The performance of Web servers plays a key role in satisfying the needs of a large and growing community of Web users. Portable high-performance Web servers reduce the hardware cost of meeting a given service demand and provide the flexibility to change hardware platforms and operating systems based on cost, availability, or performance considerations.
Web servers rely on caching of frequently-requested Web content in main memory to achieve throughput rates of thousands of requests per second, despite the long latency of disk operations. Since the data set size of Web workloads typically exceed the capacity of a server's main memory, a high-performance Web server must be structured such that it can overlap the serving of requests for cached content with concurrent disk operations that fetch requested content not currently cached in main memory.
Web servers take different approaches to achieving this concurrency. Servers using a single-process event-driven (SPED) architecture can provide excellent performance for cached workloads, where most requested content can be kept in main memory. The Zeus server [32] and the original Harvest/Squid proxy caches employ the SPED architecture1.
On workloads that exceed that capacity of the server cache, servers with multi-process (MP) or multi-threaded (MT) architectures usually perform best. Apache, a widely-used Web server, uses the MP architecture on UNIX operating systems and the MT architecture on the Microsoft Windows NT operating system.
This paper presents a new portable Web server architecture, called asymmetric multi-process event-driven (AMPED), and describes an implementation of this architecture, the Flash Web server. Flash nearly matches the performance of SPED servers on cached workloads while simultaneously matching or exceeding the performance of MP and MT servers on disk-intensive workloads. Moreover, Flash uses only standard APIs and is therefore easily portable.
Flash's AMPED architecture behaves like a single-process event-driven architecture when requested documents are cached and behaves similar to a multi-process or multi-threaded architecture when requests must be satisfied from disk. We qualitatively and quantitatively compare the AMPED architecture to the SPED, MP, and MT approaches in the context of a single server implementation. Finally, we experimentally compare the performance of Flash to that of Apache and Zeus on real workloads obtained from server logs, and on two operating systems.
The rest of this paper is structured as follows: Section 2 explains the basic processing steps required of all Web servers and provides the background for the following discussion. In Section 3, we discuss the asynchronous multi-process event-driven (AMPED), the single-process event-driven (SPED), the multi-process (MP), and the multi-threaded (MT) architectures. We then discuss the expected architecture-based performance characteristics in Section 4 before discussing the implementation of the Flash Web server in Section 5. Using real and synthetic workloads, we evaluate the performance of all four server architectures and the Apache and Zeus servers in Section 6.