Misplaced Pages

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64-541: The chip set consisted of the R2000 microprocessor, R2010 floating-point accelerator, and four R2020 write buffer chips. The core R2000 chip executed all non-floating-point instructions with a simple short pipeline. This chip also controlled the external code and data caches, made of fast standard SRAM chips organized with direct indexing and one-cycle read latency. The R2000 chip contained a small translation lookaside buffer for mapping virtual memory addresses. The R2010 chip held

128-446: A data translation lookaside buffer (DTLB). Various benefits have been demonstrated with separate data and instruction TLBs. The TLB can be used as a fast lookup hardware cache. The figure shows the working of a TLB. Each entry in the TLB consists of two parts: a tag and a value. If the tag of the incoming virtual address matches the tag in the TLB, the corresponding value is returned. Since

192-464: A 2.0 μm double-metal CMOS process. MIPS was a fabless semiconductor company, that is, they did not have the capability to fabricate integrated circuits. The chip set was initially fabricated for MIPS by Sierra Semiconductor and Toshiba . In December 1987, MIPS licensed Integrated Device Technology , LSI Logic , and Performance Semiconductor to also fabricate and market the R2000. Sierra and Toshiba continued to serve as foundries. LSI fabricated

256-415: A MIPS-style software-filled TLB. The IPT combines a page table and a frame table into one data structure. At its core is a fixed-size table with the number of rows equal to the number of frames in memory. If there are 4,000 frames, the inverted page table has 4,000 rows. For each row there is an entry for the virtual page number (VPN), the physical page number (not the physical address), some other data and

320-395: A TLB: The average effective memory cycle rate is defined as m + ( 1 − p ) h + p m {\displaystyle m+(1-p)h+pm} cycles, where m {\displaystyle m} is the number of cycles required for a memory read, p {\displaystyle p} is the miss rate, and h {\displaystyle h}

384-403: A byte. First, the page table is looked up for the frame number. Second, the frame number with the page offset gives the actual address. Thus, any straightforward virtual memory scheme would have the effect of doubling the memory access time. Hence, the TLB is used to reduce the time taken to access the memory locations in the page-table method. The TLB is a cache of the page table, representing only

448-445: A context switch, the entire TLB has to be flushed. Maintaining a tag that associates each TLB entry with an address space in software and comparing this tag during TLB lookup and TLB flush is very expensive, especially since the x86 TLB is designed to operate with very low latency and completely in hardware. In 2008, both Intel ( Nehalem ) and AMD ( SVM ) have introduced tags as part of the TLB entry and dedicated hardware that checks

512-603: A dirty bit is not used, the backing store need only be as large as the instantaneous total size of all paged-out pages at any moment. When a dirty bit is used, at all times some pages will exist in both physical memory and the backing store. In operating systems that are not single address space operating systems , address space or process ID information is necessary so the virtual memory management system knows what pages to associate to what process. Two processes may use two identical virtual addresses for different purposes. The page table must supply different virtual memory mappings for

576-430: A hard disk drive, can be used to augment physical memory. Pages can be paged in and out of physical memory and the disk. The present bit can indicate what pages are currently present in physical memory or are on disk, and can indicate how to treat these different pages, i.e. whether to load a page from disk and page another page in physical memory out. The dirty bit allows for a performance optimization. A page on disk that

640-414: A master page table, effectively creating a tree data structure. There need not be only two levels, but possibly multiple ones. For example, a virtual address in this schema could be split into three parts: the index in the root page table, the index in the sub-page table, and the offset in that page. Multilevel page tables are also referred to as "hierarchical page tables". It was mentioned that creating

704-478: A means for creating a collision chain, as we will see later. Searching through all entries of the core IPT structure is inefficient, and a hash table may be used to map virtual addresses (and address space/PID information if need be) to an index in the IPT - this is where the collision chain is used. This hash table is known as a hash anchor table . The hashing function is not generally optimized for coverage - raw speed

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768-435: A page belongs to, statistics information, or other background information. The page table is an array of page table entries. Each page table entry (PTE) holds the mapping between a virtual address of a page and the address of a physical frame. There is also auxiliary information about the page such as a present bit, a dirty or modified bit, address space or process ID information, amongst others. Secondary storage, such as

832-453: A page table structure that contained mappings for every virtual page in the virtual address space could end up being wasteful. But, we can get around the excessive space concerns by putting the page table in virtual memory, and letting the virtual memory system manage the memory for the page table. However, part of this linear page table structure must always stay resident in physical memory in order to prevent circular page faults and look for

896-482: A page walk, requiring several memory accesses. The flowchart provided explains the working of a TLB. If it is a TLB miss, then the CPU checks the page table for the page table entry. If the present bit is set, then the page is in main memory, and the processor can retrieve the frame number from the page-table entry to form the physical address. The processor also updates the TLB to include the new page-table entry. Finally, if

960-529: A process can't access data stored in memory pages of another process. Memory isolation is especially critical during switches between the privileged operating system kernel process and the user processes – as was highlighted by the Meltdown security vulnerability. Mitigation strategies such as kernel page-table isolation (KPTI) rely heavily on performance-impacting TLB flushes and benefit greatly from hardware-enabled selective TLB entry management such as PCID. With

1024-414: A process requests access to data in its memory, it is the responsibility of the operating system to map the virtual address provided by the process to the physical address of the actual memory where that data is stored. The page table is where mappings of virtual addresses to physical addresses are stored, with each mapping also known as a page table entry (PTE). The memory management unit (MMU) inside

1088-425: A short time and jumps back to a first process, the TLB may still have valid entries, saving the time to reload them. Other strategies avoid flushing the TLB on a context switch: (a) A single address space operating system uses the same virtual-to-physical mapping for all processes. (b) Some CPUs have a process ID register, and the hardware uses TLB entries only if they match the current process ID. For example, in

1152-547: A small L1 TLB (potentially fully associative) that is extremely fast, and a larger L2 TLB that is somewhat slower. When instruction-TLB (ITLB) and data-TLB (DTLB) are used, a CPU can have three (ITLB1, DTLB1, TLB2) or four TLBs. For instance, Intel 's Nehalem microarchitecture has a four-way set associative L1 DTLB with 64 entries for 4 KiB pages and 32 entries for 2/4 MiB pages, an L1 ITLB with 128 entries for 4 KiB pages using four-way associativity and 14 fully associative entries for 2/4 MiB pages (both parts of

1216-482: A software-managed TLB. The SPARC V9 architecture allows an implementation of SPARC V9 to have no MMU, an MMU with a software-managed TLB, or an MMU with a hardware-managed TLB, and the UltraSPARC Architecture 2005 specifies a software-managed TLB. The Itanium architecture provides an option of using either software- or hardware-managed TLBs. The Alpha architecture has a firmware-managed TLB, with

1280-451: A subset of the page-table contents. Referencing the physical memory addresses, a TLB may reside between the CPU and the CPU cache , between the CPU cache and primary storage memory, or between levels of a multi-level cache. The placement determines whether the cache uses physical or virtual addressing. If the cache is virtually addressed, requests are sent directly from the CPU to the cache, and

1344-467: A two-way set-associative TLB for data loads and stores. Some processors have different instruction and data address TLBs. A TLB has a fixed number of slots containing page-table entries and segment-table entries; page-table entries map virtual addresses to physical addresses and intermediate-table addresses, while segment-table entries map virtual addresses to segment addresses, intermediate-table addresses and page-table addresses. The virtual memory

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1408-432: Is a part of the chip's memory-management unit (MMU). A TLB may reside between the CPU and the CPU cache , between CPU cache and the main memory or between the different levels of the multi-level cache. The majority of desktop, laptop, and server processors include one or more TLBs in the memory-management hardware, and it is nearly always present in any processor that uses paged or segmented virtual memory . The TLB

1472-434: Is full, one or more pages in physical memory will need to be paged out to make room for the requested page. The page table needs to be updated to mark that the pages that were previously in physical memory are no longer there, and to mark that the page that was on disk is now in physical memory. The TLB also needs to be updated, including removal of the paged-out page from it, and the instruction restarted. Which page to page out

1536-445: Is more desirable. Of course, hash tables experience collisions. Due to this chosen hashing function, we may experience a lot of collisions in usage, so for each entry in the table the VPN is provided to check if it is the searched entry or a collision. In searching for a mapping, the hash anchor table is used. If no entry exists, a page fault occurs. Otherwise, the entry is found. Depending on

1600-461: Is no match, which is called a TLB miss , the MMU, the system firmware, or the operating system's TLB miss handler will typically look up the address mapping in the page table to see whether a mapping exists, which is called a page walk . If one exists, it is written back to the TLB, which must be done because the hardware accesses memory through the TLB in a virtual memory system, and the faulting instruction

1664-416: Is paged in to physical memory, then read from, and subsequently paged out again does not need to be written back to disk, since the page has not changed. However, if the page was written to after it is paged in, its dirty bit will be set, indicating that the page must be written back to the backing store. This strategy requires that the backing store retain a copy of the page after it is paged in to memory. When

1728-430: Is restarted, which may happen in parallel as well. The subsequent translation will result in a TLB hit, and the memory access will continue. The page table lookup may fail, triggering a page fault , for two reasons: When physical memory is not full this is a simple operation; the page is written back into physical memory, the page table and TLB are updated, and the instruction is restarted. However, when physical memory

1792-477: Is sometimes implemented as content-addressable memory (CAM). The CAM search key is the virtual address, and the search result is a physical address . If the requested address is present in the TLB, the CAM search yields a match quickly and the retrieved physical address can be used to access memory. This is called a TLB hit. If the requested address is not in the TLB, it is a miss, and the translation proceeds by looking up

1856-533: Is the hit time in cycles. If a TLB hit takes 1 clock cycle, a miss takes 30 clock cycles, a memory read takes 30 clock cycles, and the miss rate is 1%, the effective memory cycle rate is an average of 30 + 0.99 × 1 + 0.01 × 30 {\displaystyle 30+0.99\times 1+0.01\times 30} (31.29 clock cycles per memory access). On an address-space switch, as occurs when context switching between processes (but not between threads), some TLB entries can become invalid, since

1920-412: Is the memory space as seen from a process; this space is often split into pages of a fixed size (in paged memory), or less commonly into segments of variable sizes (in segmented memory). The page table, generally stored in main memory , keeps track of where the virtual pages are stored in the physical memory. This method uses two memory accesses (one for the page-table entry, one for the byte) to access

1984-463: Is the subject of page replacement algorithms . Some MMUs trigger a page fault for other reasons, whether or not the page is currently resident in physical memory and mapped into the virtual address space of a process: The simplest page table systems often maintain a frame table and a page table. The frame table holds information about which frames are mapped. In more advanced systems, the frame table can also hold information about which address space

R2000 microprocessor - Misplaced Pages Continue

2048-474: Is used to disambiguate the pages of different processes from each other. It is somewhat slow to remove the page table entries of a given process; the OS may avoid reusing per-process identifier values to delay facing this. Alternatively, per-process hash tables may be used, but they are impractical because of memory fragmentation , which requires the tables to be pre-allocated. Inverted page tables are used for example on

2112-449: Is useful since often the top-most parts and bottom-most parts of virtual memory are used in running a process - the top is often used for text and data segments while the bottom for stack, with free memory in between. The multilevel page table may keep a few of the smaller page tables to cover just the top and bottom parts of memory and create new ones only when strictly necessary. Now, each of these smaller page tables are linked together by

2176-403: Is where the desired information itself actually is in a cache, but the information for virtual-to-physical translation is not in a TLB. These are all slow, due to the need to access a slower level of the memory hierarchy, so a well-functioning TLB is important. Indeed, a TLB miss can be more expensive than an instruction or data cache miss, due to the need for not just a load from main memory, but

2240-517: The Alpha 21264 , each TLB entry is tagged with an address space number (ASN), and only TLB entries with an ASN matching the current task are considered valid. Another example in the Intel Pentium Pro , the page global enable (PGE) flag in the register CR4 and the global (G) flag of a page-directory or page-table entry can be used to prevent frequently used pages from being automatically invalidated in

2304-647: The PowerPC , the UltraSPARC and the IA-64 architecture. The inverted page table keeps a listing of mappings installed for all frames in physical memory. However, this could be quite wasteful. Instead of doing so, we could create a page table structure that contains mappings for virtual pages. It is done by keeping several page tables that cover a certain block of virtual memory. For example, we can create smaller 1024-entry 4 KB pages that cover 4 MB of virtual memory. This

2368-476: The hash function . Tree-based designs avoid this by placing the page table entries for adjacent pages in adjacent locations, but an inverted page table destroys spatial locality of reference by scattering entries all over. An operating system may minimize the size of the hash table to reduce this problem, with the trade-off being an increased miss rate. There is normally one hash table, contiguous in physical memory, shared by all processes. A per-process identifier

2432-455: The memory management unit or by low-level system software or firmware. In operating systems that use virtual memory, every process is given the impression that it is working with large, contiguous sections of memory. Physically, the memory of each process may be dispersed across different areas of physical memory, or may have been moved ( paged out ) to secondary storage, typically to a hard disk drive (HDD) or solid-state drive (SSD). When

2496-403: The page table in a process called a page walk . The page walk is time-consuming when compared to the processor speed, as it involves reading the contents of multiple memory locations and using them to compute the physical address. After the physical address is determined by the page walk, the virtual address to physical address mapping is entered into the TLB. The PowerPC 604 , for example, has

2560-411: The CPU stores a cache of recently used mappings from the operating system's page table. This is called the translation lookaside buffer (TLB), which is an associative cache. When a virtual address needs to be translated into a physical address, the TLB is searched first. If a match is found, which is known as a TLB hit , the physical address is returned and memory access can continue. However, if there

2624-496: The ITLB divided statically between two threads) and a unified 512-entry L2 TLB for 4 KiB pages, both 4-way associative. Some TLBs may have separate sections for small pages and huge pages. For example, Intel Skylake microarchitecture separates the TLB entries for 1 GiB pages from those for 4 KiB/2 MiB pages. Three schemes for handling TLB misses are found in modern architectures: The MIPS architecture specifies

R2000 microprocessor - Misplaced Pages Continue

2688-458: The R2000 was followed by the R3000 , using a similar overall system design but faster chip implementation. Translation lookaside buffer A translation lookaside buffer ( TLB ) is a memory cache that stores the recent translations of virtual memory to physical memory . It is used to reduce the time taken to access a user memory location. It can be called an address-translation cache. It

2752-465: The TLB is accessed only on a cache miss . If the cache is physically addressed, the CPU does a TLB lookup on every memory operation, and the resulting physical address is sent to the cache. In a Harvard architecture or modified Harvard architecture , a separate virtual address space or memory-access hardware may exist for instructions and data. This can lead to distinct TLBs for each access type, an instruction translation lookaside buffer (ITLB) and

2816-413: The TLB lookup is usually a part of the instruction pipeline, searches are fast and cause essentially no performance penalty. However, to be able to search within the instruction pipeline, the TLB has to be small. A common optimization for physically addressed caches is to perform the TLB lookup in parallel with the cache access. Upon each virtual memory reference, the hardware checks the TLB to see whether

2880-548: The TLB miss handling code being in PALcode , rather than in the operating system. As the PALcode for a processor can be processor-specific and operating-system-specific, this allows different versions of PALcode to implement different page-table formats for different operating systems, without requiring that the TLB format, and the instructions to control the TLB, to be specified by the architecture. These are typical performance levels of

2944-453: The TLBs on a task switch or a load of register CR3. Since the 2010 Westmere microarchitecture Intel 64 processors also support 12-bit process-context identifiers (PCIDs), which allow retaining TLB entries for multiple linear-address spaces, with only those that match the current PCID being used for address translation. While selective flushing of the TLB is an option in software-managed TLBs,

3008-407: The advent of virtualization for server consolidation, a lot of effort has gone into making the x86 architecture easier to virtualize and to ensure better performance of virtual machines on x86 hardware. Normally, entries in the x86 TLBs are not associated with a particular address space; they implicitly refer to the current address space. Hence, every time there is a change in address space, such as

3072-413: The architecture, the entry may be placed in the TLB again and the memory reference is restarted, or the collision chain may be followed until it has been exhausted and a page fault occurs. A virtual address in this schema could be split into two, the first half being a virtual page number and the second half being the offset in that page. A major problem with this design is poor cache locality caused by

3136-608: The chip set in its 2.0 μm double-metal CMOS process and marketed it as the LR2000. Performance Semiconductor fabricated the chip set in its PACE-I 0.8 μm double-metal CMOS process and marketed it as the PR2000. In 1988, an improved version was introduced, the R2000A. It was composed of the R2000A and R2010A ICs. It operated at 12.5 and 16.67 MHz. It has been used extensively in embedded applications such as printer controllers. In 1988,

3200-469: The first Acorn RISC Machine (ARM) evaluation kits shipping to developers. Overall speed was limited by the cache size and cache cycle time. The R2000 chip set and SRAM was initially sold only as a complete circuit board to ensure good cache bus timings. In 1987 system builders began using the chip set in arbitrary new board designs. The R2000 was available in 8.3, 12.5 and 15 MHz grades. The die contained 110,000 transistors and measured 80 mm in

3264-474: The floating point registers, floating point data paths, and their longer simple pipeline. Writes to main memory DRAM took tens of cycles to fully complete. But the R2020 chips queued and completed up to 4 pending writes to main memory, allowing the R2000 core to proceed without stalling itself. In the absence of cache misses, this chip set sustained an instruction completion rate of one instruction per ALU cycle. This

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3328-551: The only option in some hardware TLBs (for example, the TLB in the Intel 80386 ) is the complete flushing of the TLB on an address-space switch. Other hardware TLBs (for example, the TLB in the Intel 80486 and later x86 processors, and the TLB in ARM processors) allow the flushing of individual entries from the TLB indexed by virtual address. Flushing of the TLB can be an important security mechanism for memory isolation between processes to ensure

3392-558: The page number and frame number to the TLB, so that they will be found quickly on the next reference. If the TLB is already full, a suitable block must be selected for replacement. There are different replacement methods like least recently used (LRU), first in, first out (FIFO) etc.; see the address translation section in the cache article for more details about virtual addressing as it pertains to caches and TLBs. The CPU has to access main memory for an instruction-cache miss, data-cache miss, or TLB miss. The third case (the simplest one)

3456-428: The page number is held therein. If yes, it is a TLB hit, and the translation is made. The frame number is returned and is used to access the memory. If the page number is not in the TLB, the page table must be checked. Depending on the CPU, this can be done automatically using a hardware or using an interrupt to the operating system. When the frame number is obtained, it can be used to access the memory. In addition, we add

3520-404: The page table itself may occupy a different virtual-memory page for each process so that the page table becomes a part of the process context. In such an implementation, the process's page table can be paged out whenever the process is no longer resident in memory. There are several types of page tables, which are optimized for different requirements. Essentially, a bare-bones page table must store

3584-416: The present bit is not set, then the desired page is not in the main memory, and a page fault is issued. Then a page-fault interrupt is called, which executes the page-fault handling routine. If the page working set does not fit into the TLB, then TLB thrashing occurs, where frequent TLB misses occur, with each newly cached page displacing one that will soon be used again, degrading performance in exactly

3648-423: The program executed by the accessing process , while physical addresses are used by the hardware, or more specifically, by the random-access memory (RAM) subsystem. The page table is a key component of virtual address translation that is necessary to access data in memory. The page table is set up by the computer's operating system , and may be read and written during the virtual address translation process by

3712-403: The same way as thrashing of the instruction or data cache does. TLB thrashing can occur even if instruction-cache or data-cache thrashing are not occurring, because these are cached in different-size units. Instructions and data are cached in small blocks ( cache lines ), not entire pages, but address lookup is done at the page level. Thus, even if the code and data working sets fit into cache, if

3776-480: The tag during lookup. Not all operating systems made full use of these tags immediately, but Linux 4.14 started using them to identify recently used address spaces, since the 12-bits PCIDs (4095 different values) are insufficient for all tasks running on a given CPU. Page table A page table is a data structure used by a virtual memory system in a computer to store mappings between virtual addresses and physical addresses . Virtual addresses are used by

3840-495: The two processes. This can be done by assigning the two processes distinct address map identifiers, or by using process IDs. Associating process IDs with virtual memory pages can also aid in selection of pages to page out, as pages associated with inactive processes, particularly processes whose code pages have been paged out, are less likely to be needed immediately than pages belonging to active processes. As an alternative to tagging page table entries with process-unique identifiers,

3904-450: The virtual address, the physical address that is "under" this virtual address, and possibly some address space information. An inverted page table (IPT) is best thought of as an off-chip extension of the TLB which uses normal system RAM. Unlike a true page table, it is not necessarily able to hold all current mappings. The operating system must be prepared to handle misses, just as it would with

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3968-410: The virtual-to-physical mapping is different. The simplest strategy to deal with this is to completely flush the TLB. This means that after a switch, the TLB is empty, and any memory reference will be a miss, so it will be some time before things are running back at full speed. Newer CPUs use more effective strategies marking which process an entry is for. This means that if a second process runs for only

4032-437: The working sets are fragmented across many pages, the virtual-address working set may not fit into TLB, causing TLB thrashing. Appropriate sizing of the TLB thus requires considering not only the size of the corresponding instruction and data caches, but also how these are fragmented across multiple pages. Similar to caches, TLBs may have multiple levels. CPUs can be (and nowadays usually are) built with multiple TLBs, for example

4096-539: Was more efficient than non-RISC microprocessors of that time, which needed several cycles per instruction. The initial R2000A, clocked at 12.5 MHz, offered 8-10 Million integer Instructions Per Second (MIPS), or 0.9 Million FLoating Point Operations Per Second (MFLOPS), and would appear in the like of the 1987 SGI IRIS 4D and 1988 DECstation 2100 workstations. 1986 also saw similar technology in Sun's first SPARC microprocessor, Hewlett Packard's first PA-RISC microprocessor, and

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