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Next: TIP Up: Automatic I/O Hint Generation Previous: Introduction

Prefetching background

As mentioned in the introduction, applications can be manually modified to control I/O prefetching. For example, programmers can explicitly separate a request for data from the requirement that the data be available by issuing an asynchronous I/O call. However, there is a serious drawback to using asynchronous I/O. The size of the file cache, the latency and bandwidth of the I/O system, and the level of contention for the file cache and I/O system all affect the ideal scheduling of I/O requests. Issuing an asynchronous read call, however, causes the operating system to immediately issue a disk request for any uncached data specified by the call. Therefore, in redesigning an application to issue asynchronous I/O calls, a programmer implicitly makes assumptions about the characteristics of the systems on which the application will be executed.

Programmers can address this issue by using more sophisticated prefetching mechanisms, e.g. by modifying applications to issue hints for future read requests to a module that considers the dynamic I/O and caching behavior of the system before acting on the hint [Patterson94] (discussed further in Section 2.1). However, this does not avoid the higher-level problems with manual modification. First, manual modification requires that source code be available. Second, manual modification can involve formidable programming effort, both in understanding how the code currently generates read requests and in determining how the code should be modified so that the application will benefit from I/O prefetching. While some applications will only require the insertion of a few lines of code in a few strategic locations, other applications may require significant structural reorganization to support accurate and timely I/O prefetching [Patterson97]. Accordingly, we expect such modifications to be made only by a small fraction of programmers on a small fraction of programs. Therefore, automatic approaches are desirable.

The most widespread form of automatic I/O prefetching is the sequential read-ahead performed by most operating systems [Feiertag71, McKusick84] that exploits the preponderance of sequential whole-file reads [Ousterhout85, Baker91]. However, sequential read-ahead has limited utility when files are small. Furthermore, sequential read-ahead will not help, and may hurt, when access patterns are nonsequential.

In a more sophisticated history-based approach for automating I/O prefetching, the operating system gathers information about past file accesses and uses it to infer future file requests [Kotz91, Curewitz93, Griffioen94, Kroeger96, Lei97]. History-based prefetching is particularly well-suited for discovering and exploiting access patterns that span multiple applications. For example, it may implicitly recognize the edit-compile-run cycle and prefetch the appropriate compiler, object files, or libraries while a user is editing a source file. When applied to disk-bound applications such as those used in our experiments, however, history-based approaches are less appropriate. These approaches are inherently limited by the tradeoff between the amount of history information retained and the achievable resolution in prefetching decisions. High resolution prediction - the ability to anticipate irregular block accesses in long-running disk-bound applications, for example - could require prohibitively large traces of prior executions. By whatever measures a particular history-based prefetching system reduces the amount of information it retains - e.g. by tracking only certain types of events or only the most frequently occurring events - the system will also sacrifice its ability to predict the accesses of applications whose access patterns vary widely between runs and/or applications that heavily exercise the I/O system but recur infrequently.

For these types of applications, we need a different approach for automating I/O prefetching. We would like an approach that considers precisely the factors which determine a specific application's stream of read requests, without burdening the operating system by requiring it to maintain long-term application-specific information. One such approach is for a tool, generally a compiler, to statically analyze an application in order to determine how read requests will be generated, and then transform the application so that the appropriate I/O prefetching will occur [Mowry96, Trivedi79, Cormen94, Thakur94, Paleczny95]. Such static approaches have proven extremely effective at reducing execution times for loop-intensive, array-based applications. However, these approaches are limited by hard interprocedural static analysis problems, especially because I/O is often an "outer loop" activity separated from the core computation by many layers of abstraction (procedure calls and jump tables, for example).

Our approach is based on having applications perform speculative execution, which is essentially a form of dynamic self-analysis. As with static approaches, we are able to capture application-specific factors which are expensive for history-based prefetching systems to extract and retain. Unlike static approaches, however, we do not require detailed understanding of the control and data flow of the application. Instead, our approach requires only a few simple static analyses and transformations. In addition, by relying on dynamic analysis, our approach can easily take advantage of input data values as they becomes available during the course of execution.

next up previous
Next: TIP Up: Automatic I/O Hint Generation Previous: Introduction

Fay Chang
Tue Jan 5 18:05:04 EST 1999