When playing any 60fps videos on YouTube, we can see a fair amount of frames being dropped. It's even more obvious with high resolution videos. Turning off e10s fixes the issue. This is unlikely to be decoder related, as the decoder code path is common between the two. Note that disabling the Gpu process (or the Gpu remote decoder) doesn't fix the problem. Only disabling e10s completely does. This is a recent regression, I didn't experience those issues a couple of month back. Disabling advanced layers doesn't make a difference so it's not that either.

The frames are dropped in the compositor, as we don't see the number of frames dropped changing in the VideoPlaybackQuality attribute. (bug 1245400)

For the bug related to e10s, like[1], it could be the same cause, since both Jya and I can see this problem on our 4k panel. [1]https://bugzilla.mozilla.org/show_bug.cgi?id=1383292#c18

Per comment 1, it looks like a gfx bug. Add the keyword in the whiteboard.

(In reply to Jean-Yves Avenard [:jya] from comment #0) > This is a recent regression, I didn't experience those issues a couple of > month back. > Disabling advanced layers doesn't make a difference so it's not that either. Have you tried bisecting?

Here is a stream where it's easy to test: go to about:config set media.mediasource.webm.enabled to true Open: https://www.youtube.com/watch?v=R3AKlscrjmQ Switch to full screen, and select the 2160p/60fps stream. Result on my 2013 mac Pro (8 cores, 32GB of RAM): e10s you see about 1/3rd of the frames being dropped with e10s off, it's smooth as butter Note that we do not report frames dropped by the compositor, so YouTube "stats for nerds" right click on the player window and select that, will show 0 frames being dropped. But you can visually see them being dropped.

(In reply to Marco Castelluccio [:marco] from comment #4) > (In reply to Jean-Yves Avenard [:jya] from comment #0) > > This is a recent regression, I didn't experience those issues a couple of > > month back. > > Disabling advanced layers doesn't make a difference so it's not that either. > > Have you tried bisecting? I'll tried... bisecting with e10s on/off is a pain however.. the pref name changing over time, the method to disabling it changing etc.

(In reply to Jean-Yves Avenard [:jya] from comment #5) > Here is a stream where it's easy to test: > > go to about:config > set > media.mediasource.webm.enabled to true > > Open: > https://www.youtube.com/watch?v=R3AKlscrjmQ > > Switch to full screen, and select the 2160p/60fps stream. > > Result on my 2013 mac Pro (8 cores, 32GB of RAM): > e10s you see about 1/3rd of the frames being dropped > with e10s off, it's smooth as butter Weird... I re-ran it and with e10s on in nightly it was perfect all of a sudden....