Tag Archives: xbl

master control pecobro, the overkill performance code browser

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The title isn’t true yet, but it’s disturbingly almost-true.  Pecobro’s hackish underpinnings have been “accidentally” replaced with surprisingly forward-looking code capable of supporting a much fancier feature set.  (I didn’t mean to, but I got tricked because the incremental cost to doing the ‘right thing’ was always so low.)

The trace that you can see here, by clicking on any of this text what has coloring (and using firefox 3 in a session that you don’t mind if it crashes), is of running Mark Banner (Standard8)‘s Thunderbird bloatTest on OS X with DTrace probes enabled, but without actually doing the bloat testing.  So Thunderbird starts up, opens the address book, closes it, opens the compose window, closes it, and then quits.

Here is a preliminary processed trace from a run triggering bug 296453.  Be forewarned that there is some missing information from the underlying trace and so it’s not all that it could be.  I think the XPConnect probes need to be fleshed out slightly more (and processed).

The code (both pecobro and mozilla codebase patches) is at: http://hg.mozilla.org/users/bugmail_asutherland.org/

What crazy overkill things can pecobro do now?

  • Parse all of the javascript code used in Thunderbird, including nearly all (maybe all?) of the 1.x enhancements.  The parser is still a bit hackish, especially when it comes to support of regexes, but it actually parses things.  Thanks to Chris Lambrou for posting his initial JavaScript.g antlr3 grammar (and BSD licensing it) that I used as a basis for my hacky version.  This was certainly a learning experience; I now know that javascript can never replace python in my heart…
  • Find all of the places functions can be defines in javascript code and do a pretty good job at figuring out reasonable names for them.  This even includes “(new)? Function()”.  This allows us to use the function-info DTrace probes to use line number information to identify functions.  We do this so that we can identify anonymous functions.
  • Handle XBL as well as we handle pure-javascript files.  (Parsing/AST fix-ups/syntax highlighting with AST traversal/etc.)
  • Parse/interpret disturbingly large portions of makefiles.  Sure, it ignores the rules, and I’ve only implemented the functions I require, but it does all the conditional and include stuff with variable expansion, including some key functions like foreach/filter/etc.
  • Parse jar manifests sufficiently well to figure out what file ends up at what chrome path.
  • Cache its javascript parse results at both the AST and (sorta-semantic) processing level, among other things.  Hooray for cerealizer which dares persist/de-persist that which pickle dare not.  (Nested classes.)  (Note: We don’t cache XBL stuff yet, and the ASTs are large enough that de-persisting them can be quite perceptible.)

What is not crazy, but still nice:

  • The traces are now much more reliable thanks to modifications to the Mozilla USDT probes to provide sufficient information for us to distinguish between JavaScript execution contexts (namely, JSContext).  Modifications were also made to avoid the ‘guessing’/ghost value problems that happen when native functions were called.
  • A foolish bug in the sparkbar calculation has been fixed.  aka “Now With Accuracy”!
  • If a function in a trace called another function or was called by another function, this will be displayed inline in the source display.
  • Infer ‘native’ functions (with the above additional modifications to the Mozilla USDT probes), assigning them to an object.  This ends up being Object, ChromeWindow, or other native Class type.  Some of this may be mooted by getting the XPConnect probes reliably in the mix.

What doesn’t it do?

  • Add the XPConnect js_Invoke probes; I punted on that because that was turning out to be difficult to get right.
  • It ignores .xul files for now.  Although xul files primarily appears in a caller context (as told by function-info probes), they can also be a callee when someone pulls a fast one and inlines some simple code in a script tag.  We brutally mis-attribute the call to the previous function when this happens.  This will eventually get resolved because we will need to understand .xul files for namespace reasons.  Also, people sound like they’re interested in who overlays what and the like, and that’s sorta right up our alley thanks to all the overkill stuff we do.
  • Exhaustively determine reads from/contributions to the ‘global’ (window or what not) namespace/scope.  The groundwork is there in terms of understanding the contribution of top-level statements to the namespace or its reads from it, but we don’t traverse into functions.
  • Associate functions with an object/type (ignoring the native function case).  This requires more semantic understanding.
  • Clicking on functions still doesn’t do anything.  I disabled that before my first post on pecobro due to firefox-crashing issues, and still haven’t gotten around to resolving it.
  • A usable overview visualization.  The overview diagram has become cluttered by the presence of so many ‘relevant’ (touched) files.

pecobro: the performance code browser (early stage)

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At the beginning of last week, I had gotten dtrace working on a Mac Mini using the Mozilla javascript-provider probes.  Very cool stuff, but it left me with several questions about what would really be best to do next:

  • How do I best understand what I’m seeing?  (Most of the codebase is brand new to me…)
  • How do I share the data with others in a way that is both comprehensible and allows them to draw their own conclusions from the data?
  • What can I do to reduce the effort required to work on performance problems?

I was tempted to just try and just dig into the system so I could have something to show immediately, but I knew it would still take a while to see the big picture just using an editor/ctags/lxr/opengrok, even informed by dtrace.  And even then, that big picture doesn’t scale well; whatever picture I managed to formulate would be stuck inside my head…

So my solution was to try and build a tool that could help me accomplish my short-term goals soon, and have the potential to grow into a usable solution to all of the above… eventually.  The goal, in a nutshell, is to provide a code browser for javascript that is able to integrate performance information (retrieved from traces) alongside the code.  Seeing that lxr/mxr and opengrok didn’t understand javascript or XBL all that well, it also seemed feasible to try and improve on their browsing capabilities for javascript.  A far-down-the-road goal is also to be able to pull in information from the underlying C++ code as well, potentially leveraging dehydra, etc.  (This would primarily be for understanding what happens when we leave the javascript layer, not trying to be the same solution for C++ space.)

So what can it do so far?  You can go try it for yourself if you like as long as you keep your expectations very low and realize the current state does not reflect all of the bullet points below.  Also, you probably want firefox 3.0.  Or you can read my bullet points:

  • Parse custom DTrace script output!  The Mozilla DTrace probe points could probably use a little love to improve what we are able to get out.  Also, I think it’s betraying us somewhere.
  • Parse JavaScript! Sorta!  (I hacked in support for the regular expression syntax, but I haven’t corrected the ambiguity with division, so things with division break.  Also, there’s at least one or two other glitches that cause early termination.) [Yay antlr!]
  • Parse XBL!  Even with entity inlining!  Even when people put #ifdefs in the XML document! Sorta!  We don’t actually do anything intelligent with the XBL right now or with its JavaScript, but it won’t take much to get that much improved. [Yay elementtree!]
  • Visualize some stuff!  Inter-file relationship graph in the overview.  In the code and ‘Funcs’ sidebar tab you get a sparkbar where each bar represents a time interval.  The height of the par is the percentage of possible time we could have spent in that time interval.  Red means we belive that time was spent in the function itself, green means we think we spent that time in calls to other functions. [Yay visophyte!]
  • Navigate with history!  Click on the overview graph and you go to things.  Click on the file names in the ‘Files’ list and you go to the files.  I tried to make it so you could click on function names in the side bars to go to them, but jquery.scrollTo and/or firefox 3.0b5 had serious crashing issues.  [Yay jquery, jquery.history!]
  • See syntax-highlighted code with random headings intertwined (shows the parser worked) and potentially a visualization.  [Yay pygments!]

My hope in the near-term is to fix the outright bugs (parsing issues), get XBL going, and then augment the function information with more trace-derived data including more traditional call-stacks, etc.  Then the tool should be sufficiently usable that my immediate focus can change to creating automated tests to actually gather performance/execution traces so we can use the tool for what I started it for.  This may also get shelved for a while if it turns out that we need action (patches) immediately.