[webkit-dev] High web process CPU usage

Benjamin Poulain benjamin at webkit.org
Wed Jul 20 20:26:53 PDT 2016

I tried locally with ToT but I cannot reproduce.
I load huffingtonpost.com in a tab->it takes an (in)decent amount of 
CPU. Then I switch to an about:blank tab->the CPU time goes back to zero.

The Idle Wake Ups you are getting are pretty crazy too.

Can you please file a bug for this?
Next time you see the issue, Sample the faulty process. We may find 
something interesting in there.


On 7/20/16 7:30 PM, Chris Aljoudi wrote:
> Hi all,
> In the Safari shipped with the latest macOS Sierra beta, as well as
> Safari Tech Preview 9, there's an odd CPU usage issue with individual
> tab's web processes.
> For example, if one navigates to huffingtonpost.com
> <http://huffingtonpost.com>, Activity Monitor will show the tab web
> process continually using 5-10% of CPU, along with hundreds of idle
> wake-ups. Even more concerning, this continues to happen with
> the huffingtonpost.com <http://huffingtonpost.com> web process even if
> one picks another tab in the window.
> Here's a screenshot from Activity Monitor; one Safari window is open. It
> has two tabs: polymail.io <http://polymail.io>,
> and about:blank. about:blank is the one currently in focus.
> To investigate, I opened Web Inspector and recorded the JS/event
> timeline. I found that there were timers firing quite frequently.
> Now, I know Safari/WebKit /should/ be throttling timers of background
> tabs *anyway*, but I figured I'd `clearInterval` all the timers to see
> if that reduces the CPU usage. Nope (even though I verified no timers
> are firing any longer after clearing all the intervals — no activity on
> the timeline whatsoever; CPU usage and idle wake ups still way too high).
> This doesn't happen with *every* page, but it happens with most that are
> reasonably complex (can't tell what the trigger is yet; perhaps the use
> of JavaScript timers?)
> The previous Sierra beta's Safari and Tech Preview 8 both showed this
> problem. I don't know whether it goes further back.
> Any ideas what's going on? Is this 'expected behavior' and I'm missing
> something obvious?
> Thanks,
> Chris
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