For many users, the perceived speed of computing is increasingly dependent on the performance of networked server systems, underscoring the need for high performance servers. Unfortunately, general purpose operating systems provide inadequate support for server applications, leading to poor server performance and increased hardware cost of server systems.
One source of the problem is lack of integration among the various input-output (I/O) subsystems and the application in general-purpose operating systems. Each I/O subsystem uses its own buffering or caching mechanism, and applications generally maintain their own private I/O buffers. This approach leads to repeated data copying, multiple buffering of I/O data, and other performance-degrading anomalies.
Repeated data copying causes high CPU overhead and limits the throughput of a server. Multiple buffering of data wastes memory, reducing the space available for the document cache. This size reduction causes higher cache miss rates, increasing disk accesses and reducing throughput. Finally, lack of support for application-specific cache replacement policies [9] and optimizations like TCP checksum caching [15] further reduce server performance.
We present the design, the implementation, and the performance of IO-Lite, a unified I/O buffering and caching system for general-purpose operating systems. IO-Lite unifies all buffering and caching in the system to the extent permitted by the hardware. In particular, it allows applications, interprocess communication, the file cache, the network subsystem, and other I/O subsystems to share a single physical copy of the data safely and concurrently. IO-Lite achieves this goal by storing buffered I/O data in immutable buffers, whose locations in physical memory never change. The various subsystems use mutable buffer aggregates to access the data according to their needs.
The primary goal of IO-Lite is to improve the performance of server applications such as those running on networked (e.g., Web) servers, and other I/O-intensive applications. IO-Lite avoids redundant data copying (decreasing I/O overhead), avoids multiple buffering (increasing effective file cache size), and permits performance optimizations across subsystems (e.g., application-specific file cache replacement and cached Internet checksums).
A prototype of IO-Lite was implemented in FreeBSD. In keeping with the goal of improving performance of networked servers, our central performance results involve a Web server, in addition to other benchmark applications. Results show that IO-Lite yields a performance advantage of 40 to 80% on real workloads. IO-Lite also allows efficient support for dynamic content using third-party CGI programs without loss of fault isolation and protection.