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.
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.
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.
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.
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.
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.
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!
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.
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.
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 2560×1440 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.
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.
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!
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.
The minimum FPS percentile graphic shows the same story – the pink link representing the HD 7970s with Vsync turned on looks solid.
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.
I decided to run through the same Crysis 3 sequences at 1920×1080 on the GTX 680s in SLI with all four NVIDIA options enabled: Vsync off, Vsync on, Adaptive Vsync and Smooth Vsync.
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.
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.
The observed FPS doesn’t change at all.
Another view here shows the same thing with a fixed frame rate of 30 FPS for adaptive and smooth Vsync options.
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.