Flash: An Efficient and Portable Web Server

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Flash An Efficient and Portable Web Server

• Based on paper by Pai et al.

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Web Server Processing Steps
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Multiple Process Architecture
Process 1

separate address spaces
Process N

• Advantages
• Simple programming while addressing blocking
  issue
• Disadvantages
• Many processes large context switch overheads
• Consumes much memory
• Optimizations involving sharing information among
  processes (e.g., caching) harder

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Multiple Thread Architecture

• Advantages
• Lower context switch overheads
• Shared address space simplifies optimizations
  (e.g., caches)
• Disadvantages
• Need kernel level threads (why?)
• Some extra memory needed to support multiple
  stacks
• Need thread-safe programs, synchronization

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Single Process Event Driven (SPED)
Accept Conn
Read Request
Find File
Send Header
Read File Send Data
Event Dispatcher

• Single threaded
• Asynchronous (non-blocking) I/O
• Advantages
• Single address space
• No synchronization
• Disadvantages
• In practice, disk reads still block

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Asynchronous Multi-Process Event Driven (AMPED)
Accept Conn
Read Request
Find File
Send Header
Read File Send Data
Event Dispatcher
Helper 1
Helper 1
Helper 1

• Like SPED, but use helper processes/thread for
  disk I/O
• Use IPC to communicate with helper process
• Advantages
• Shared address space for most web server
  functions
• Concurrency for disk I/O
• Disadvantages
• IPC between main thread and helper threads

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Comparisons
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Flash Implementation

• Single main thread
• Helper Processes
• Implemented as processes for portability
• Perform filename translation and reading disk
  blocks into memory
• Spawned dynamically as needed kept on reserve
  when not active

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Caches in Flash

• Pathname translation - map requested filenames
  (e.g., /bob) and actual names (e.g.,
  /home/users/bob/public_html/index.html)
• Response header - reuse response header info when
  same files requested repeatedly
• Mapped files - reuse memory mapping of frequently
  used files

Accept Client Connection
Read HTTP Request Header
Pathname Translation Cache
Helper
Find File
Response Header Cache
Send HTTP Response Header
Read File Send Data
Mapped File Cache
Helper
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Summary

• Several architectural alternatives
• Multi-process / Multi-threaded
• Avoids blocking on network and disk operations
• Single process event driven (SPED)
• Efficiency via non-blocking calls
• Fast but limited I/O concurrency
• AMPED
• Hybrid combining SPED and threads/processes for
  increased I/O concurrency
• Empirical measurements indicate good performance