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Frame Rating Dissected: Full Details on Capture-based Graphics Performance Testing

Vsync and its Effect on Frame Rating – Does it fix CrossFire?

After publishing the Frame Rating Part 3 story, I started to see quite a bit of feedback from readers and other enthusiasts with many requests for information about Vsync and how it might affect the results we are seeing here.  Vertical Sync is the fix for screen tearing, a common artifact seen in gaming (and other mediums) when the frame rendering rate doesn’t match the display’s refresh rate.  Enabling Vsync will force the rendering engine to only display and switch frames in the buffer to match the vertical refresh rate of the monitor or a divisor of it.  So a 60 Hz monitor could only display frames at 16ms (60 FPS), 33ms (30 FPS), 50ms (20 FPS), and so on with a 120 Hz monitor could also being capable of 8ms (120 FPS), etc. 

Many early readers hypothesized that simply enabling Vsync would fix the stutter and runt issues that Frame Rating was bringing to light.  To test this we looked for a game that ran right around the 60 FPS mark in our in normal testing with Vsync disabled and then set about to re-run results with it on.  We are using a standard 60 Hz monitor with the goal of being able to test some 120 Hz capability soon after we figure out a final bug or two with our capture configuration. 

First up, let’s take a look at the NVIDIA GeForce GTX 680 and GTX 680 SLI and see what shows up.

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Because the average frame rate per second graph averages out the frame times for a total of one second of time, the averages won’t quite be the straight lines you might have expected.  Looking at the GTX 680 SLI Vsync enabled results the only key item is that the frame rate doesn’t go above 60 FPS like it does with Vsync disabled.

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The single card and SLI configurations without Vsync disabled look just like they did on previous pages but the graph for GTX 680 SLI with Vsync on is very different.  Frame times are only switching back and forth between 16 ms and 33 ms, 60 and 30 instantaneous FPS due to the restrictions of Vsync.  What might not be obvious at first is that the constant shifting back and forth between these two rates (two refresh cycles with one frame, one refresh cycle with one frame) can actually cause more stuttering and animation inconsistencies than would otherwise appear.

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Based on our graph here we found that with Vsync enabled we had about 87% of our frames running at 60 FPS (16 ms) and 13% at 30 FPS (33 ms).  You might be curious how there could be 60 FPS frame rate so often with Vsync on but very few frames at 60 FPS with Vsync off, and the answer lies in the rate limiting caused by Vsync.  Because of the back pressure on the game engine caused by the longer frame times (30 FPS, 33 ms) from Vsync there is more time for the GPUs to “catch up” and render another frame at 16 ms. 

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Our ISU graph on stutter potential tells the story in a more damning light; starting at the 30th percentile the Vsync enabled setup of GTX 680s in SLI are already running at much higher frame variances and it only gets worse as we hit the 60s, 80s and 90s.  At the 90th percentile we are seeing frame variances over 12 ms, which is nearly a complete monitor refresh cycle!

 

Now let’s see how the AMD Radeon HD 7970 results change.

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Something interesting is already happening here – the Vsync enabled results from the HD 7970 CrossFire configuration are running at HIGHER average frame rates per second than with Vsync disabled!  The orange line clearly never hits the 60 FPS mark while the black line (Vsync) does. 

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Without Vsync we clearly see the runts affecting the plot of frame times here on the HD 7970s in CrossFire but enabling Vsync does appear to eliminate them! 

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With our observed frame rate we have the same results for the HD 7970 CrossFire as we did with our FRAPS results, indicating no dropped frames or runt frames.  Standard CrossFire mode still shows the horrible results we have come to expect from our analysis today.

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Our Min FPS percentile graph shows us that we are running at 60 FPS (16 ms) 85% of the time and 30 FPS (33 ms) the rest.  Because our data here is based the observed frame rates and not the FRAPS frame rates, there is no correlation between the two CrossFire runs.

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The ISU graph of stutter potential again indicates that the Vsync enabled option is introducing higher frame variances than we would like and it is doing it more dramatically and earlier than the GTX 680s in SLI. 

It does appear that enabling Vsync will help alleviate the runts issue seen with AMD Radeon cards in CrossFire but at the cost of much more frame variance and stuttered animation on games that previously didn’t exhibit that problem. 

Let's take a look at another example using CrossFire that has another particular set of circumstances.  I theorized that in a gaming scenario that bordered just under 60 FPS with a single GPU, we would still see problematic results when jumping to HD 7970s in CrossFire.  Take our Battlefield 3 2560x1440 testing: with only one HD 7970 we are running just under 60 FPS most of the time which would, with Vsync enabled, force the game to run at 30 FPS with 33ms frame times.  Ideally we would like to see that move from 33ms frame times to 16ms frame times when adding in another HD 7970 in CrossFire due to the extra performance pushing the card over 60 FPS steady.

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Our FRAPS graphs looks how we would hope and expect real-world performance to look.  While the single HD 7970 ran at a non-standard frame rate when performance was under 60 FPS, towards the end (50 sec point) where it could, we see a flat line that is partially hidden behind the pink line.  That pink line represents CrossFire HD 7970s and by doubling the number of GPUs we expected to maximize performance at 60 Hz with Vsync enabled, and we have. 

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Observed frame rates calculated by removing runts are showing the Vsync DISABLED results on the HD 7970s in CrossFire mirror what we have seen before with much lower performance.  However, the Vsync ENABLED results did not change! 

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The somewhat complicated plot diagram of frame times indicates that at no time did the frame rate of the HD 7970 cards in CrossFire go below 60 FPS or above the 16ms mark - even though there are thousands of frames under 16ms (runts) when Vsync is disabled.  Not only that but performance over the single HD 7970 with Vsync enabled is improved - rather than having jumps between the 16ms and 33ms frame times, we are locked in at 16ms - matching the 60 Hz refresh of our panel. 

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The minimum FPS percentile graphic shows the same story - the pink link representing the HD 7970s with Vsync turned on looks solid.

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Notice as well that with a static 16ms frame time we see no frame time variance at all in our ISU graph indicating that the kinds of stutter we are searching for are not showing up at all.

How is this happening?  How is enabling Vsync 'fixing' the runts and frame time issues of CrossFire?  The secret lies in the inherent back pressure of vertical sync to pace the graphics card and AMD's CrossFire engines even against its own will.  By forcing the GPUs to only render one frame every 16ms (at the maximum), Vsync is able to force the GPU to pace itself in a way that it would otherwise not.  This doesn't happen in every game though as we saw in the Crysis results first, and there is a lot more testing that needs to be done with Vsync to make a firm decision.

 

NVIDIA has a couple of different solutions in the NVIDIA Control Panel that might help: Adaptive Vsync and Smooth Vsync.  Adaptive Vsync was released with the first Kepler GPUs last year and we found it to be very effective at reducing stutter while also eliminating tearing.  Smooth Vsync is a little known feature that only exists in the driver when SLI is enabled as it takes advantage of many of the same frame metering features that SLI uses.  It attempts to keep frame rates “settled” at a level until it decides it has enough horsepower to move up to the next frame rate option for an extended period of time.  It is a very dubious description at best and NVIDIA didn’t go into much detail on how they decide if they have enough GPU overhead remaining or how long that “period of time” really is.

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I decided to run through the same Crysis 3 sequences at 1920x1080 on the GTX 680s in SLI with all four NVIDIA options enabled: Vsync off, Vsync on, Adaptive Vsync and Smooth Vsync. 

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Our FRAPS based results show the same similar looking results for standard Vsync on and off, but the adaptive and smooth Vsync options appear to be fixed at 30 FPS with the occasional hiccup on the Smooth Vsync.

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The plot of frame times is kind of confusing but the important data is to compare standard Vsync On to Adaptive and Smooth.  With the exception of the 6 or so spikes on the smooth configuration the frames are basically fixed at 33 ms, resulting in a perfectly smooth gameplay experience but at the expensive of limiting performance. 

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The observed FPS doesn’t change at all.

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Another view here shows the same thing with a fixed frame rate of 30 FPS for adaptive and smooth Vsync options.

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NVIDIA’s Adaptive Vsync shows basically 0 variance and only very minimal variance on the Smooth Vsync option at the 96th percentile.  So even though performance is lower on average, the experience is smoother.

 

NVIDIA’s additional Vsync options are definitely a strong point in favor of its technology though the Smooth Vsync only exists on SLI configurations.  I have been told that they were considering adding it to single graphics card configurations and I certainly hope they do as it adds some significant value in the same way Adaptive Vsync and Frame Rate Limiting do.

For both NVIDIA and AMD multi-GPU solutions with standard Vsync, enabling it definitely changes the story.  NVIDIA’s cards pretty much perform as we expected but for CrossFire we didn’t really know what expect with the various visual concerns.  It does appear that the runts problem was at least mostly solved with the enabling of Vsync though to be clear we are only testing a couple of game at this point – much more needs to be done. 

However, enabling Vsync creates a whole host of other potential issues that gamers have to deal with.  Even though the goal of removing visual tearing is met with the option turned on, you do add latency to the gameplay experience, as much as 60ms in some cases, from input to display.  Putting back pressure on the GPU pipeline, for both NVIDIA and AMD, means that some frames are going to be running behind schedule or behind the input timing of the game itself.  Many gamers won't want to deal with those kind of input problems and that is why many still play games with Vsync disabled.  Turning on Vsync does help AMD's CrossFire performance but it isn't the final answer just yet.

March 27, 2013 | 04:34 PM - Posted by Ryan Shrout

I'll see if we can take a look at that.

March 29, 2013 | 12:31 PM - Posted by jgstew

The whole time I was reading this article, I was more and more curious how Virtu's technology would effect things. I'm curious about more than just their Virtual V-sync, but their other options as well for both single and multiple GPUs. Virtu has not had the scaling that SLI & Crossfire have had, but perhaps their technology would show well in other areas with this analysis.

I do feel that Frame Rating & the input to display latency are much more interesting metrics.

Great work on the article.

March 27, 2013 | 02:32 PM - Posted by serpinati of the wussu (not verified)

I've read somewhere that this will not be the norm for pcper to do this type of testing with all video card reviews (too labor intensive). Is this true?

If it is true, will pcper at least record the frame data and simply give it a quick look to make sure video cards aren't doing something absolutely crazy (for example, if you were reviewing those 7970's crossfire you might not plan to actually to analyze the frame times, but you would at least look over the recorded frames anyways and catch the crazy runt frames and mention it in your reviews.)

March 27, 2013 | 04:34 PM - Posted by Ryan Shrout

No, my plan is to take this route going forward.

March 27, 2013 | 02:57 PM - Posted by Anonymous (not verified)

Greatly appreciate the work behind this! And opening up the tools / scripts to everyone, pushing it to other hardware magazines. Huge kudos! Oh, and you should slap Anandtech over the head for not even mentioning your involvment into the new metering method in their introductory article, that only mentions nVidia...

Is there any way to determine latency between t_present and t_display? If not, it should be - maybe some kind of timestamp could be worked into the overlay? Because that would be interesting not only in the context of VSync, but also regarding nVidias frame metering, which must take some time analyzing the frametimes. Supposedly, AMD has some smoothing algorithm coming up for Crossfire as well, so there it would also be valuable information.

Regarding Adaptive / Smooth VSync: They clearly come off too good in this article. They're not the focus of the article of course, still a little more thought should have been put behind this, considering the amount of time taken to create this awe inspiring effort of an article and the tools behind it. Adaptive VSync turns VSync off when tearing is most annoyingly visible, i.e. at framerates below monitor refresh rate - true triple buffering (instead of the queue nowadays called triple buffering) would be the good solution here. Smooth Vsync does not seem to do anything particularly positive judging from what little measurement is available in the article - only time (further tests that is) will tell what it actually does...

March 27, 2013 | 04:36 PM - Posted by Ryan Shrout

We have more research into the different NVIDIA Vsync options coming up, stay tuned.

As for the timestamp different to check for the gaps between t_present and t_display...we are on that same page as well.  :)

March 27, 2013 | 02:57 PM - Posted by Mawrtin (not verified)

Have you tried CF using radeonpro? Apparently it offers some kinda of Dynamic V-sync similar to nVidias adaptive if I'm not mistaken.

March 27, 2013 | 05:03 PM - Posted by Marty (not verified)

No, it does not. It's a frame limiter, which eliminates some of the stutter, but causes more lag (increases latency).

March 27, 2013 | 03:16 PM - Posted by Mangix

Hmmm. I wonder if there is a difference between Double and Triple Buffered VSync. Newer Valve games support both. Would appreciate testing there.

Also, are there driver settings that help/harm frame rating? Nvidia's Maximum Pre-rendered frames setting sounds like something that can have an effect.

March 27, 2013 | 04:25 PM - Posted by xbeaTX (not verified)

AMD has complained on Anandtech for this type of test using fraps ... now anyone know the real situation and it's shocking
this article sets the new stantard of excellence ... congratulations and thanks for the enormous work done... keep it up! :)

March 27, 2013 | 04:37 PM - Posted by Ryan Shrout

Thank you!  Help spread the word!

March 27, 2013 | 10:55 PM - Posted by TinkerToyTech

Posted to my facebook page

March 27, 2013 | 04:40 PM - Posted by Marty (not verified)

Ryan, I've a suspicion that you wanted to select Adaptive VSync on NVidia, but have selected Adaptive (half refresh rate) instead in the control panel. Would you please check it out.

March 28, 2013 | 02:20 AM - Posted by showb1z (not verified)

²

Other than that great article. Would also be interested in results with frame limiters.

March 27, 2013 | 05:23 PM - Posted by Dan (not verified)

This site is so awesome that it is one of th eonly ones that I disable Adblock in Chrome for. You deserve the ad $'s.

Rock on, Ryan and crew!

March 29, 2013 | 07:37 AM - Posted by Ryan Shrout

Thank you we appreciate it!

March 27, 2013 | 05:45 PM - Posted by Mike G (not verified)

Thanks for the all of the time taken to accurately test and then explain your frame rating methods to us. I wonder if AMD would be willing to have a representative come on and speak on how they will be addressing this issue. I for one will be holding off purchasing an additional 7970 at this time.

March 27, 2013 | 06:43 PM - Posted by bystander (not verified)

They have, with AnandTech, though the person they spoke to was the single GPU driver guy, they did mention they have a plan to offer fixes in July.
http://www.anandtech.com/tag/gpus

March 27, 2013 | 06:51 PM - Posted by Soulwager (not verified)

Is your capture hardware capable of capturing 1080p @ 120hz? The data rate should be less than 1440p@60hz.

Also, I would like to see some starcraft 2 results. It's frequently CPU limited, and I'm wondering how that impacts the final experience when compared to a gpu limited situation. I'd recommend the "unit preloader" map as a benchmark run, once to pre-load all the assets and again for the capture.

March 29, 2013 | 07:38 AM - Posted by Ryan Shrout

Actually, I think 1080p@120 is a higher pixel clock than 25x14@60; we tried to do basic 120 Hz testing right before publication without luck but we'll be trying again soon.

As for SC2, we'll take a look.

March 29, 2013 | 12:00 PM - Posted by Soulwager (not verified)

You're right, the pixel clock is higher. I guess I was only thinking about total number of pixels that need to be recorded, but the smaller resolution is more heavily impacted by overscan.

March 27, 2013 | 09:28 PM - Posted by SPBHM

great stuff.

I would be interested in seeing some results with some framerate limit, not from vsync, but another limit, something like FPS max 45 for Crysis 3 (higher than a single card, around the average for the CF), you can easily do that with dxtory

March 27, 2013 | 09:49 PM - Posted by Trey Long (not verified)

Its nice to see the truth come out. Save the excuses and fix it AMD.

March 28, 2013 | 03:41 AM - Posted by Carol Smith (not verified)

Why did you completely ignore AMD's RadeonPro freeware ?
Which offers and has offered Dynamic V-Sync that's superior to Nvidia's Adaptive V-Sync for years now.
Tom's Hardware seems to be the only site that actually used this utility to make a fair comparison between SLI and Crossfire.

RadeonPro's Dynamic V-Sync was shown to be clearly superior to Nvidia's Adapative V-Sync Implementation .

Here is the Tom's link
http://www.tomshardware.com/reviews/radeon-hd-7990-devil13-7970-x2,3329-...

I hope that you learn from your fellow journalist's to include this fantastic solution in your upcoming article.
Thanks for your hard work and good luck.

March 28, 2013 | 06:27 AM - Posted by Marty (not verified)

A frame limiter improves stuttering, but increases lag. You loose some Fps too. So you have to decide between the devil and lucifer in the case of Crossfire.

March 29, 2013 | 07:01 AM - Posted by Carol Smith (not verified)

Exact same thing with Nvidia SLI and Adaptive V-Sync, if you don't want stuttering you have to sacrifice framrates.

March 29, 2013 | 07:39 AM - Posted by Ryan Shrout

I'm going to take a look, but this is NOT "AMD's" software.

March 28, 2013 | 03:50 AM - Posted by Andre3000 (not verified)

Thanks for the eye opener! Which AMD drivers have been used for this review? I have read the article.. but i might have missed it?

March 29, 2013 | 07:39 AM - Posted by Ryan Shrout

For AMD we used 13.2 beta 7 and for NVIDIA we used 314.07.

March 29, 2013 | 08:27 AM - Posted by Steve (not verified)

Ryan: Is it possible to test AMD's CF with older drivers to see if this problem has been around for a long time or if it is a more recent problem with AMD's continuous driver upgrades to improve speeds?

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