CSE 120 Principles of Operating Systems Spring 2009Lecture OverviewManaging Page TablesTwo-Level Page TablesTwo-Level Page TablesAddressing Page TablesEfficient TranslationsTLBsManaging TLBsManaging TLBs (2)Paged Virtual MemoryPage FaultsAddress Translation ReduxThe Common CaseTLB MissesReloading the TLBTLB Misses (2)Page FaultsAdvanced FunctionalitySharingSharing (2)Copy on WriteMapped FilesMapped Files (2)SummaryNext time…TodoCSE 120Principles of Operating SystemsSpring 2009Lecture 10: PagingGeoffrey M. VoelkerMay 12, 2009 CSE 120 – Lecture 10 – Paging 2Lecture OverviewToday we’ll cover more paging mechanisms: Optimizations Managing page tables (space) Efficient translations (TLBs) (time) Demand paged virtual memory (space) Recap address translation Advanced Functionality Sharing memory Copy on Write Mapped filesMay 12, 2009 CSE 120 – Lecture 10 – Paging 3Managing Page Tables Last lecture we computed the size of the page table for a 32-bit address space w/ 4K pages to be 4MB This is far far too much overhead for each process How can we reduce this overhead? Observation: Only need to map the portion of the address space actually being used (tiny fraction of entire addr space) How do we only map what is being used? Can dynamically extend page table… Does not work if addr space is sparce (internal fragmentation) Use another level of indirection: two-level page tablesMay 12, 2009 CSE 120 – Lecture 10 – Paging 4Two-Level Page Tables Two-level page tables Virtual addresses (VAs) have three parts:» Master page number, secondary page number, and offset Master page table maps VAs to secondary page table Secondary page table maps page number to physical page Offset indicates where in physical page address is located Example 4K pages, 4 bytes/PTE How many bits in offset? 4K = 12 bits Want master page table in one page: 4K/4 bytes = 1K entries Hence, 1K secondary page tables. How many bits? Master (1K) = 10, offset = 12, inner = 32 – 10 – 12 = 10 bitsMay 12, 2009 CSE 120 – Lecture 10 – Paging 5Two-Level Page TablesPage tableMaster page number SecondaryVirtual AddressMaster Page TablePage frame OffsetPhysical AddressPhysical MemoryOffsetPage frameSecondary Page TableMay 12, 2009 CSE 120 – Lecture 10 – Paging 6Addressing Page TablesWhere do we store page tables (which address space)? Physical memory Easy to address, no translation required But, allocated page tables consume memory for lifetime of VAS Virtual memory (OS virtual address space) Cold (unused) page table pages can be paged out to disk But, addressing page tables requires translation How do we stop recursion? Do not page the outer page table (called wiring) If we’re going to page the page tables, might as well page the entire OS address space, too Need to wire special code and data (fault, interrupt handlers)May 12, 2009 CSE 120 – Lecture 10 – Paging 7Efficient Translations Our original page table scheme already doubled the cost of doing memory lookups One lookup into the page table, another to fetch the data Now two-level page tables triple the cost! Two lookups into the page tables, a third to fetch the data And this assumes the page table is in memory How can we use paging but also have lookups cost about the same as fetching from memory? Cache translations in hardware Translation Lookaside Buffer (TLB) TLB managed by Memory Management Unit (MMU)May 12, 2009 CSE 120 – Lecture 10 – Paging 8TLBs Translation Lookaside Buffers Translate virtual page #s into PTEs (not physical addrs) Can be done in a single machine cycle TLBs implemented in hardware Fully associative cache (all entries looked up in parallel) Cache tags are virtual page numbers Cache values are PTEs (entries from page tables) With PTE + offset, can directly calculate physical address TLBs exploit locality Processes only use a handful of pages at a time» 16-48 entries/pages (64-192K)» Only need those pages to be “mapped” Hit rates are therefore very importantMay 12, 2009 CSE 120 – Lecture 10 – Paging 9Managing TLBs Address translations for most instructions are handled using the TLB >99% of translations, but there are misses (TLB miss)… Who places translations into the TLB (loads the TLB)? Hardware (Memory Management Unit) [x86]» Knows where page tables are in main memory» OS maintains tables, HW accesses them directly» Tables have to be in HW-defined format (inflexible) Software loaded TLB (OS) [MIPS, Alpha, Sparc, PowerPC]» TLB faults to the OS, OS finds appropriate PTE, loads it in TLB» Must be fast (but still 20-200 cycles)» CPU ISA has instructions for manipulating TLB» Tables can be in any format convenient for OS (flexible)May 12, 2009 CSE 120 – Lecture 10 – Paging 10Managing TLBs (2) OS ensures that TLB and page tables are consistent When it changes the protection bits of a PTE, it needs to invalidate the PTE if it is in the TLB Reload TLB on a process context switch Invalidate all entries Why? What is one way to fix it? When the TLB misses and a new PTE has to be loaded, a cached PTE must be evicted Choosing PTE to evict is called the TLB replacement policy Implemented in hardware, often simple (e.g., Last-Not-Used)May 12, 2009 CSE 120 – Lecture 10 – Paging 11Paged Virtual Memory We’ve mentioned before that pages can be moved between memory and disk This process is called demand paging OS uses main memory as a page cache of all the data allocated by processes in the system Initially, pages are allocated from memory When memory fills up, allocating a page in memory requires some other page to be evicted from memory» Why physical memory pages are called “frames” Evicted pages go to disk (where? the swap file/backing store) The movement of pages between memory and disk is done by the OS, and is transparent to the applicationMay 12, 2009 CSE 120 – Lecture 10 – Paging 12Page Faults What happens when a process accesses a page that has been evicted?1. When it evicts a page, the OS sets the PTE as invalid and stores the location of the page in the swap file in the PTE2. When a process accesses the page, the invalid PTE will cause a trap (page fault)3. The trap will run the OS page fault handler4. Handler uses the invalid PTE to locate page in swap
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