CSC369: Operating System

1. CSC369: Operating System Tutorial 8 March 9, 2012 TA: Europa Shang

2. Assignment 3 • Implement paging by writing the following functions: • lpage_fault: handles a page fault • lpage_evit: evicts an logical page from physical memory • page_replace: implements the page replacement policy • Sequential Replacement: FIFO • Second Chance Replacement: Clock • No code reading questions

3. Virtual Memory • Processes need more logical address space than physically available • Need to manage what is in memory • Swap pages on request • When a page is requested but not in memory? • Page fault • When there is no more space in main memory to bring in pages? • Replace and evict

4. MIPS TLB • TLB keeps track of mapping from virtual to physical pages • On memory read/write, check the entries in the TLB in parallel (associative lookup): • Entry is found: TLB hit • Entry not found: TLB miss • Causes EX_TLBL for loads: reads • Causes EX_TLBS for stores: writes • Protection fault: trying to write to read-only memory causes EX_MOD (modify) • TLB Exceptions • mips_trap calls vm_fault for any TLB exception • Different types of VM_FAULT_* are passed on • Eventually gets to lpage_fault

5. LPAGE • The lpage entry maps the virtual address to the address in physical memory and in swap space • On creation, • Request a block of swap space • Set physical address to INVALID • Update swap pointer • The lpage structure keeps track of where the page stores • In physical memory : lp_paddr • On disk: lp_swapaddr • Low bits of lp_addr used to hold flags, e.g. DIRTY, PINNED • Read comments in src/kern/include/vmprivate.h

6. LPAGE Operations • lpage_create: creates an logical page object • lpage_destroy: deallocates a logical page, releases any RAM or swap pages involved • lpage_lock: acquires the lock on an lpage • lpage_unlock: releases the lock on an lpage • lpage_lock_and_pin: locks the lpage page and pins the underlying physical page in the coremap. • lpage_copy: creates a new lpage and copy data from another lpage • lpage_zerofill: creates a new lpage and arrange for it to be cleared to all zeros • lpage_fault: handles a fault on a specific lpage • lpage_evict: evicts an lpage from physical memory

7. VM_OBJECT • A vm_object is a data structure that defines a a valid block of process virtual memory • Defiend in src/kern/include/vmprivate.h • Created by as_define_region • Each vm_objectcontains a base virtual address and an array of logical pages • The address space is a collection of vm_objects • Redzone: a possible guard band against other vm_objects • Simple operations: create, destroy, copy, etc. • lpage and vm_object • From virtual address, can the corresponding vm_object • In the vm_object, can find the index of the corresponding lpage structure

8. Coremap • Logical pages are nice, but we ultimately need to work with physical memory • Need to keep track of physical pages • Coremap maps pages in memory to their virtual address. • Reverse page table • One entry per page frame • Indexed by physical page number • Contain the virtual page number of address space that is occupying the page frame.

9. Coremap • TLB management and handling • Page replacement policy • Keep track of the number of • Maximum number of physical pages • Number of kernel pages • Number of user pages • Number of pages that are free • Maintain information whether a page has been pinned for manipulation

10. COREMAP_ENTRY • A coremap entry has bit flags that indicate if pages are kernel pages, pinned (busy), etc. • May add other flags to support your design structcoremap_entry { structlpage *cm_lpage; volatile int cm_tlbix:7; unsigned cm_cpunum:5; unsigned cm_kernel:1, cm_notlast:1, cm_allocated:1; volatile unsigned cm_pinned:1; };

11. COREMAP_ENTRY v.s. LPAGE • Each lpage is a logical piece of memory • The memory may be in memory or in swap (on disk) • A lpage points to the location of its data • The coremap maps the physical memory storage • When you need physical memory, consult the coremap to see what memory is free. • A coremap entry points to a lpage

12. COREMAP Operations • tlb_replace: returns index of TLB entry to replace • TLB replacement algorithm. • tlb_invalidate: marks a given TLB entry as invalid • tlb_clear: flushes the TLB by loading it with invalid entries • tlb_unmap: searches the TLB for a specific virtual address translation and invalidates it if it exits • mips_getslot: gets a TLB slot for use, replacing an existing one if necessary and performing any at-replacement actions • page_replace: returns an index into the coremap for the selected victim page

13. COREMAP Operations • coremap_bootstrap: allocates memory for the coremap and initlaizes its entries • do_evict: performs the eviction of the specific slot in coremap • do_page_replace: starting point for the page replacement code, calls page_replace and do_evict • mark_page_allocated: update coremap entries for newly allocated pages and update coremap counters • Called from coremap_alloca_one_page and coremap_alloc_multipages • coremap_free: deallocates a specific physical address and any subsequent pages allocated in the same block • coremap_pin: marks a specific page pinned for manipulation of its contents • coremap_unpin: unpins a specific page • mmu_map: enters a translation into the MMU.

14. Page Faults • Minor fault: desired page is in memory but not found in TLB • Update the TLB and coremap • coremap.c : mmu_map • Major fault: desired page is not in memory • It’s either in swap space, or hasn’t been created yet • How do you know if it’s a major fault? • lp_paddr is INVALID_PADDR if the page is not in memory • Page hasn’t been created yet • A new page is allocated to the process and initialized (zero-filled) • In src/kern/vm/addrspace.c: as_fault • Page is on disk • We need to swap the page into memory from swap space

15. Page Replacement • Updating the victim’s PTE to show that it is in swap • Evicting/invalidating victim’s PTE from the TLB • Copying it to disk iff its dirty • Loading the new page into memory • Updating the new page’s PTE and inserting it into the TLB

16. Synchronization • OS/161 assumes that lpage, vm_objects and address spaces are not shared • But one thread may access an lpage belonging to another thread, in order to evict a page • Don’t need locks when accessing address space and vm_objects, but lpages do need synchronization • Bit lock is used to save space: lpage_lock, lpage_unlock • Locks for the Virtual Memory system • global_paging_lock: limits number of pages pinned at any one time • swaplock: synchronizes swapmap and counters • Used by swap_alloc, swap_free, swap_reserve, swap_unreserve • cm_pinned: locks pages that are being used • One bit lock per lpage • Lock ordering, i.e., you should acquire in this order • global_paging_lock BEFORE coremap pages BEFORE lpages

17. PAGE_REPLACE • page_replace takes no arguments and returns an index into the coremap for the selected victim page. • You will write two versions of this function. • Sequential Page Replacement • Clock Page Replacement • Configuration • ASST3-FIFO • ASST3-CLOCK

18. PAGE_REPLACE: FIFO • Sequential Page Replacement • Select page to be evicted from the coremap sequentially. • Skip pinned pages and kernel pages • Tips • Map need a global variable for the head of the queue

19. PAGE_REPLACE: LRU • Clock Page Replacement • Scans pages in coremap sequentially. • Skip pinned pages, kernel pages, and pages with the reference bit set. • Turn off the reference bit if a page is skipped because the reference bit was 1. • Tips • May need a bit field in coremap_entry for the reference bit • May need a global variable for the clock hand • May need to modify several functions for the reference bit

20. LPAGE_FAULT and LPAGE_EVICT • lpage_fault: handles a fault on a specific lpage. • If a page is resident in memory, get a physical page from coremap and swap it in. • lpage_evict: evicts an lpage from physical memory to swap space • Action: • Evict the contents of the page at lp_paddr by writing it to lp_swapaddr on the swap device (if it is dirty), and mark lp_paddr as invalid • Called by do_evict when a physical page is chosen for eviction • Synchronization • Stats counters should be updated under global lock protection • Read the comments in the code