Quantum Flow Engineering Newsletter #6

Hi everyone,

I would like to share some updates about some of the ongoing performance
related work.

We have started looking at the native stack traces that are submitted
through telemetry from the Background Hang Reports that take more than 8
seconds.  (We were hoping to have been able to reduce this threshold to
256ms <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1346415> for a while
now, but the road has been bumpy -- but this should land really soon now!)
Michael Layzell put together a telemetry analysis job that creates a
symbolicated version of this data here:
https://people-mozilla.org/~mlayzell/bhr/.  For example, this
<https://people-mozilla.org/%7Emlayzell/bhr/20170405.html> is the latest
generated report.  The grouping of this data is unfortunate, since the data
is collected based on the profiler pseudo-stack labels, which is captured
after 128ms, and then native stack (if the hang continues for 8 seconds)
gets captured after that, so the pseudo-stack and the native stack may or
may not correspond, and this grouping also doesn't help going through the
list of native stacks and triage them more effectively.  Work is under way
to create a nice dashboard
<https://bugzilla.mozilla.org/show_bug.cgi?id=3D1344003> out of this data,
but in the mean time this is an area where we could really use all of the
help that we can get.  If you have some time, it would be really nice if
you can take a look at this data and see if you can make sense of some of
these call stacks and find some useful bug reports out of them.  If you do
end up filing bugs, these are super important bugs to work on, so please
make sure you add "[qf]" to the status whiteboard so that we can track the

Another item worthy of highlight is Mike Conley's Oh No! Reflow! add-on
<https://mikeconley.github.io/ohnoreflow/>.  Don't let the simple web page
behind this link deceive you, this add-on is really awesome!  It generates
a beep every time that a long running reflow happens in the browser UI
(which, of course, you get to turn off when you don't need to hunt for
bugs!), and it logs the sync reflows that happened alongside the JS call
stack to the code that triggered them, and it also gives you a single link
that allows you to quickly file a bug with all of the right info in it,
pre-filled!  In fact you can see the list of already filed bugs
through this add-on!

Another issue that I want to bring up is the [qf:p1] bugs.  As you have
noticed, there are a lot of them.  :-)  It is possible that some of these
bugs aren't important to work on, for example because they only affect edge
case conditions that affects a super small subset of users and that wasn't
obvious when the bug was triaged.  In some other cases it may turn out that
fixing the bug requires massive amounts of work that is unreasonable to do
in the amount of time we have, or that the right people for it are doing
more important work and can't be interrupted, and so on.  Whatever the
issue is, whether the bug was mis-triaged, or can't be fixed, please make
sure to raise it on the bug!  In general the earlier these issues are
uncovered the better it is, because everyone can focus their time on more
important work.  I wanted to make sure that this wasn't lost in all of the
rush around our communication for Quantum Flow, my apologies if this hasn't
been clear before.

On to the acknowledgement section, I hope I'm not forgetting to mention
anyone's name here!

   - Bas Schouten made it so that we don't clear the compositor background
   immediately before drawing into it
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1352442>.  This made som=
   painting and scrolling related benchmarks faster
   <https://treeherder.mozilla.org/perf.html#/alerts?id=3D5801>, and
fixed a flickering
   issue <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1330814> in the me=
   - Mason Chang made the Youtube settings widget less janky
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1351733> on Windows with
   D2D when the video is fullscreen.
   - David Baron made the flushes caused by the code that watches your
   mouse to know which side of the window to put the status bar when you ho=
   a link less severe <https://bugzilla.mozilla.org/show_bug.cgi?id=3D13071=
   - Neil Deakin removed a synchronous layout flush
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1334635> that used to
   happen when closing a window which would slow down the window going away=
   - D=C3=A3o Gottwald removed some obsolete tab animation telemetry
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1345315> which could slo=
   down tab animations (yes!).  D=C3=A3o also removed a synchronous layout =
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1356655> which could slo=
   down detaching a tab into a new window and he also lazified some
code to avoid
   some more layout flushes
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1356663> which were
   related to the status panel.
   - Markus Stange made the Gecko Profiler not kill your battery after you
   stop profiling <https://bugzilla.mozilla.org/show_bug.cgi?id=3D830990>,
   and also made it so that once you start profiling a thread, you can
   actually stop profiling it
   - Michael Layzell excluded some common file JS file extensions
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1352684> from the
   expensive computation that we run on Windows to determine the mimetype o=
f a
   file from its extension.
   - Kris Maglione made it possible to load add-on SDK modules lazily
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1314861>.  He also
made loading
   ExtensionContent.jsm lazy
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1317697>.  These two
   changes together (but probably mostly the former) showed great
   improvements on Talos
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1314861#c52>.  This is
   even true for users without any add-ons using the add-on SDK since these
   modules are also used in our internal code.
   - Robert Helmer created a Go Faster add-on
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1356676> to quickly depl=
   the fix of bug 1353216
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1353216> to Firefox 52
   users even before they upgrade to Firefox 53 which includes the fix!
   - Edouard Oger made Firefox Sync UI code cache the DateTimeFormat object=
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1350613> where possible.
   - Alastor Wu made some Fennec media controls code that could be
   expensive not run on Firefox desktop
   - Makoto Kato added a cache for document encoder objects used by
   TextEditor <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1352882>, in
   order to improve the performance of setting the value of
   - Tom Schuster optimized Object.hasOwnProperty()
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1344469>.  This is a
   commonly used JavaScript built-in on real web pages, really nice
   optimization to have!
   - Florian Qu=C3=A8ze removed a synchronous layout flush
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1353563> that used to
   happen when displaying each item in the awesome bar.  He improved the
   awesomebar even more by caching the one-off search buttons instead of
   regenerating them every time we open the popup
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1312999>.  Furthermore, =
   ensured that we don't pay the cost of initializing UITour.jsm for pages
   that do not display a UI tour
   - Wei-Cheng Pan added telemetry probes for navigation timings and time
   to first byte measures
   - Shu-yu Guo improved the timing of script source compression by
   delaying it until GC
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1348134>.  Previously we
   used to do this when parsing JavaScript (on a separate thread) which is
   wasteful because it would mean that we'd have to decompress the compress=
   source during page load sometimes, and also burning needless CPU cycles
   during JS parsing isn't wise.
   - J. Ryan Stinnett made the Developer Toolbar not create a network
   listener for every single tab that you have open
   <https://bugzilla.mozilla.org/show_bug.cgi?id=3D1346854>.  Besides perf
   wins, this means fewer OOM crashes too!

Thanks for reading!  Until next time, happy hacking!

4/21/2017 6:25:52 AM
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