Architecture/Proposal/Advanced Threading-Architecture

Type: Proposal State: draft

The advanced threading-architecture aims to solve OOos scalability and responsiveness problems.

Problem

A brief list of problems we face:

• Sometimes unresponsive user interface, e.g. trying to connect to a particular web server may take some minutes, without repaint and everything (it can take more time, if you have more than one http:// reference in your document) ;-).
• Polling / busy waiting, e.g. frequent re-schedule or yield calls, while loading or saving a document.
• Scalability with multiple clients doing API calls, only one API call can be in execution at any time.
• Most long lasting operations such as loading, saving, printing etc. are not interruptible.

Solution

OOo must be changed to be purely event / callback driven. Please see Wikipedia for what event-driven programming is.

The following list gives a first idea, of what should be done,

• all (potentially) blocking calls need to be event-driven,
• all long lasting calls need to be executed by dedicated threads, notifying the consumers via events / callbacks, in case data is available (thus basically creating event sources and event sinks),
• UNIX asynchronous signals, being an process interface by their own, need to be mapped to events,
• Windows window messages need to be mapped to events,
• the threading-architecture must be defined high level, e.g.
  • concurrency per application, or
  • concurrency per document.

Pros

• Not calling potential blocking system calls leads to 'short' lasting mutex acquisitions.
• No hand crafted reschedules necessary anymore.
• Easy utilization of hyper threading, multiple cores and SMP.
• Controllable CPU utilization and possible avoidance of over utilization.
• 'Simple' architecture.
• Potentially high level thread abstraction.

Cons

• (assumed to be) Hard to implement.

Pseudo Code for event loop:

void dispatch(int signal) {
  switch(signal) {
  case SIGIO:
    fileHandler(getHandle());
    break;
  case SIGTERM:
    ...
  }
}

int quit;
sigset_t sigset;

int main(void) {
  int signal;

  while(!quit) {
    sigwait(&sigset, &signal);
    dispatch(signal);
  }

  return 0;
}

Graphical overview: