In computer science, persistent memory is any method or apparatus for efficiently storing data structures such that they can continue to be accessed using memory instructions or memory APIs even after the end of the process that created or last modified them.[1]
Often confused with non-volatile random-access memory (NVRAM), persistent memory is instead more closely linked to the concept of persistence in its emphasis on program state that exists outside the fault zone of the process that created it. (A process is a program under execution. The fault zone of a process is that subset of program state which could be corrupted by the process continuing to execute after incurring a fault, for instance due to an unreliable component used in the computer executing the program.)
Efficient, memory-like access is the defining characteristic of persistent memory.[2] It can be provided using microprocessormemory instructions, such as load and store. It can also be provided using APIs that implement remote direct memory access (RDMA) actions, such as RDMA read and RDMA write. Other low-latency methods that allow byte-grain[clarification needed] access to data also qualify.
Persistent memory capabilities extend beyond non-volatility of stored bits. For instance, the loss of key metadata, such as page table entries or other constructs that translate virtual addresses to physical addresses, may render durable bits non-persistent. In this respect, persistent memory resembles more abstract forms of computer storage, such as file systems. In fact, almost all existing persistent memory technologies implement at least a basic file system that can be used for associating names or identifiers with stored extents, and at a minimum provide file system methods that can be used for naming and allocating such extents.
The read-of-non-persistent-write problem is found for lock-free programs on persistent memory. As compare-and-swap (CAS) operations do not persist the written values to persistent memory, the modified data can be made visible by the cache coherence protocol to a concurrent observer before the modified data can be observed by a crash observer at persistent memory. If a power failure happens right after the write is made visible but not yet persistent, the read-of-non-persistent-write problem can occur, i.e., a data variable that is modified by a compare-and-swap operation can be made visible to a concurrent observer before a crash observer, causing potential crash inconsistencies.
To illustrate the problem: for a singly linked lock-free list, a node can be inserted by a producer thread A after the head node, the next pointer of the head node gets atomically switched (CAS) to point to the new node A, however, this CAS is not persisted. Then, another node gets inserted by producer thread B after node A, as CAS for node A is already visible to all concurrent threads. CAS atomically switches the next pointer of node A to point to node B, and this CAS gets persisted. If a power failure happens at this point, the application that uses the linked list would be left in an inconsistent state, with both node A and node B lost, as the next pointer from the head node to node A has not been persisted. As node B has been published but can’t be accessed after a reboot, and other data may have been persisted that are accessed through or dependent on node B, all subsequent accesses to such data will not be possible, causing data loss.[3]
The read-of-non-persistent-write problem is not limited to lock-free linked lists, it can be found in any lock-free data structures where the potential gap between concurrent visibility and persistent visibility can exist. For instance, a similar problem can occur with persistent circular buffers.[4]
See alsoedit
NOVA (filesystem) or "non-volatile memory accelerated", an open-source log-structured file system for byte-addressable persistent memory
Persistent data, information that is infrequently accessed and not likely to be modified
Persistent data structures, a data structure that always preserves the previous version of itself when it is modified (effectively immutable)
^Satish M. Thatte. 1986. Persistent memory: a storage architecture for object-oriented database systems. In Proceedings on the 1986 international workshop on Object-oriented database systems (OODS '86). IEEE Computer Society Press, Los Alamitos, CA, USA, 148-159.
^P. Mehra and S. Fineberg, "Fast and flexible persistence: the magic potion for fault-tolerance, scalability and performance in online data stores," 18th International Parallel and Distributed Processing Symposium, 2004. Proceedings., Santa Fe, NM, USA, 2004, pp. 206-. doi: 10.1109/IPDPS.2004.1303232
^Wang, William; Diestelhorst, Stephan (June 17, 2019). "Persistent Atomics for Implementing Durable Lock-Free Data Structures for Non-Volatile Memory (Brief Announcement)". The 31st ACM Symposium on Parallelism in Algorithms and Architectures. Association for Computing Machinery. pp. 309–311. doi:10.1145/3323165.3323166. ISBN9781450361842. S2CID 195064876 – via ACM Digital Library.
^Wolczko, Mario (April 26, 2019). "Non-Volatile Memory and Java: Part 2". Medium.
External linksedit
Persistent Memory Programming, a collection of resources related to persistent memory programming
Containers and persistent data, LWN.net, May 28, 2015, by Josh Berkus
Asynchronous DRAM Refresh (ADR), SNIA, January 2014 (applies to DRAM as well)
Twizzler: An Operating System for Next-Generation Memory Hierarchies, University of California, Santa Cruz Technical Report UCSC-SSRC-17-01, December 5, 2017, by Daniel Bittman, Matt Bryson, Yuanjiang Ni, Arjun Govindjee, Isaak Cherdak, Pankaj Mehra, Darrell D. E. Long, and Ethan L. Miller
January 01, 1970
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This article is about computer science concept For other uses see The Persistence of Memory disambiguation In computer science persistent memory is any method or apparatus for efficiently storing data structures such that they can continue to be accessed using memory instructions or memory APIs even after the end of the process that created or last modified them 1 Often confused with non volatile random access memory NVRAM persistent memory is instead more closely linked to the concept of persistence in its emphasis on program state that exists outside the fault zone of the process that created it A process is a program under execution The fault zone of a process is that subset of program state which could be corrupted by the process continuing to execute after incurring a fault for instance due to an unreliable component used in the computer executing the program Efficient memory like access is the defining characteristic of persistent memory 2 It can be provided using microprocessor memory instructions such as load and store It can also be provided using APIs that implement remote direct memory access RDMA actions such as RDMA read and RDMA write Other low latency methods that allow byte grain clarification needed access to data also qualify Persistent memory capabilities extend beyond non volatility of stored bits For instance the loss of key metadata such as page table entries or other constructs that translate virtual addresses to physical addresses may render durable bits non persistent In this respect persistent memory resembles more abstract forms of computer storage such as file systems In fact almost all existing persistent memory technologies implement at least a basic file system that can be used for associating names or identifiers with stored extents and at a minimum provide file system methods that can be used for naming and allocating such extents Contents 1 The read of non persistent write problem 2 See also 3 References 4 External linksThe read of non persistent write problem editThe read of non persistent write problem is found for lock free programs on persistent memory As compare and swap CAS operations do not persist the written values to persistent memory the modified data can be made visible by the cache coherence protocol to a concurrent observer before the modified data can be observed by a crash observer at persistent memory If a power failure happens right after the write is made visible but not yet persistent the read of non persistent write problem can occur i e a data variable that is modified by a compare and swap operation can be made visible to a concurrent observer before a crash observer causing potential crash inconsistencies To illustrate the problem for a singly linked lock free list a node can be inserted by a producer thread A after the head node the next pointer of the head node gets atomically switched CAS to point to the new node A however this CAS is not persisted Then another node gets inserted by producer thread B after node A as CAS for node A is already visible to all concurrent threads CAS atomically switches the next pointer of node A to point to node B and this CAS gets persisted If a power failure happens at this point the application that uses the linked list would be left in an inconsistent state with both node A and node B lost as the next pointer from the head node to node A has not been persisted As node B has been published but can t be accessed after a reboot and other data may have been persisted that are accessed through or dependent on node B all subsequent accesses to such data will not be possible causing data loss 3 The read of non persistent write problem is not limited to lock free linked lists it can be found in any lock free data structures where the potential gap between concurrent visibility and persistent visibility can exist For instance a similar problem can occur with persistent circular buffers 4 See also editNOVA filesystem or non volatile memory accelerated an open source log structured file system for byte addressable persistent memory Persistent data information that is infrequently accessed and not likely to be modified Persistent data structures a data structure that always preserves the previous version of itself when it is modified effectively immutable Phantom OS a persistent operating systemReferences edit Satish M Thatte 1986 Persistent memory a storage architecture for object oriented database systems In Proceedings on the 1986 international workshop on Object oriented database systems OODS 86 IEEE Computer Society Press Los Alamitos CA USA 148 159 P Mehra and S Fineberg Fast and flexible persistence the magic potion for fault tolerance scalability and performance in online data stores 18th International Parallel and Distributed Processing Symposium 2004 Proceedings Santa Fe NM USA 2004 pp 206 doi 10 1109 IPDPS 2004 1303232 Wang William Diestelhorst Stephan June 17 2019 Persistent Atomics for Implementing Durable Lock Free Data Structures for Non Volatile Memory Brief Announcement The 31st ACM Symposium on Parallelism in Algorithms and Architectures Association for Computing Machinery pp 309 311 doi 10 1145 3323165 3323166 ISBN 9781450361842 S2CID 195064876 via ACM Digital Library Wolczko Mario April 26 2019 Non Volatile Memory and Java Part 2 Medium External links editPersistent Memory Programming a collection of resources related to persistent memory programming Containers and persistent data LWN net May 28 2015 by Josh Berkus Asynchronous DRAM Refresh ADR SNIA January 2014 applies to DRAM as well Twizzler An Operating System for Next Generation Memory Hierarchies University of California Santa Cruz Technical Report UCSC SSRC 17 01 December 5 2017 by Daniel Bittman Matt Bryson Yuanjiang Ni Arjun Govindjee Isaak Cherdak Pankaj Mehra Darrell D E Long and Ethan L Miller Retrieved from https en wikipedia org w index php title Persistent memory amp oldid 1144357434, wikipedia, wiki, book, books, library,
