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Process working set and page replacement in WinNT

Process working set and page replacement in WinNT. Practical session #4 バリ　ゲローフィ. Outline. Process working set Working set implementation Aging, trimming and replacement Modifying the replacement algorithm Priority inversion user-land . Process Working Set. Working set:

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Process working set and page replacement in WinNT

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  1. Process working set and page replacement in WinNT Practical session #4 バリ　ゲローフィ Advanced operating systems course, The University of Tokyo

  2. Outline • Process working set • Working set implementation • Aging, trimming and replacement • Modifying the replacement algorithm • Priority inversion user-land

  3. Process Working Set • Working set: • All the physical pages “owned” by a process • Essentially, all the pages the process can reference without incurring a page fault • Working set limit: • The maximum pages the process can own • When limit is reached, a page must be released for every page that’s brought in (“working set replacement”) • Default upper limit on size for each process • System-wide maximum calculated & stored in MmMaximumWorkingSetSize • Approximately RAM minus 512 pages (2 MB on x86) minus min size of system working set (1.5 MB on x86) • Interesting to view (gives you an idea how much memory you’ve “lost” to the OS) • True upper limit: 2 GB minus 64 MB for 32-bit Windows

  4. Working Set List • A process always starts with an empty working set • It then incurs page faults when referencing a page that isn’t in its working set • Many page faults may be resolved from memory (to be described later) • Just an array of entries maintaining some statistics for each page mapped in to the process’ address space newer pages older pages PerfMon Process “WorkingSet”

  5. Working Set Replacement • When working set max reached (or working set trim occurs), must give up pages to make room for new pages • Local page replacement policy (most Unix systems implement global replacement) • Means that a single process cannot take over all of the physical memory unless other processes aren’t using it • Page replacement algorithm is an approximation of the least recently used (LRU) method • Pages are aged based on access bit PerfMon Process “WorkingSet” to standby or modified page list

  6. Working set implementation:data structures

  7. MMSUPPORT • MinimumWorkingSetSize: minimum size of working set • MaximumWorkingSetSize: maximum size of working set • WorkingSetSize: current size of working set • VmWorkingSetList: pointer to the working set list • NextAgingSlot: index from where aging algorithm starts next time

  8. _MMWSLE – Working set list • FirstDynamic: the first working set index that could be removed • LastEntry: the last valid entry • NextSlot: index from where trimming starts (i.e. the last scanned index + 1) • FirstFree: the first free index (if there is any) • Wsle: the actual array of working set entries

  9. Working set manager source files

  10. Where is the decision made which pages to remove? • Global method (BalanceSetManager): • Iterates a list of working sets (processes) and calls MiProcessWorkingSet() -> MiTrimWorkingSet() - removes multiple pages • Per-process method during page fault: • MmAccessFault() -> MiAddValidPageToWorkingSet() -> MiAllocateWsle() -> MiDoReplacement() -> MiReplaceWorkingSetEntry() – drops one page

  11. MiTrimWorkingSet() • Iterates the working set and looks for pages: • That are older than a certain age (LRU!) • Collects them to a list (called WsleFlushList) • Calls MiRemoveWorkingSetPages() for actual removal

  12. MiReplaceWorkingSetEntry() • Called in the page fault handler • Looks for a page in the working set that can be removed, only one page: • It scans working set and tries to find a page that is older than the specified limit (also keeps track the oldest during the search) • it doesn’t check more than a predefined limit (MM_WORKING_SET_LIST_SEARCH = 17) so that the time spent on search is reasonable • It frees the entry

  13. How is age maintained? (the foundation of LRU) • MiAgeWorkingSet() function: • Walks trough a portion of a working set • Checks PTE access bit • If not set: • Increases the age field of the WSLENTRY VOID MiAgeWorkingSet ( IN PMMSUPPORT VmSupport, IN LOGICAL DoAging, IN PWSLE_NUMBER WslesScanned, IN OUT PPFN_NUMBER TotalClaim, IN OUT PPFN_NUMBER TotalEstimatedAvailable )

  14. Experiment with page removal • Test scenario: • Use SetProcessWorkingSetSizeEx() to set working set size to max 256 pages • Allocate more memory than 256 pages • Refer the pages periodically and see which ones get removed during page faults

  15. Let’s modify the paging algorithm • Modify MiReplaceWorkingSetEntry() so that it decides which page to drop based on some other policy (not the pages’ age), for instance: • FIFO • LIFO (does it make sense?) • Random • Page out modified pages first • Clock algorithm • Suggestions?

  16. Log from default policy: MiFreeWsle: (pid:1604) drops wset ID: 6, address: 0x318 MiFreeWsle: (pid:1604) drops wset ID: 7, address: 0x319 MiFreeWsle: (pid:1604) drops wset ID: 8, address: 0x31a MiFreeWsle: (pid:1604) drops wset ID: 9, address: 0x31b MiFreeWsle: (pid:1604) drops wset ID: 10, address: 0x31c MiFreeWsle: (pid:1604) drops wset ID: 11, address: 0x31d MiFreeWsle: (pid:1604) drops wset ID: 12, address: 0x7c81f MiFreeWsle: (pid:1604) drops wset ID: 13, address: 0x7c812 MiFreeWsle: (pid:1604) drops wset ID: 15, address: 0x31e MiFreeWsle: (pid:1604) drops wset ID: 16, address: 0x7c82e MiFreeWsle: (pid:1604) drops wset ID: 17, address: 0x7ffdf MiFreeWsle: (pid:1604) drops wset ID: 18, address: 0x31f MiFreeWsle: (pid:1604) drops wset ID: 19, address: 0x430 MiFreeWsle: (pid:1604) drops wset ID: 20, address: 0x431

  17. Same scenario (referring same addresses) with FIFO: MiFreeWsle: (pid:1952) drops wset ID: 582, address: 0x316 MiFreeWsle: (pid:1952) drops wset ID: 504, address: 0x317 MiFreeWsle: (pid:1952) drops wset ID: 503, address: 0x77e6f MiFreeWsle: (pid:1952) drops wset ID: 455, address: 0x318 MiFreeWsle: (pid:1952) drops wset ID: 413, address: 0x319 MiFreeWsle: (pid:1952) drops wset ID: 409, address: 0x31a MiFreeWsle: (pid:1952) drops wset ID: 402, address: 0x31b MiFreeWsle: (pid:1952) drops wset ID: 396, address: 0x31c MiFreeWsle: (pid:1952) drops wset ID: 362, address: 0x77eaa MiFreeWsle: (pid:1952) drops wset ID: 360, address: 0x7c81f MiFreeWsle: (pid:1952) drops wset ID: 356, address: 0x7c812 MiFreeWsle: (pid:1952) drops wset ID: 331, address: 0x31e

  18. Let’s see the code…

  19. Assignment #3 • Pick a method and change the page replacement algorithm! • Print out the page replacements and explain how the log corresponds to your method!

  20. Priority inversion user-space application • Anybody implemented already? • Problems? • Question?

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