Showing papers on "Cache algorithms published in 1982"

Effects of Cache Coherency in Multiprocessors

Abstract: In many commercial multiprocessor systems, each processor accesses the memory through a private cache. One problem that could limit the extensibility of the system and its performance is the enforcement of cache coherence. A mechanism must exist which prevents the existence of several different copies of the same data block in different private caches. In this paper, we present an in-depth analysis of the effects of cache coherency in multiprocessors. A novel analytical model for the program behavior of a multitasked system is introduced. The model includes the behavior of each process and the interactions between processes with regard to the sharing of data blocks. An approximation is developed to derive the main effects of the cache coherency contributing to degradations in system performance.

Effects of cache coherency in multiprocessors

Abstract: In many commercial multiprocessor systems, each processor accesses the memory through a private cache. One problem that could limit the extensibility of the system and its performance is the enforcement of cache coherence. A mechanism must exist which prevents the existence of several different copies of the same data block in different private caches. In this paper, we present an indepth analysis of the effect of cache coherency in multiprocessors. A novel analytical model for the program behavior of a multitasked system is introduced. The model includes the behavior of each process and the interactions between processes with regard to the sharing of data blocks. An approximation is developed to derive the main effects of the cache coherency contributing to degradations in system performance.

Method and apparatus for grouping asynchronous recording operations

Abstract: The disclosure relates to demotion of data to a backing store (disk storage apparatus--DASD) from a random access cache in a peripheral data storage system. A cache replacement control list, such as a least recently used (LRU) list is scanned in a soon-to-be replaced first portion (portion closest to LRU entry) to identify first data to be demoted. Then the control list is scanned in first and second portions to identify further data to be demoted with the first data as a single group of data. In DASD, such data is all storable in the same cylinder of the DASD.

Analysis of Multiprocessors with Private Cache Memories

Abstract: This paper presents an approximate analytical model for the performance of multiprocessors with private cache memories and a single shared main memory. The accuracy of the model is compared with simulation results and is found to be very good over a broad range of parameters. The parameters of the model are the size of the multiprocessor, the size and type of the interconnection network, the cache miss-ratio, and the cache block transfer time. The analysis is extended to include several different read/write policies such as write-through, load-through, and buffered write-back. The analytical technique presented is also applicable to the performance of interconnection networks under block transfer mode.

Page controlled cache directory addressing

Abstract: The described embodiment modifies cache addressing in order to decrease the cache miss rate based on a statistical observation that the lowest and highest locations in pages in main storage page frames are usually accessed at a higher frequency than intermediate locations in the pages. Cache class addressing controls are modified to change the distribution of cache contained data more uniformly among the congruence classes in the cache (by comparison with conventional cache class distribution). The cache addressing controls change the congruence class address as a function of the state of a higher-order bit or field in any CPU requested address.

Multilevel cache system with graceful degradation capability

Abstract: The directory and cache store of a multilevel set associative cache system are organized in levels of memory locations. Round robin replacement apparatus is used to identify in which one of the multilevels information is to be replaced. The directory includes parity detection apparatus for detecting errors in the addresses being written in the directory during a cache memory cycle of operation. Control apparatus combines such parity errors with signals indicative of directory hits to produce invalid hit detection signals. The control apparatus in response to the occurrence of a first invalid hit detection signal conditions the round robin apparatus as well as other portions of the cache system to limit cache operation to those sections whose levels are error free thereby gracefully degrading cache operation.

Shared interface apparatus for testing the memory sections of a cache unit

Abstract: The circuits of a cache unit constructed from a single board are divided into a cache memory section and a controller section. The cache unit is connectable to the central processing unit (CPU) of a data processing system through the interface circuits of the controller section. Test mode logic circuits included within the cache memory section enable cache memories to be tested without controller interference utilizing the same controller interface circuits.

Cache/disk file status indicator with data protection feature

Abstract: A data processing system has a host processor, a RAM, a cache memory for storing segments of data, a plurality of disk drive devices and a storage control unit for controlling data transfers, data from the host processor being written to the cache memory and subsequently destaged to the disks. The storage control unit continuously updates a variable indicating the number of written-to segments resident in the cache memory that have not been destaged to the disks. The variable is stored in the RAM. A File Status flipflop is responsive to the variable to produce a "good file" signal when the variable is zero. The File Status flipflop is provided with an auxiliary power supply to maintain it in its present state in the event all power to the cache memory is terminated. Upon restoration of power after a complete power loss, the output of the status flipflop may be sampled by a command from the host processor to find out if any data was lost because written-to segments were present in the cache memory at the time all power was lost. The output of the File Status flipflop is also utilized to inhibit manually actuated functions which would destroy or invalidate written-to segments in the cache memory if the power to the cache memory were turned off, the cache memory were taken off line, or the port select switches between cache memory and the storage control unit were actuated to change port connections.

Apparatus for cache clearing

Abstract: A cache clearing apparatus for a multiprocessor data processing system having a cache unit and a duplicate directory associated with each processor. The duplicate directory, which reflects the contents of the cache directory within its associated cache unit, and the cache directory are connected through a system controller unit. Commands affecting information segments within the main memory are transferred by the system controller unit to each of the duplicate directories to determine if the information segment affected is stored in the cache memory of its associated cache memory. If the information segment is stored therein the duplicate directory issues a clear command through the system controller to clear the information segment from the associated cache unit.

Cache arrangement for direct memory access block transfer

Abstract: A cache memory system reduces cache interference during direct memory access block write operations to main memory. A control memory within cache contains in a single location validity bits for each word in a memory block. In response to the first word transferred at the beginning of a direct memory access block write operation to main memory, all validity bits for the block are reset in a single cache cycle. Cache is thereafter free to be read by the central processor during the time that the remaining words of the block are written without the need for additional cache invalidation memory cycles.

Cache management by the compiler

Abstract: An ideal high performance computer includes a fast processor and a multi-million byte memory of comparable speed. Since it is currently economically infeasible to have large memories with speeds matching the processor, hardware designers have included the cache. Because of its small size, and its effectiveness in eliminating the speed mismatch, the cache has become a common feature of high performance computers. Enhancing cache performance proved to be instrumental in the speed up of cache-based computers. In most cases enhancement methods could be classified as either software based, or hardware controlled. In most cases, software based improvement methods that proved to be very effective in main memory were considered to be inapplicable to the cache. A main reason has been the cache's transparency to programs, and the fast response time of main memory. This resulted in only hardware enhancement features being considered, and implemented for the cache. Developments in program optimization by the compiler were successful in improving the program's performance, and the understanding of program behavior. Coupling the information about a program's behavior with knowledge of the hardware structure became a good approach to optimization. With this premise we developed two cache management models: the prompting model, and the explicit management model. Both models rely on the underlying concepts of: prefetching, clustering (packing), and loop transformations. All three are software based enhancement methods that proved to be successful in boosting main memory performance. In analyzing these methods for possible implementation in the cache we found that optimal data packing is a hard problem. Nevertheless, we suggested various heuristic methods for effective packing. We then set forth a number of conditions for loop transformations. The aim of these transformations is to facilitate prefetching (preloading) of cache blocks during loop execution. In both models the compiler places preload requests within the program's code. These requests are serviced in parallel with program execution. Replacement decisions are determined at compile time in the explicit model, but are fully controlled by the hardware in the prompting model. In this model special tag bits are introduced to each cache block in order to facilitate replacement decisions. The handling of aggregate data elements (arrays) are also discussed in the thesis. In the explicit model a special indexing scheme is introduced for controlling array access in the cache. In addition, main memory addresses are only generated for block load requests, all other addresses are for the cache.

I/o controller with a dynamically adjustable cache memory

Abstract: A controller I/O (20) for transferring data between a host processor (10) and a plurality of attachment devices (16) comprises a cache memory (42) provided for storing blocks of data which are most likely to be needed in the near future. When transferring data to cache memory (42) from an attachment device (16), additional unrequested information can be transferred at the same time if it is likely that this additional data will soon be requested. Further memory (47) includes a directory table wherein all data in cache memory (42) is listed at a «home" position and, if more than one block of data in cache memory (42) have the same home position, a conflict chain is set-up so that checking the contents of the cache memory (42) can be done simply and quickly.

Hierarchical data storage system for digital computers

Abstract: Transfers of data to a backing store (disk storage apparatus 16) from a random access cache (40) in a hierarchical data storage system are grouped to reduce access demands on the data transfer paths. A cache replacement control list, such as a least recently used (LRU) list is scanned in a soon-to-be replaced first portion (portion closest to LRU entry) to identify first data to be transferred. Then, the control list is scanned in a first and second portion to identify further data to be grouped with the first data for transfer. In the case of a backing store consisting of multiple magnetic disks, such identification is carried out on the basis of the relationship that all data to be transferred will go to the same cylinder of backing storage.

On-chip microprocessor cache memory and its operating method

Abstract: 1. A cache store comprising an associative store (CAM) and a write/read store (RAM), characterized by the following features : - the cache store is integrated in a micropressor chip, - it can be divided on the logic level into a programme cache store, a micro-programme cache store and a data cache store of variable size.

Cache memories: A tutorial and survey of current research directions

Abstract: The tutorial presents a unified nomenclature for the description of cache memory systems. Using this foundation, examples of existing cache memory systems are detailed and compared.The second presentation discusses a programmable cache memory architecture. In this architecture, intelligence is added to the cache to direct the activity between the cache and the main memory. Also to be described are heuristics for programming the cache which allow the additional power to be exploited.The third presentation deals with innovations involving systems where the cache memory is not used as a simple high speed buffer for main memory. A straight forward example of this appears in IBM's Translation Lookaside Buffer on 370s with dynamic address translation hardware. Other examples are to be described include a cache system for the activation stack of a block structured language, a cache system to store subexpressions for an expression oriented architecture, and a multiprocessor architecture that relies on two levels of cache.

Digital processor with local storage contained in cache

Abstract: The processor contains a relatively small local storage (LS 12) which can be effectively expanded by utilizing a portion of a processor's store-in-cache (63). The cache allocates a line (i.e. block) for LS use by the instruction unit (IE) sending a special signal with an address for a line in a special area in main storage which is non-program addressable (i.e. not addressable by any of the architected instructions of the processor). The special signal suppresses the normal line fetch operation of the cache from main storage caused when the cache does not have a requested line. After the initial allocation of the line space in the cache to LS use, the normal cache operation is again enabled, and the LS line can be castout to the special area in main storage and be retrieved therefrom to the cache for LS use.

Cache control method and apparatus

Abstract: A host computer (10) is backed up by long term secondary magnetic disk storage means (14) coupled to the computer by channels (12), a storage director (16) and a control module (18). A cache memory (22) with an associated cache manager (24) are also connected to the storage director (16) for storing data which the host computer (10) is likely to require. In order to allow automatic transfer to the cache memory (22) of only that data which is likely to be required, the storage director (16) and cache manager (24) determine when accessed data from the disk storage means (14) appears to be part of sequential data because it lacks indications to the contrary, such as embedded SEEK instructions. When data lacks such counter indications, automatic transfers to the cache memory (22) occur a track at a time.

A multiple operating mode storage subsystem

Abstract: The efficient promotion of data from a backing store (disk storage apparatus 16-18 termed DASD) to a random access cache 40 in a storage system such as used for swap and paging data transfers. When a sequential access indicator (SEQ in 22) is sent to and retained in the storage system, all data specified in a subsequent read «paging mode» command is fetched to the cache from DASD. If such prefetched data is replaced from cache and the sequential bit is on, a subsequent host access request for such data causes all related data not yet read to be promoted to cache. A maximal amount only of related data may be promoted; such maximal amount is determined by cache addressing characteristics and DASD access delay boundaries. Without the sequential bit on, only the addressed data block is promoted to cache.

Clearing invalid addresses in cache memory

Abstract: A cache memory comprises a directory 202 and a data store 201 The n-bit portion of a desired address from an associated CPU selects a location in directory 202, and the m-bit address portions in the 4 levels I to IV of that location are compared at 203 with the m-bit portion of the desired address On a match, the corresponding level of the corresponding location of the data store 201 is accessed to access the desired word The cache words should mirror the contents of the main memory, but the latter may be changed by eg another CPU or an IOC, and the resulting invalid addresses must be cleared from the cache memory This is done by searching the directory 202 for an invalid address during the second half of a cache cycle, after the directory has been searched to determine whether the desired word is in the cache and while that desired word is being accessed in the cache store 201 If an invalid address is found, the second half of the next cache cycle is used to clear it from the cache, by resetting the full/empty indicator in the directory control portion C for that level and that location

Storage subsystems with arrangements for limiting data occupancy in caches thereof

Abstract: A storage hierarchy has a backing store (14) and a cache (15). During a series of accesses to the hierarchy by a user (10) write commands are monitored and analysed. Writing data to the hierarchy results in data being selectively removed from the cache. Space in the cache not being allocated to data being written, results in such data being written to the backing store to the exclusion of the cache. Writing as part of a chain or sequential set of commands causes further removal of the data from the cache at the end of the chain or sequence. Removal of data increases the probability of writing data directly to the backing store with data readings from the cache.