Virtual Page Research Articles

Sharing objects in a distributed, single address space environment

With reference to an object type defining the two basic operations, read and write, we present solutions to the object sharing problem, classified according to the migration and/or replication of the shared objects. We refer to a memory management system supporting a single address space view of storage in a distributed environment. Our system defines a small, powerful set of primitives that allow processes to explicitly control the allocation and deletion of the virtual pages in the physical storage as well as the page movements across the memory hierarchy. By using real programs, we demonstrate that these primitives make it possible to integrate an object sharing algorithm within the implementation of the given object at little programming effort. The discussion takes a number of salient issues into consideration, including network costs and a conceptual framework for actual implementation of the memory management system.

IMPROVING THE EXECUTION EFFICIENCY OF BARRIER SYNCHRONIZATION IN SOFTWARE DSM THROUGH STATIC ANALYSIS

In software Distributed Shared Memory (SDSM) systems, the large coherence granularity imposed by virtual memory page size tends to induce false sharing, which may lead to heavy network traffic or useless page misses on barrier operations. In this paper, we propose a method to alleviate the coherence overhead of barrier synchronization in the SDSM systems. It performs static analysis on a shared-memory program to examine data dependency between processors across global barriers, and then special primitives are inserted into the program in order to exploit the dependency information at run time. If the data modified before a barrier will be accessed by some of the other processors after the barrier, coherence messages are transferred only to the processors through the inserted primitives. Furthermore, if the modified data will not be used by any other processors, the primitives enforce the coherence messages to be delivered only to master process after the parallel execution of the program completes. We implemented the static analysis with SUIF parallelizing compiler and then evaluated the execution performance of modified programs in a 16-node SDSM system supporting AURC protocol. The experimental results show that our method is very effective at reducing the useless coherence messages, and also can improve the execution time substantially by reducing false sharing misses.

Atlas of Anesthesia, Volume VI: Pain Management

Atlas of Anesthesia, Volume VI: Pain Management

On Broadcast Disk Paging

Broadcast disks are an emerging paradigm for massive data dissemination. In a broadcast disk, data is divided into n equal-sized pages, and pages are broadcast in a round-robin fashion by a server. Broadcast disks are effective because many clients can simultaneously retrieve any transmitted data. Paging is used by the clients to improve performance, much as in virtual memory systems. However, paging on broadcast disks differs from virtual memory paging in at least two fundamental aspects: A page fault in the broadcast disk model has a variable cost that depends on the requested page as well as the current state of the broadcast. Prefetching is both natural and a provably essential mechanism for achieving significantly better competitive ratios in broadcast disk paging. In this paper, we design a deterministic algorithm that uses prefetching to achieve an O(n log k) competitive ratio for the broadcast disk paging problem, where k denotes the size of the client's cache. We also show a matching lower bound of $\Omega(n\log k)$ that applies even when the adversary is not allowed to use prefetching. In contrast, we show that when prefetching is not allowed, no deterministic online algorithm can achieve a competitive ratio better than $\Omega(nk)$. Moreover, we show a lower bound of $\Omega(n \log k)$ on the competitive ratio achievable by any nonprefetching randomized algorithm against an oblivious adversary. These lower bounds are trivially matched from above by known results about deterministic and randomized marking algorithms for paging. An interpretation of our results is that in the broadcast disk paging, prefetching is a perfect substitute for randomization.

The virtual museum

Jonathan Bowen is considered by many as the ‘founding father’ of the Virtual Library museums pages, one of the premier Internet sites in the museum field. He is a lecturer at the Department of Computer Science, University of Reading (United Kingdom), where he leads the Formal Methods and Software Engineering Group, and was previously a senior researcher at the Oxford University Computing Laboratory. He has worked in the field of computing in both industry and academia since 1977 and has served on more than fifteen programme committees including a major working group within the European Union information technologies programme, ESPRIT. The author of 140 publications including nine books, Jonathan Bowen won the 1994 IEE (Institution of Electrical Engineers) Charles Babbage Premium award. In 1997 he was honorary chair, workshop presenter and an invited speaker at the first ‘Museums and the Web’ conference and has been an active participant in subsequent conferences.

Only connect

Jonathan Bowen is considered by many as the ‘founding father’ of the Virtual Library museums pages, one of the premier Internet sites in the museum field. He is a lecturer at the Department of Computer Science, University of Reading (United Kingdom), where he leads the Formal Methods and Software Engineering Group, and was previously a senior researcher at the Oxford University Computing Laboratory. He has worked in the field of computing in both industry and academia since 1977 and has served on more than fifteen programme committees including a major working group within the European Union information technologies programme, ESPRIT. The author of 140 publications including nine books, Jonathan Bowen won the 1994 IEE (Institution of Electrical Engineers) Charles Babbage Premium award. In 1997 he was honorary chair, workshop presenter and an invited speaker at the first ‘Museums and the Web’ conference and has been an active participant in subsequent conferences.

Potentials and limitations of fault-based Markov prefetching for virtual memory pages

Potentials and limitations of fault-based Markov prefetching for virtual memory pages

U-cache: A cost-effective solution to the virtual cache synonym problem

This paper proposes a cost-effective solution to the virtual cache synonym problem. In the proposed solution, a minimal hardware addition guarantees correct handling of the synonym problem whereas a simple modification to the virtual-to-physical address mapping in the operating system optimizes the performance. The key to the proposed solution is a small, physically-indexed cache called a U-cache. The U-cache maintains the reverse translation information of cache blocks that belong to unaligned virtual pages, where unaligned means that the lower bits of the virtual page number that are used to index the virtual cache do not match those of the corresponding physical page number. The biggest advantage of the U-cache approach is that it leaves room for software optimization in the form of mapping alignment. Performance evaluation based on memory reference traces from a real system shows that the U-cache, with only a few entries, performs almost as well as (in some cases outperforms) a fully-configured hardware-based solution when more than 95% of mappings are aligned.

Continuous checkpointing: joining the checkpointing with virtual memory paging

SUMMARY Checkpointing is a basic mechanism for backward error-recovery in fault-tolerant systems. A checkpointed process stops execution and saves its states to files periodically. To reduce the file sizes, only data modified between two consecutive checkpoint times is saved. However, existing approaches do not consider operating system paging activities; which, if ignored may double the number of disk accesses required to checkpoint non-resident dirty pages. In this paper, we propose continuous checkpointing, which combines the checkpoint facility with virtual memory paging operations. Thus, checkpointing is continuous during the lifetime of a process without extra overhead. Checkpoint size is no longer proportional to application size, but rather is bounded by resident dirty pages. Experimental results show that disk accesses can be reduced by about 80% when checkpointing large applications. ©1997 by JohnWiley & Sons, Ltd.

Continuous checkpointing: joining the checkpointing with virtual memory paging

Checkpointing is a basic mechanism for backward error-recovery in fault-tolerant systems. A checkpointed process stops execution and saves its states to files periodically. To reduce the file sizes, only data modified between two consecutive checkpoint times is saved. However, existing approaches do not consider operating system paging activities; which, if ignored may double the number of disk accesses required to checkpoint non-resident dirty pages. In this paper, we propose continuous checkpointing, which combines the checkpoint facility with virtual memory paging operations. Thus, checkpointing is continuous during the lifetime of a process without extra overhead. Checkpoint size is no longer proportional to application size, but rather is bounded by resident dirty pages. Experimental results show that disk accesses can be reduced by about 80% when checkpointing large applications. © 1997 John Wiley & Sons, Ltd.

Stochastic Page Placement

Imperfect virtual-to-physical page mappings cause excess conflict misses in direct-mapped CPU caches. Perfect place ment minimizes the conflict miss rate and is useful for determining an upper bound on direct-mapped cache performance. In this paper, we introduce a polynomial-time alternative to perfect placement called stochastic page placement. Our method combines a general optimization technique with trace-driven simulation to find a local minimum of the perfect page placement solu tion space. Two stochastic placement policies are presented: GA placement uses a genetic algorithm to compute virtual page mappings, and SA placement uses simulated annealing. Parallel distributed simulation algorithms for perfect placement and GA placement are also presented. For our workloads, GA placement generally outperforms SA placement, but other careful placement policies that exploit workload properties usually outperform stochastic placement.

Tradeoffs between false sharing and aggregation in software distributed shared memory

Software Distributed Shared Memory (DSM) systems based on virtual memory techniques traditionally use the hardware page as the consistency unit. The large size of the hardware page is considered to be a performance bottleneck because of the implied false sharing overheads. Instead, we show that in the presence of a relaxed consistency model and a multiple writer protocol, a large consistency unit is generally not detrimental to performance. We study the tradeoffs between false sharing and aggregation effects when using large consistency units. In this context, this paper makes three separate contributions:1. We document the cost of false sharing in terms of extra messages and extra data being communicated. We find that, for the applications considered, when the virtual memory page is used as the consistency unit, the number of extra messages is small, while the amount of extra data can be substantial.2. We evaluate the performance when the consistency unit is increased to a multiple of the virtual memory page size. For most applications and data sets, the performance improves, except when the false sharing effects include extra messages or a large amount of extra data.3. We present a new algorithm for dynamically aggregating pages. In our algorithm, the aggregated pages do not necessarily need to be contiguous. In all cases, the performance of our dynamic aggregation algorithm is similar to that achieved with the best static page size.These results were obtained by measuring the performance of eight applications on the TreadMarks distributed shared memory system. The hardware platform used is a network of 166Mhz Pentiums connected by a switched 100Mbps Ethernet network.

Adaptive page replacement based on memory reference behavior

As disk performance continues to lag behind that of memory systems and processors, virtual memory management becomes increasingly important for overall system performance. In this paper we study the page reference behavior of a collection of memory-intensive applications, and propose a new virtual memory page replacement algorithm, SEQ. SEQ detects long sequences of page faults and applies most-recently-used replacement to those sequences. Simulations show that for a large class of applications, SEQ performs close to the optimal replacement algorithm, and significantly better than Least-Recently-Used (LRU). In addition, SEQ performs similarly to LRU for applications that do not exhibit sequential faulting.

Managing server load in global memory systems

New high-speed switched networks have reduced the latency of network page transfers significantly below that of local disk. This trend has led to the development of systems that use network-wide memory, or global memory, as a cache for virtual memory pages or file blocks. A crucial issue in the implementation of these global memory systems is the selection of the target nodes to receive replaced pages. Current systems use various forms of an approximate global LRU algorithm for making these selections. However, using age information alone can lead to suboptimal performance in two ways. First, workload characteristics can lead to uneven distributions of old pages across servers, causing increased contention delays. Second, the global memory traffic imposed on a node can degrade the performance of local jobs on that node.This paper studies the potential benefit and the potential harm of using load information, in addition to age information, in global memory replacement policies. Using an analytic queueing network model, we show the extent to which server load can degrade remote memory latency and how load balancing solves this problem. Load balancing requests can cause the system to deviate from the global LRU replacement policy, however. Using trace-driven simulation, we study the impact on application performance of deviating from the LRU replacement policy. We find that deviating from strict LRU, even significantly for some applications, does not affect application performance. Based upon these results, we conclude that global memory systems can gain substantial benefit from load balancing requests with little harm from suboptimal replacement decisions. Finally, we illustrate the use of the intuition gained from the model and simulation experiments by proposing a new family of algorithms that incorporate load considerations as well as age information in global memory replacement decisions.

Relational databases create online publications

Relational databases that are accessible and updatable via the Internet afford Web designers many advantages over traditional HTML documents. The automatic interactive database approach (AIDA) to Web publishing eliminates old information, creates automatic links, and reduces memory requirements by using virtual HTML pages. The pages are consistent, reliable, and up to date.

Out-of-core streamline visualization on large unstructured meshes

This paper presents an out-of-core approach for interactive streamline construction on large unstructured tetrahedral meshes containing millions of elements. The out-of-core algorithm uses an octree to partition and restructure the raw data into subsets stored into disk files for fast data retrieval. A memory management policy tailored to the streamline calculations is used such that, during the streamline construction, only a very small amount of data are brought into the main memory on demand. By carefully scheduling computation and data fetching, the overhead of reading data from the disk is significantly reduced and good memory performance results. This out-of-core algorithm makes possible interactive streamline visualization of large unstructured-grid data sets on a single mid-range workstation with relatively low main-memory capacity: 5-15 megabytes. We also demonstrate that this approach is much more efficient than relying on virtual memory and operating system's paging algorithms.

The World Wide Web and the Virtual Library museums pages

The World Wide Web (WWW) has been expanding exponentially since it was launched. It provides a unifying way to navigate and present information around the world in seconds using the Internet computer network. Many types of organization have been caught up in the revolution and are struggling to cope with the effects and keep pace with the development of the technology. One such community is that of museums. This paper presents the way museums are currently using the Web internationally, and how their use could develop in the future. Included is a description of the Virtual Library museums pages (VLmp), a leading directory of online museums, recently adopted by the International Council of Museums. Some of the ‘virtual’ visitor statistics are also presented and discussed.

Builder, an interactive molecular construction utility--structural modelling using virtual reality on the web

In this paper we discuss and show how virtual reality (VR) working over the World Wide Web (WWW) can be used by chemists and biochemists as an extremely powerful tool in analyzing data and disseminating information and for interactive learning. Examples are presented of how VR is being used to investigate and visualize otherwise complex data. The program Builder v1.2, which is an interactive WWW program that illustrates the integration of Web and VR technologies, is described. The VRML language is used to build these virtual realities and is an acronym for the Virtual Reality Modelling Language. VRML was developed originally by Gavin Bell, Anthony Parisi and Mark Pesce and is based upon Silicon Graphics, Inc. successful Open Inventor graphics language. It is used to describe scenes that are displayed within a VRML viewer. But VRML is not an ordinary graphics language because it was designed to work within the World Wide Web by including aspects of a hypertext language. Thus objects in a scene may be hyperlinks to other virtual worlds, WWW pages or MIME types, allowing the viewer to navigate the Internet in a virtual reality and it is this aspect that is explored here in relation to chemistry. Builder v1.2 is an on-line utility to create customized, 3-D models of membranes using a WWW forms interface and was one of the very first examples of interactive scene creation with VRML. The user may choose components to go in the membrane from a range of proteins, cholesterol or a glycoprotein and then specify their location in the membrane. The 3-D scene may be supplied at several levels of detail, including a full sphere representation suitable for high-end workstations, and a simple, minimum-rendering format ideal for PCs. The coordinates of the components may be downloaded in PDB format; the VRML scene has the components all hyperlinked to Web pages describing their function. In conclusion, VRML provides a medium through which data that is too difficult to portray in 2-D may be analyzed and published. This illustrates clearly the benefits of the Internet as a future scientific publishing medium and a repository for supplementary and enhanced material to traditional printed journal information.

Compiler-directed page coloring for multiprocessors

This paper presents a new technique, compiler-directed page coloring, that eliminates conflict misses in multiprocessor applications. It enables applications to make better use of the increased aggregate cache size available in a multiprocessor. This technique uses the compiler's knowledge of the access patterns of the parallelized applications to direct the operating system's virtual memory page mapping strategy. We demonstrate that this technique can lead to significant performance improvements over two commonly used page mapping strategies for machines with either direct-mapped or two-way set-associative caches. We also show that it is complementary to latency-hiding techniques such as prefetching.We implemented compiler-directed page coloring in the SUIF parallelizing compiler and on two commercial operating systems. We applied the technique to the SPEC95fp benchmark suite, a representative set of numeric programs. We used the SimOS machine simulator to analyze the applications and isolate their performance bottlenecks. We also validated these results on a real machine, an eight-processor 350MHz Digital AlphaServer. Compiler-directed page coloring leads to significant performance improvements for several applications. Overall, our technique improves the SPEC95fp rating for eight processors by 8% over Digital UNIX's page mapping policy and by 20% over a page coloring, a standard page mapping policy. The SUIF compiler achieves a SPEC95fp ratio of 57.4, the highest ratio to date.

Builder, an interactive molecular construction utility—structural modelling using virtual reality on the web

In this paper we discuss and show how virtual reality (VR) working over the World Wide Web (WWW) can be used by chemists and biochemists as an extremely powerful tool in analyzing data and disseminating information and for interactive learning. Examples are presented of how VR is being used to investigate and visualize otherwise complex data. The program Builder v1.2, which is an interactive WWW program that illustrates the integration of Web and VR technologies, is described. The VRML language is used to build these virtual realities and is an acronym for the Virtual Reality Modelling Language. VRML was developed originally by Gavin Bell, Anthony Parisi and Mark Pesce and is based upon Silicon Graphics, Inc. successful Open Inventor graphics language. It is used to describe scenes that are displayed within a VRML viewer. But VRML is not an ordinary graphics language because it was designed to work within the World Wide Web by including aspects of a hypertext language. Thus objects in a scene may be hyperlinks to other virtual worlds, WWW pages or MIME types, allowing the viewer to navigate the Internet in a virtual reality and it is this aspect that is explored here in relation to chemistry. Builder v1.2 is an on-line utility to create customized, 3-D models of membranes using a WWW forms interface and was one of the very first examples of interactive scene creation with VRML. The user may choose components to go in the membrane from a range of proteins, cholesterol or a glycoprotein and then specify their location in the membrane. The 3-D scene may be supplied at several levels of detail, including a full sphere representation suitable for high-end workstations, and a simple, minimum-rendering format ideal for PCs. The coordinates of the components may be downloaded in PDB format; the VRML scene has the components all hyperlinked to Web pages describing their function. In conclusion, VRML provides a medium through which data that is too difficult to portray in 2-D may be analyzed and published. This illustrates clearly the benefits of the Internet as a future scientific publishing medium and a repository for supplementary and enhanced material to traditional printed journal information.