Why Threads Are A Bad Idea
    (for most purposes)

          John Ousterhout
    Sun Microsystems Laboratories


    john...
Introduction
x   Threads:
     – Grew up in OS world (processes).
     – Evolved into user-level tool.
     – Proposed as ...
What Are Threads?
                                Shared state
                             (memory, files, etc.)

       ...
What Are Threads Used For?
x   Operating systems: one kernel thread for each user
    process.
x   Scientific applications...
What's Wrong With Threads?
casual                         all programmers                      wizards

                  ...
Why Threads Are Hard
x   Synchronization:
     – Must coordinate access to shared data with locks.
     – Forget a lock? C...
Why Threads Are Hard, cont'd
x   Hard to debug: data dependencies, timing dependencies.
x   Threads break abstraction: can...
Why Threads Are Hard, cont'd
x   Achieving good performance is hard:
     – Simple locking (e.g. monitors) yields low conc...
Event-Driven Programming
x   One execution stream: no CPU
    concurrency.
x   Register interest in events             Eve...
What Are Events Used For?

x   Mostly GUIs:
    – One handler for each event (press button, invoke menu
      entry, etc.)...
Problems With Events
x   Long-running handlers make application non-
    responsive.
     – Fork off subprocesses for long...
Events vs. Threads
x   Events avoid concurrency as much as possible, threads
    embrace:
     – Easy to get started with ...
Events vs. Threads, cont'd
x   Events faster than threads on single CPU:
     – No locking overheads.
     – No context sw...
Should You Abandon Threads?
x   No: important for high-end servers (e.g. databases).

x   But, avoid threads wherever poss...
Conclusions
x   Concurrency is fundamentally hard; avoid whenever
    possible.
x   Threads more powerful than events, but...
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Why threads are a bad idea

  1. 1. Why Threads Are A Bad Idea (for most purposes) John Ousterhout Sun Microsystems Laboratories john.ousterhout@eng.sun.com http://www.sunlabs.com/~ouster
  2. 2. Introduction x Threads: – Grew up in OS world (processes). – Evolved into user-level tool. – Proposed as solution for a variety of problems. – Every programmer should be a threads programmer? x Problem: threads are very hard to program. x Alternative: events. x Claims: – For most purposes proposed for threads, events are better. – Threads should be used only when true CPU concurrency is needed. Why Threads Are A Bad Idea September 28, 1995, slide 2
  3. 3. What Are Threads? Shared state (memory, files, etc.) Threads x General-purpose solution for managing concurrency. x Multiple independent execution streams. x Shared state. x Pre-emptive scheduling. x Synchronization (e.g. locks, conditions). Why Threads Are A Bad Idea September 28, 1995, slide 3
  4. 4. What Are Threads Used For? x Operating systems: one kernel thread for each user process. x Scientific applications: one thread per CPU (solve problems more quickly). x Distributed systems: process requests concurrently (overlap I/Os). x GUIs: – Threads correspond to user actions; can service display during long-running computations. – Multimedia, animations. Why Threads Are A Bad Idea September 28, 1995, slide 4
  5. 5. What's Wrong With Threads? casual all programmers wizards Visual Basic programmers C programmers C++ programmers Threads programmers x Too hard for most programmers to use. x Even for experts, development is painful. Why Threads Are A Bad Idea September 28, 1995, slide 5
  6. 6. Why Threads Are Hard x Synchronization: – Must coordinate access to shared data with locks. – Forget a lock? Corrupted data. x Deadlock: – Circular dependencies among locks. – Each process waits for some other process: system hangs. thread 1 lock A lock B thread 2 Why Threads Are A Bad Idea September 28, 1995, slide 6
  7. 7. Why Threads Are Hard, cont'd x Hard to debug: data dependencies, timing dependencies. x Threads break abstraction: can't design modules independently. x Callbacks don't work with locks. T1 T2 T1 deadlock! calls Module A Module A deadlock! Module B Module B callbacks sleep wakeup T2 Why Threads Are A Bad Idea September 28, 1995, slide 7
  8. 8. Why Threads Are Hard, cont'd x Achieving good performance is hard: – Simple locking (e.g. monitors) yields low concurrency. – Fine-grain locking increases complexity, reduces performance in normal case. – OSes limit performance (scheduling, context switches). x Threads not well supported: – Hard to port threaded code (PCs? Macs?). – Standard libraries not thread-safe. – Kernel calls, window systems not multi-threaded. – Few debugging tools (LockLint, debuggers?). x Often don't want concurrency anyway (e.g. window events). Why Threads Are A Bad Idea September 28, 1995, slide 8
  9. 9. Event-Driven Programming x One execution stream: no CPU concurrency. x Register interest in events Event (callbacks). Loop x Event loop waits for events, invokes handlers. x No preemption of event Event Handlers handlers. x Handlers generally short-lived. Why Threads Are A Bad Idea September 28, 1995, slide 9
  10. 10. What Are Events Used For? x Mostly GUIs: – One handler for each event (press button, invoke menu entry, etc.). – Handler implements behavior (undo, delete file, etc.). x Distributed systems: – One handler for each source of input (socket, etc.). – Handler processes incoming request, sends response. – Event-driven I/O for I/O overlap. Why Threads Are A Bad Idea September 28, 1995, slide 10
  11. 11. Problems With Events x Long-running handlers make application non- responsive. – Fork off subprocesses for long-running things (e.g. multimedia), use events to find out when done. – Break up handlers (e.g. event-driven I/O). – Periodically call event loop in handler (reentrancy adds complexity). x Can't maintain local state across events (handler must return). x No CPU concurrency (not suitable for scientific apps). x Event-driven I/O not always well supported (e.g. poor write buffering). Why Threads Are A Bad Idea September 28, 1995, slide 11
  12. 12. Events vs. Threads x Events avoid concurrency as much as possible, threads embrace: – Easy to get started with events: no concurrency, no preemption, no synchronization, no deadlock. – Use complicated techniques only for unusual cases. – With threads, even the simplest application faces the full complexity. x Debugging easier with events: – Timing dependencies only related to events, not to internal scheduling. – Problems easier to track down: slow response to button vs. corrupted memory. Why Threads Are A Bad Idea September 28, 1995, slide 12
  13. 13. Events vs. Threads, cont'd x Events faster than threads on single CPU: – No locking overheads. – No context switching. x Events more portable than threads. x Threads provide true concurrency: – Can have long-running stateful handlers without freezes. – Scalable performance on multiple CPUs. Why Threads Are A Bad Idea September 28, 1995, slide 13
  14. 14. Should You Abandon Threads? x No: important for high-end servers (e.g. databases). x But, avoid threads wherever possible: – Use events, not threads, for GUIs, distributed systems, low-end servers. – Only use threads where true CPU Event-Driven Handlers concurrency is needed. – Where threads needed, isolate usage in threaded application kernel: keep most of code single-threaded. Threaded Kernel Why Threads Are A Bad Idea September 28, 1995, slide 14
  15. 15. Conclusions x Concurrency is fundamentally hard; avoid whenever possible. x Threads more powerful than events, but power is rarely needed. x Threads much harder to program than events; for experts only. x Use events as primary development tool (both GUIs and distributed systems). x Use threads only for performance-critical kernels. Why Threads Are A Bad Idea September 28, 1995, slide 15
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