I've got a Firefox process with nothing in it but the Google home page (the "search" page, not the "homey" home page with news etc.) and this site. I've killed it and restarted it multiple times. It's burning massive amounts of CPU cycles doing ... what?

I notice that Firefox, over time, leaks memory terribly and uses more and more CPU, but not anything close to this much. What is it doing? At present it's the primary process loading down the system, and that's new behavior. Eating memory and getting bloated is usually something that makes it sluggish. Now it's just hogging resources and spinning my fan.

I'm running 12.04 on an HP laptop but that shouldn't matter of course.

Are there some Firefox tools for looking at its internal thread activity or something?

edit newly-created profiles show exactly the same behavior ...

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    Firefox is an excellent browser that does not exhibit the symptoms you describe. Your problem is most likely with one or more add-ons. What add-ons do you have, and what version of Firefox? – Tom Brossman Jul 1 '12 at 6:05
  • Are you using the latest version of Firefox? (take a look into Help->About Firefox). I had similar behaviour with the previous version, which suddenly went havoc eating all my RAM + Swap within a few minutes until I killed it. This however did not repeat (yet) with 13.0.1. As new profiles (without addons I hope?) show the same problem, addons are unlikely to be the cause for you. – Izzy Jul 1 '12 at 11:13
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    Yes, I've found the issue and it was not a Firefox problem. It's a kernel issue caused by the 2012 leap second insertion, and I've found a fix. See this LKML thread. – Pointy Jul 1 '12 at 13:12
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    @TomBrossman no addons - it was a kernel problem that victimized the browser. See my answer. – Pointy Jul 1 '12 at 13:16

This LKML thread describes a kernel issue that appeared due to the 2012 leap second insertion (1 Jul 2012). I don't know exactly what they're talking about, but it seems that the clock update didn't do some internal bookkeeping, with the result being that some synchronization calls weren't resulting in the ordinary process blocking that they'd cause. Thus, some user-mode processes that rely on the kernel service were just spinning the CPU endlessly.

The fix, however, is extremely simple:

sudo date -s "`date`"

Immediately after doing that — and I did nothing else — the Firefox process calmed down. I could see the sharp "cliff" on my CPU monitor.

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