Startup performance of Addon SDK-based extensions

The addons.mozilla.org roadmap for 2011 has improving the add-ons performance as one of this year's goals, so after finishing with my XUL extensions project for the weekend, I did some quick measurements to see what effect add-ons built with the SDK have on the Firefox startup time.

I posted this message to the Add-on SDK discussion group, but reproducing it here anyway.

According to my tests (Mac OS X (10.5) with the latest Minefield and the latest Add-on SDK using getStartupInfo()):

• an empty SDK-based extension (empty main.js) adds ~20 ms to the 700 ms baseline warm startup time (3%), which is tolerable, but
• the default extension created with 'cfx init' (adds a simple widget and loads the tabs module) adds more than 200 ms (28%) to the warm startup!
  

I ran some poor man profiling to check a hypothesis that most of this cost is in the module system (require()). It is very rough, but it showed how most (75%) of the time was spent:

• 25% (50ms) running es5.js in each of the 38 distinct modules loaded
• 15% (30ms) is other initialization operations with the sandbox (creating sandboxes, setting properties)
• 10% (20ms) is spent in fs.getFile(), presumably loading the modules from disk
  
• 25% (50ms) spent evaluating the module code itself, not counting the require() costs listed above.

Here are the details of my instrumentation and the results: legend, data:

Counter	Hit count	Time spent, ms	Attributed to the securable-module infrastructure / to the user-level code?
start loading	38	37	user
fs.getFile()	38	19	s-m
up to createSandbox	38	0	s-m
createSandbox	38	17	s-m
setupSandbox, part 1	38	1	s-m
setupSandbox, part 2 - modifyModuleSandbox	38	54	s-m
setupSandbox, part 3	38	15	s-m
before evaluate	38	0	s-m
after evaluate	38	52	user
finished	38	0	s-m

The list of all 38 modules loaded by a simple extension using widget and tabs (indenting shows require() dependencies):

• plain-text-console
• memory
• unload
• main
• widget
• xul-app
• api-utils
• panel
• content
• content/loader
• events
• traits
• traits/core
• url
• file
• byte-streams
• text-streams
• xpcom
• content/symbiont
• ./worker
• cuddlefish
• securable-module
• timer
• hidden-frame
• errors
• observer-service
• window-utils
• tabs
• windows
• list
• windows/tabs
• tabs/tab
• utils/function
• tabs/events
• utils/thumbnail
• utils/data
• windows/dom
• windows/loader