Firefox 57 delays requests to tracking domains

Firefox Quantum – version 57 – introduced number of changes to the network requests scheduler.  One of them is using data of the Tracking Protection database to delay load of scripts from tracking domains when possible during the time a page is actively loading and rendering – I call it tailing.

This has a positive effect on page load performance as we save some of the network bandwidth, I/O and CPU for loading and processing of images and scripts running on the site so the web page is complete and ready sooner.

Tracking scripts are not disabled, we only delay their load for few seconds when we can.  Requests are kept on hold only while there are site sub-resources still loading and only up to about 6 seconds.  The delay is engaged only for scripts added dynamically or as async.  Tracking images and XHRs are always delayed, as well as any request made by a tracking script.  This is legal according all HTML specifications and it’s assumed that well built sites will not be affected regarding functionality.

To make it more clear what we exactly do for site and tracking requests, this is how scheduling roughly looks like when tailing is engaged:

Firefox Quantum Tracker Tailing OK

And here with the tailing turned off:

Firefox Quantum Tracker Tailing OFF

This is of course not without problems.  For sites that are either not well built or their rendering is influenced by scripts from tracking domains there can be a visible or even functional regression.  Simply said, some sites need to be fixed to be able to adopt this change in scheduling.

One example is Google’s Page-Hiding Snippet, which may cause a web page to be blank for whole 4 seconds since the navigation start.  What happens?  Google’s A/B testing initially hides the whole web page with opacity: 0.  The test script first has to do its job to prepare the page for the test and only then it unhides the page content.  The test script is dynamically loaded by the analytics.js script.  Both the analytics.js and the test script are loaded from, a tracking domain, for which we engage the tailing delay.  As the result the page is blank until one of the following wins: 4 seconds timeout elapses or we load both the scripts and execute them.  For a common user this appears as a performance drawback and not a win.

Other example can be a web page referring an API of an async tracking script from a sync script, which obviously is a race condition, since there is no guarantee that an async script loads before a sync script.  There is a real life example of such not-well-built site using a Twitter API – window.twttr.  The twttr object is simply not there when the site’s script calls on it.  An exception is thrown and the rest of the site script is not executed breaking some of the page’s functionality.  That effected web page worked before tailing just because Twitter’s servers were fast to respond and executed sooner than the site script using the window.twttr object.  Hence, worked only by a lucky accident.  Note that sites with such race condition issues are 100% broken also when opened in Private Browsing windows or when Tracking Protection with just the default list is turned on.

To conclude on how useful the tailing feature is – unfortunately, at the moment I don’t have enough data to provide (it’s on its way, though.)  So far testing was made mostly locally and on our Web Page Test internal testing infrastructure.  The effect was unfortunately just hidden in the overall noise, hence more scientific and wide testing needs to be done.


EDIT: Interesting reactions on and Hacker News.


(Note: few somewhat off-topic comments have been trashed in case you wonder why they don’t appear here ; I will only accept comments bringing a benefit to discussion of this feature and its issues, thanks for understanding)

Mozilla Log Analyzer added basic network diagnostics

Mozilla Log Analyzer objects search results

Few weeks ago I’ve published Mozilla Log Analyzer (logan).  It is a very helpful tool itself when diagnosing our logs, but looking at the log lines doesn’t give answers about what’s wrong or right with network requests scheduling.  Lack of other tools, like Backtrack, makes informed decisions on many projects dealing with performance and prioritization hard or even impossible.  The same applies to verification of the changes.

Hence, I’ve added a simple network diagnostics to logan to get at least some notion of how we do with network request and response parallelization during a single page load.  It doesn’t track dependencies, by means of where from exactly a request originates, like which script has added the DOM node leading to a new request (hmm… maybe bug 1394369 will help?) or what all has to load to satisfy DOMContentLoaded or early first paint.  That’s not in powers of logan right now, sorry, and I don’t much plan investing time in it.  My time will be given to Backtrack.

But what logan can give us now is a breakdown of all requests being opened and active before and during a request you pick as your ‘hero request.’  May tell you what the concurrent bandwidth utilization was during the request in question, or what lower priority requests have been scheduled, been active or even done before the hero request.  What requests were blocking the socket where your request was finally dispatched on, and so on…

To obtain this diagnostic breakdown, use the current Nightly (at this time its Firefox 57) and capture logs from the parent AND also child processes with the following modules set:


(sync is optional, but you never know.)

Make sure you let the page you are analyzing to load, it’s OK to cancel too.  It’s best to close the browser then and only after that load all the produced logs (parent + children) to logan.  Find your ‘hero’ nsHttpChannel.  Expand it and then click its breadcrumb at the top of the search results.  There is a small [ diagnose ] button at the top.  Clicking it brings you to the breakdown page with number of sections listing the selected channel and also all concurrent channels according few – I found interesting – conditions.

This all is tracked on github and open to enhancements.

Automatically attaching child and test-spawned Firefox processes in Visual Studio IDE

Did you ever dream of debugging Firefox in Visual Studio with all its child processes attached automatically?  And also when being started externally from a test suit like mochitest or browsertest?  Tired of finding the right pid and time to attach manually?  Here is the solution for you!

Combination of the following two extensions to Visual Studio Community 2015 will do the trick:

  1. Spawned Process Catcher X – attaches automatically to all child processes the debugee (and its children) spawns
  2. Entrian Attach – attaches the IDE automatically to an instance of a process spawned FROM ANYWHERE, e.g. when running tests via mach where Firefox is started by a python script – yes, magic happens ;)

Spawned Process Catcher X works automatically after installation without a need for any configuration.

Entrian Attach is easy to configure: In the IDE, in the main menu go to TOOLS/Entrian Attach: Configuration…, you’ll get the following window:

UPDATE: It’s important to enter the full path for the executable.  The Windows API for capturing process spawning is stupid – it only takes name of an executable, not a full path or wildchars.  Hence you can only specify names of executable files you want Entrian Attach to automatically attach to.  Obviously, when Visual Studio is running with Entrian Attach enabled and you start your regular browser, it will attach too.  I’ve added a toolbar button EntrianAttachEnableDisable to the standard toolbar for a quick switch and status visibility.

Other important option is to set Attach at process start when to “I’m not already debugging its exe”.  Otherwise, when firefox.exe is started externally, a shim process is inserted between the parent and a child process which breaks our security and other checks for expected pid == actual pid.  You would just end up with a MOZ_CRASH.

Note that the extension configuration and the on/off switch are per-solution.

Entrian Attach developer is very responsive.  We’ve already cooked the “When I’m not already debugging its exe” option to allow child process attaching without the inserted shim process, took just few days to release a fixed version.

Entrian Attach is a shareware with 10-day trial.  Then a single developer license is for $29.  There are volume discounts available.  Hence, since this is so super-useful, Mozilla could consider buying a multi-license.  Anyway, I believe it’s money very well spent!

Intel Rapid Storage Technology disappears on Windows 10 and more

This started with the INACCESSIBLE_BOOT_DEVICE doom I wrote about before.  But here I want to treat the story after that.

When I solved the above mentioned problem, I ended up with 14.6 version of the driver in the system.  Everything worked well.

Until few months ago, the Intel Rapid Storage Technology UI completely disappeared from my system.  Only an empty folder under Program Files left.  Like somebody would steal it… Despite that, it still was listed as an installed software in Control Panels.

Since the UI part is important, I decided to gain it back.

But before running any SetupRST installers I rather inspected and prepared the system.  And what I found to my surprise!  The Intel’s driver for the RAID storage was missing.  I was on the Microsoft’s one (EhStorClass.sys).  That was very interesting thing I didn’t expect.

The preparation part #1 – create a restore point!

And preparation part #2 – check that restore point is accessible from the Recovery mode.  And here it starts :)  Pressing F8 during Windows 10 boot no longer works.  Hence, on this very same machine, I created a restore USB drive.  Booted from it.  Trying to access the list of restore points – “first select a system.”  What?  Aha!  The system drive didn’t mount in the Recovery mode!  This means that there is something wrong with this affected system.  But I don’t have intentions finding out what.

Fortunately, having a laptop with Windows 10 helped here.  That laptop didn’t have iRST installed, ever.  Creating a restore drive on a different machine and booting back on the ill machine makes the list of restore points visible.  Now I can proceed.

I decided to install only the iRST UI.  As reported in the original blog post comments, version is considered safe to install regarding the INACCESSIBLE_BOOT_DEVICE error.  Hence, I downloaded the setup from Intel.

Running it started to complain that iRST was already installed.  I exited the setup and uninstalled the iRST using Control Panels.  Then restarted with fingers crossed.  And the system… booted!  And the Microsoft’s driver didn’t move a bit.

Next step.  Running the setup again, now it complains that there is a newer Intel Rapid Storage driver, version 14.6.x.x.  If it’s there and it used to work, then why not to keep it?

Hence, this time I run SetupRST with -Nodrv command line argument to don’t install the driver.  According to my understanding, it should only install the missing UI.  Setup installs, asks for restart.  I do it, and the system… still boots!

The Intel Rapid Storage Technology UI is there after boot, as used to be!  There is also still the Microsoft driver.  The iRST UI works with it as expected and is also able to check the volume for errors.  Nice.

But somehow I’d rather have iRST UI, the service and the driver from one company.  One never knows if when changing the volume parameters like changing the drives, enlarging etc would break in any bad combination.

Hence, I run SetupRST again, no arguments.  It asks for either uninstall or repair.  I choose repair.  It does its job, asks for reboot.  I do it, and the system… yes, still boots :)

The final result: I have both the iRST driver and the iRST UI back.
User interface version:
Driver version:

This seems to work well.  I can again see status of all volumes and disks, and hopefully also manage volumes safely as before.

I hope this may help anyone experiencing iRST UI being mysteriously ripped off the Windows 10 system.

Moz logging (former NSPR logging) file now has a size limit option

There are lot of cases of mainly networking issues rare enough to reproduce making users have their Firefox run for long hours to hit the problem.  Logging is great in bringing the information to us when finally reproduced, but after few hours the log file can be – well – huge.  Easily even gigabytes.

But now we have a size limit, all you need to do:

Adding rotate module to the list of modules will engage log file size limit:

MOZ_LOG=rotate:200,log modules...

The argument is the limit in megabytes.

This will produce up to 4 files with names appended a numbering extension, .0, .1, .2, .3.  The logging back end cycles the files it writes to while sum of these files’ sizes will never go over the specified limit.

The patch just landed on mozilla-central (version 51), bug 1244306.

Note 1: the file with the largest number is not guarantied to be the last file written.  We don’t move the files, we only cycle.  Using the rotate module automatically adds timestamps to the log, so it’s always easy to recognize which file keeps the most recent data.

Note 2: rotate doesn’t support append.  When you specify rotate, on every start all the files (including any previous non-rotated log file) are deleted to avoid any mixture of information.  The append module specified is then ignored.