dGPU improve UI animation smoothness and external display performance?

There are still a lot of unknowns as to what exactly causes the poor UI performance on rMBPs.

A lot of people automatically assume the limiting component is the GPU, but it's almost always completely baseless.

Anandtech investigated a bit on the issue and seemed to conclude the issue was mainly CPU-bound and caused by the lack of software GPU-acceleration and parallelized threading, but he admittedly wasn't really sure of anything.

Whether you're on Mountain Lion or Mavericks, it seems that manually switching between the integrated graphics and discrete graphics with an utility like gfxCardStatus has no observable effect on UI performance.

Even with a 2012 15" rMBP with a HD 4000 and GT 650M (huge performance gap between the two), it seems forcing discrete graphics has absolutely no effect on scroll lag whatsoever, which seems to validate Anandtech's theory that it's entirely CPU-bound. The fact Mavericks improved the situation over Mountain Lion, which itself improved the situation over Lion, also suggests that software optimization has a large role in improving it, probably moreso than improving hardware. For example, Mavericks + HD 4000 is significantly smoother than Mountain Lion + GT 650M.

If the issue is indeed CPU-bound, one might expect the 15" rMBP to still perform better than a 13" rMBP because its CPU is significantly more powerful as well. The thing is, what mainly gives the 15" rMBP's CPU an edge is the fact that it's quad-core. Assuming Anand is right about the fact a single CPU thread is getting all the UI-related load (and bottlenecking the whole UI stack), the 2 extra cores the 15" rMBP offer would be effectively useless in that situation. I should note that Anandtech only tested this on Mountain Lion and didn't post any follow-up regarding Mavericks since then.

From Anandtech last year:

In your specific situation, I assume that the reason your Mac performs better in clamshell mode with an external display compared to when using its internal display + an external display at the same time is simply that the Mac has much less pixels to push and HiDPI is off. I think it has nothing to do with the fact you're using integrated or discrete graphics and that you could reproduce the exact same effect with a GT 750M.

All we can do as consumers is patiently wait for Apple to improve software optimization, which should have a greater effect than any hardware upgrade money could buy right now or in the next few years. As someone who got a 15" rMBP at launch last year, it's frustrating to see things improve so slowly. I guess it's the price of being an early adopter, but at the same time I can't help but think the whole situation shouldn't be happening considering Apple products are usually synonymous with a polished experience. A bit disappointing to say the least.

 

I think it's mainly software that is the problem.

The other day I connected my rMBP to a 30" display (2560x1600) while running the rMBP's screen at the scaled 1920x1200 resolution.

I had iTunes open and on the 30" it was very smooth to grab the iTunes window and move it around. When I dragged that window on to my rMBP's display however it lagged like crazy, dragging the window around and it was moving very delayed a long time after the mouse and it was jittery.

But here's the thing, if the iTunes window clipped any side of the screen at all, even just 1mm of the border of the iTunes window, it didn't lag at all. Just as smooth as it was on my 30" display.

Strange right? 100% of the window visible = Insane lag. 99.99999% of the window visible = No lag at all.

I then after finding this strange behaviour restarted iTunes and it no longer lagged on my rMBP screen even if it was in full view.

And I find strange behaviour like this in a few apps. It gives me the sense that the problem isn't the hardware but OS X. They clearly have a lot more work to do with Mavericks to fix all these issues, smooth scrolling is great but it's not perfect like it is on the non-retina models and I truly believe it's down to software.

 

It's plugged in right now and iStat Menus says the CPU is at 53c and that is currently the hottest temperature on my Notebook. It reaches the same temperature when it's not connected to my 30" display with identical workload (mostly idle with just 8 apps open).

Honestly I've not noticed any difference in the temperature of the computer plugging it in to my external display.

 

I don't think the quad-core CPU would help much either. I just updated my original post, see the paragraph right above the quote. Unless it has changed since Anandtech conducted his test last year, a single CPU core seems to be getting pretty much all the load, and thus creating the stuttering issues. The two extra cores the 15" rMBP offers would be effectively useless in that situation.

 

This post should be referenced every time someone asks about Retina UI lag. Well done.

 

I have the late 2013 15" rMBP with the GT 750m and I can say that yes, the dGPU is (in general) a more fluid experience. I also run Adobe CS6 and while the Iris Pro is relatively smooth using PS and AI, the dGPU is still a bit more fluid. That said, I'm betting most of the differences I've mentioned are driver bound, the Iris Pro is very capable, and should (in theory) be able to power UI animations without breaking a sweat.

There are some interesting bugs I've noticed, with the Dock set to the left or right of screen (and set to a smaller size), there is a slight delay when entering Mission Control. This can only be seen with the Iris Pro and must be software related, as when the Dock is on the bottom of the screen, there is no lag at all.

 

I cannot really say the same. Usually on one monitor with nothing in the background doing anything. Everything is smooth. If I do play some video or flash in a different app, scrolling gets very choppy. Somehow sometimes more choppy on the 750M than the Iris Pro. To me this feels like the 750M not switching power states as quick as Iris Pro which makes it feel a bit more laggy, or it may just not switch to higher power states as aggressively.
Either way the behavior is very strange and the only thing I can say for certain is that the 750M gets you absolutely nothing in fixing choppy behavior when it occurs. The choppiness I notice isn't really a constant thing, it shows up at certain load scenarios. Usually those scenarios should leave 70-90% of CPU and GPU performance free so in theory there really shouldn't be a problem.

I don't know how a 13" with the Dual Core Iris compares as it is slower in everything. It might not make a difference because I experience choppiness like I never had on my old much much slower 2010 15" in Mavericks, even at low load scenarios. I think it is a software related responsivness problem. I am guessing a 15" will not do any better because its theoretical speed isn't really in use when it already has issues.

I had the odd issue that switching the GPU to Iris Pro sometimes actually helps choppiness a bit, so I doubt the 750M will save the day. It isn't a performance problem. In Windows the 750M never even leaves its lowest power state on normal browser scrolling and scrolling is butter smooth even in 2880x1800 at any DPI setting. It isn't the resolution, it is some code path in the whole output scaling that causes this problem.

ad External display:
That really doesn't cause an issue. I was worried about that (as my old 2010 got noisy at any thing other than the lowest loads with an external) but right now I play a twich.tv flash video on the external screen in safari and the notebook stay cool and quite at 2000 rpm. Temps now in the 58-59 range. Temps will grow a little higher when the notebook is online longer but the fans never ramp up until you actually do something more serious. The external alone and any sort of medium/low load doesn't seem to be any cause for noise concerns at all.

 

I gave it a shot. I tried to recreate similar situations that usually led to lag after I disabled HiDPI with SwitchResX Trial.
I first tried 1680x1050. Butter smooth without the least amount of a performance issue noticeable.
I then switched to 2880x1800 but zoomed webpages to the same size (about 250% zoom) it still appeared a lot smoother than with HiDPI. Very light stutter noticeable but about like running on 5 times faster hardware.
I then tried some more demanding webpages (the verge as one mentioned).
That ones still wasn't all smooth in both Safari and Chrome.
Switching to Iris Pro doesn't really change anything. I think the instances where it helps is when some software path gets reintialized and therefore runs cleaner or better.
MacRumors is butter smooth on either GPU. But sometimes lags severely in HiDPI mode when there is background load. It should be a simple enough website. When it stutters as I described in HiDPI mode the difference is huge, nothing even a twice as fast hardware could fix. Between the two GPUs the difference isn't really noticeable at least not for me, in any configuration. (I can only compare on one monitor as the external only works with the dGPU.)

BTW scrolling a pdf in preview never causes an issue on any setting. I will later try Windows and see how that one fares on the verge website which seems to be really demanding.

Browsing alone works actually very well without HiDPI in 2880x1800 . Finder is a bit hard to read.

Windows at full 2880x1800 in Chrome has no performance issues whatsoever even on verge.com.
I am not sure how compareable that really is as OSX scrolling is per pixel line while in Windows the smallest increment is rather big. Scrolling quickly and far up down still looks bad in OSX while perfectly fine in Windows.

Note: Whenever I used HiDPI I used the more space option. The equivalent of 1680x1050 as coming from a HR AG 2010 15" I consinder best for retina (1440x900) unbearable. max space 1920x1200 is okay but I would never use this display on anything less than 1680x1050, so I consider any better performance at best for Retina if it exists (in an other thread somebody said that, I am in Windows now and cannot test), pointless to even explore.