Flash: An Efficient and Portable Web Server

Introduction

This paper presents the design of a new Web server architecture called the asymmetric multi-process event-driven (AMPED) architecture, and evaluates the performance of an implementation of this architecture, the Flash Web server.

在这之前，主要存在3种不同的web server architecture

The single-process event-driven (SPED) architecture

The multi-process (MP) architecture

The multi-threaded (MT) architecture

Simplified Request Processing Steps

All of these steps involve operations that can potentially block.

read data or accept connections from a socket may block if the expected data has not yet arrived from the client.

write to a socket may block if the TCP send buffers are full due to limited network capacity.

test a file’s validity (using stat()) or open the file (using open()) can block until any necessary disk accesses complete.

reading a file (using read()) or accessing

data from a memory-mapped file region can block while data is read from disk.

3种不同的web server architecture存在的问题

The single-process event-driven (SPED) architecture

single process of execution.

using non-blocking system calls to perform I/O operations(An operation like select or poll).

non-blocking read and write operations work as expected on network sockets and pipes, but may actually block when used on disk files.

read,write,open and stat operations may still be blocking.



The multi-process (MP) architecture

each process handles one request.

disadvantages

each process has its separate address space.

cannot share data: separate cache.

context switch overhead（上下文切换所带来的开销）.



The multi-threaded (MT) architecture

each thread handles one request.

advantages

one address space: all threads share one cache.

less context switch overhead.

OS has to support kernel threads

Asymmetric Multi Process Event Driven

Combination of MP and SPED.

Use non-blocking calls to perform network and pipe operations.

Use helper process to perform blocking disk I/O operations,Helper process can be a separate thread or process.

Master and Helper process communicate through IPC

Master and Helper mmap() the same request file.

Helper process reads file from disk and brings into memory.

Helper notifies master that file is ready.

Avoid data transfer between processes.

Flash web server

Implementation of the AMPED architecture.

Uses aggressive caching

The helper processes are responsible for performing

pathname translations and for bringing disk blocks into memory.

Response header caching

Caching of already mapped files.

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