere)
  • “Pseudo”: Look and feel of any other filesystem
    • Subdirectories and files
    • However, files are no “real” files but meta-data
  • Interface to internal kernel data structures
    • One subdirectory per process ID
    • OS identifies process by integer number
    • Here and elsewhere, <pid> is meant as placeholder for such a number

1.1.1. Video about /proc

1.1.2. Drawing about /proc

1.1.3. Drawing about man pages

1.2. Linux Kernel Memory Interface

• Memory allocation (and much more) visible under /proc/<pid>
• E.g.:
  • /proc/<pid>/pagemap: One 64-bit value per virtual page
    • Mapping to RAM or swap area
  • /proc/<pid>/maps: Mapped memory regions
  • /proc/<pid>/smaps: Memory usage for mapped regions
• Notice: Memory regions include shared libraries that are used by lots of processes

1.3. GNU/Linux Reporting: smem

• User space tool to read smaps files: smem
  • See https://linoxide.com/memory-usage-reporting-smem/
• Terminology
  • Virtual set size (VSS): Size of virtual address space
  • Resident set size (RSS): Allocated main memory
    • Standard notion, yet overestimates memory usage as lots of memory is shared between processes
      • Shared memory is added to the RSS of every sharing process
  • Unique set size (USS): memory allocated exclusively to process
    • That much would be returned upon process’ termination
  • Proportional set size (PSS): USS plus “fair share” of shared pages
    • If page shared by 5 processes, each gets a fifth of a page added to its PSS

1.3.1. Sample smem Output

$ smem -c "pid command uss pss rss vss" -P "bash|xinit|emacs"
  PID Command                          USS      PSS      RSS      VSS
  765 /usr/bin/xinit /etc/X11/Xse      220      285     2084    15952
 1390 /bin/bash -c libreoffice5.3      240      510     2936    13188
  826 /bin/bash /usr/bin/qubes-se      256      524     3008    13204
  750 -su -c /usr/bin/xinit /etc/      316      587     3368    21636
 1251 bash                            4864     5136     7900    26024
 2288 /usr/bin/python /usr/bin/sm     5272     6035     9432    24688
 1145 emacs                          90876    93224   106568   662768

1.3.2. Sample smem Graph

License Information

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