Performance:Home Page

This page is meant to provide a starting point for developers who want to improve performance in Gecko and Firefox. The list below focuses on the performance projects that we think will have the biggest impact. Each task includes a brief description and links to additional resources. The tasks are grouped by performance area but are otherwise in no particular order.

Startup Performance

Reduce startup workload

We should be able to greatly reduce the amount of work needed to bring up the Firefox browser window. Today, it loads a bunch of extra stuff that really could be delayed until it is needed. Phoenix 0.1 started up very quickly because it was a simple XUL browser! ;-)

Possible areas for improvement:

  * Put components which aren't typically needed at startup into external dlls, to reduce the amount of code that has to be paged in
  * Fastload XBL bindings
  * ...

Move FF to XULRunner (investigate at least)

A trimmed down XUL browser application based on xulrunner loads much more quickly than FF 1.5, and uses significantly less memory (on the order of 14 vs 26 megabytes of memory). We need to understand why, and more importantly figure out how to trim the size of FF's memory footprint accordingly. This may mean moving FF to xulrunner, or it may mean applying something else that we learn from the investigation. At any rate, we know it should be possible to show the user's homepage using significantly less memory than we do now, which should help startup performance.

Preload FF at system start

Bring back Mozilla quick launch, but do it better ;-) Possibly build an extension that could be used with FF 1.5 to enable this functionality.

Javascript Component Loading

Even with fastloaded (precompiled) javascript components, we spend a fair amount of startup time deserializing large scripts. Things to do:

  * Refactor large components (nsExtensionManager.js, nsUpdateService.js) to load the minimal amount of code at startup
  * Eliminate locking in the JS engine when deserializing scripts, see bug http://bugzilla.mozilla.org/show_bug.cgi?id=312238
  * Use a large pool of memory for allocating strings during deserialization, see https://bugzilla.mozilla.org/show_bug.cgi?id=279839#c11

Function Ordering

All of the major platforms support having the linker reorder functions in the binary. We can exploit this to move the functions that are needed at startup into a contiguous section at the start of the binary, which should reduce the time spent reading from disk.

  * Profile the startup process to generate an optimal function ordering
  * Integrate the ordering into the build system

Improve Fastload system

The Fastload system is used to load precompiled javascript, preparsed XUL, and soon preparsed CSS at startup. The system is currently designed to be architecture-independent, which makes it impossible to write large chunks of memory at once (differences in endianness, data sizes, and struct packing). We could drastically reduce the time spent reading the fastload file by changing to an architecture-dependent, memory-mapped file that could be accessed directly.

Page load and DHTML Performance

Optimize XPConnect Locking

XPConnect does a lot of locking for threadsafety despite the fact that 99% of callers are on the main thread. We should optimize this for the common case. This should directly impact DHTML performance. For more information, see https://bugzilla.mozilla.org/show_bug.cgi?id=307953 .

Optimize for the main application thread

99% of the work done by Gecko happens on the main thread, and yet many of the core pieces of Gecko are written to be fully threadsafe. This includes core XPCOM components, XPConnect, and portions of Necko. There is also an overuse of threadsafe reference counting (using atomic increment/decrement). We can shift more of the performance burden on background threads by using proxy objects when accessing "main thread only" objects on background threads. We can do this today with some hardcoding of NS_GetProxyForObject, but it would be better if object proxification were automated. For example, getting a component from the component manager on a background thread should result in a proxy to the real object if the object only wishes to be invoked on the main thread. This will allow us to more easily streamline main thread object access while preserving support for those infrequent background thread use cases.

Request Prioritization

Necko has support for prioritizing requests to the same host relative to each other. We can make use of this to give on-screen content higher priority than off-screen content. Images, plugins, iframes, etc. could benefit from this.

Other areas for investigation

These ideas are less well-defined than the ones above, but may be good starting points:

  * Reuse the scrollbars and associated objects between pages.  Currently, these are recreated on each load.
  * Experiment with the timing of initial layout and paint.  We may be able to help at least perceived performance by trying to get a full screen of content displayed as fast as possible (but without "jumping").
  * Optimize the way that HTML reflow works.  See Gecko:Reflow_Refactoring.
  * Investigate better strategies for allocating memory for DOM and layout objects.
  * Other items listed at Gecko:Layout_Improvements.