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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