In this paper, we examine the performance tradeoffs that are raised by caching data in the client workstations of a client-server DBMS. We begin by presenting a range of lock-based cache consistency algorithms that arise by viewing cache consistency as a v~iant of the well-understood problem of replicated data management. We then use a detailed simulation model to study the performance of these algorithm over a wide range of workloads end system resource configurations. The results illustrate the key performance tradeoffs related to clientserver cache consistency, and should be of use to designers of next-generation DBMS prototypes and products.

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Data caching tradeoffs in client-server DBMS architectures

Client-server caching revisited

One of the most important resources that an operating system manages is main memory Systems often provide virtual memory, using this main memory to cache the contents of a larger virtual storage Unfortunately, existing systems hide the algorithms for controlling the contents of the main memory cache and for managing the external storage used to represent that virtual memory within the operating system kernel, making them unavailable to important systems applications
My thesis is that it is desirable and practical to provide an external memory management interface that allows user-level programs to define and manage the contents of memory objects that may be mapped into virtual address spaces Exporting this interface simplifies the construction of complex virtual memory applications, allowing them to manage the sharing, consistency, and permanent storage of their data without being embedded in the operating system kernel These systems applications can also take advantage of the performance benefits of using main memory as a cache of the contents of their data objects
This dissertation describes the Mach external memory management interface and implementation in detail The interface design is based on communication: memory objects are communication channels, on which the operating system sends messages to read or write the contents of pages of virtual memory The implementation addresses the modularity and security issues that arise from exporting this interface to user-level programs Several systems applications that have been constructed using the Mach interface are described, including a user-level file system, a transaction-based data management system, and a network shared virtual memory service An evaluation of the Mach system analyzes the cost of using this interface, and shows how it makes a fast path to the main memory cache available to systems applications Experiments also show that the overall performance of the Mach system does not suffer appreciably under normal load, and that this system still outperforms commercially available operating systems under identical benchmarks I conclude that an external memory management interface is a useful, affordable component that should be provided by future operating systems

Exporting a user interface to memory management from a communication-oriented operating system

There is an ever-increasing demand for more complex transactions and higher throughputs in transaction processing systems leading to higher degrees of transaction concurrency and, hence, higher data contention. The conventional two-phase locking (2PL) Concurrency Control (CC) method may, therefore, restrict system throughput to levels inconsistent with the available processing capacity. This is especially a concern in shared-nothing or data-partitioned systems due to the extra latencies for internode communication and a reliable commit protocol. The optimistic CC (OCC) is a possible solution, but currently proposed methods have the disadvantage of repeated transaction restarts. We present a distributed OCC method followed by locking, such that locking is an integral part of distributed validation and two-phase commit. This method ensures at most one re-execution, if the validation for the optimistic phase fails. Deadlocks, which are possible with 2PL, are prevented by preclaiming locks for the second execution phase. This is done in the same order at all nodes. We outline implementation details and compare the performance of the new OCC method with distributed 2PL through a detailed simulation that incorporates queueing effects at the devices of the computer systems, buffer management, concurrency control, and commit processing. It is shown that for higher data contention levels, the hybrid OCC method allows a much higher maximum transaction throughput than distributed 2PL in systems with high processing capacities. In addition to the comparison of CC methods, the simulation study is used to study the effect of varying the number of computer systems with a fixed total processing capacity and the effect of locality of access in each case. We also describe several interesting variants of the proposed OCC method, including methods for handling access variance, i.e., when rerunning a transaction results in accesses to a different set of objects.

/pdf/distributed-optimistic-concurrency-control-methods-for-high-qg0mibin3s.pdf

Distributed optimistic concurrency control methods for high performance transaction processing

The present invention discloses a modified computer architecture (50, 71, 72) which enables an applications program (50) to be run simultaneously on a plurality of computers (M1, ... Mn). Shared memory at each computer is updated with amendments and/or overwrites so that all memory read requests are satisfied locally. During initial program loading (75), or similar, instructions which result in the application program (50) acquiring (or releasing) a lock on a particular asset (50A, 50X-50Y) (synchronization) are identified. Additional instructions are inserted (162, 163) to result in a modified synchronization routine with which all computers are updated.

Multiple computer architecture with synchronization

A distributed memory hierarchy (DMH) is a memory system consisting of storage modules distributed over a high-bandwidth local area network. It provides for transaction applications an abstraction of single virtual memory space to which shared data are mapped. As in a conventional memory hierarchy (MH) in a single-machine system, a DMH is responsible for locating, migrating, and caching data pages; however, unlike a conventional MH, a DMH must do so across the storage modules in a network. In addition, a DMH must handle the problem of propagation of transaction updates preserving serializability of transactions. The performance of a DMH system is strongly influenced by concurrency control and update propagation. It is also crucial that performance analysis accounts for memory resources and network requirements. A DMH system is presented, the tradeoffs between conservative and aggressive update propagation strategies are defined, and promising new strategies are identified. >

V.-O. Tam

Papers

Update propagation in distributed memory hierarchy