Jan 11

Builtin Startup Measurement

I got used to measuring startup the complicated way (example here). It’s complicated enough that many people prefer to use stopwatches.

Turns out modern operating systems can help applications self-diagnose startup speed. Thanks to landing bug 522375 we now provide an API for measuring startup speed. For example, now I know that xpcshell takes forever to startup on mac

./xpcshell -e 'print(new Date() - Components.classes["@mozilla.org/toolkit/app-startup;1"].getService(Components.interfaces.nsIAppStartup_MOZILLA_2_0).getStartupInfo().process)'

At any point one can now go to the error console and type in



var si=Components.classes["@mozilla.org/toolkit/app-startup;1"].getService(Components.interfaces.nsIAppStartup_MOZILLA_2_0).getStartupInfo(); si.sessionRestored-si.process

to get various interesting timestamps. So next time you see a startup take a surprising amount of time, you can go and poke around to see where that time was spent. At the moment there are 4 datapoints:

  1. .process – Process creation timestamp. This is cool because this happens before any library code is executed.
  2. .main – XRE_main timestamp. My favourite thing to do is to subtract .process from .main. This demonstrates huge overheads that many application programmers refuse to believe in.
  3. .firstPaint – Timestamp of the first intended paint. This coincides with when the user sees the first sign of life.
  4. .sessionRestore – Timestamp of session restore, ie when the browser becomes useful.

Lots of people helped in getting this feature landed, but two people stand out. Daniel Brooks originally figured how to expose Windows/Linux startup speed in Mozilla. Mike Hommey devised a morally-reprehensible way to get precise startup speed out of the idiotic way that Linux presents it.

Linux Sucks

Mac and Windows expose process creation times via human time. It’s just like any other API with time in it. Linux provides process startup speed in imbecile jiffies-since-boot. That’d be irritating enough, but to piss people off further there is no way to convert that into human time. Clever ps developers resort to calculating jiffies/second by comparing against seconds-since-start in /proc/uptime. Unfortunately that does not even come close to providing anything close to millisecond resolution needed for useful numbers. Mike’s idea of timing startup of another thread/task to get a known jiffy-stamp got us precise-enough numbers (around 10ms resolution with most kernels, see patch for details). At least Linus was kind enough to let mere user-space devs obtain the current tick-rate.


Linux doesn’t suck anymore, a commenter pointed me to a relatively new(2006) taskstats interface which appears to work sanely like the BSD one.

Mike Hommey made an about:startup extension using this API.