The issue of frame-latency problems with single-GPU setups (particularly Radeon) recently has sparked a lot talk on our forums, and for even longer, the issues of microstuttering with Crossfire and SLI setups has long been an issue that has not been addressed. In a very lenghthy discussion (worth the read if you're interested in the interworkings of everything, as I am) Anandtech spoke with AMD about why the problem is there, how it can be resolved and some nifty features they will be adding to their drivers later this year.
The full read is here: http://www.anandtech.com/show/6857/amd-stuttering-issues-driver-roadmap-fraps
Here is a recap for those who don't want to read.
1. Radeon has latency issues.
- This is true, and AMD now evaluated frame latency for every game they optimize.
- As a result of stuttering optimzations, AMD has squeezed untapped potential from their GPUs. (ex. 13% in Borderlands!)
- Expect better consistency and performance from all games from here on out.
- FRAPS is can sometimes provide false positives of stuttering because it measures the wrong part of the pipeline
- FRAPS measures time between "present()" calls, which is not a good indicator of when the frame actually reaches your screen.
- FRAPS may indicate latency spikes even when frames are hitting your screen in a consistent manner.
- More in depth tools, like GPUView are needed to truly asses frame latency.
2. Crossifire Microstuttering.
- The primary method for Crossfire/SLI is AFR (alternate frame rendering).
- GPUs render every other frame.
- Best results happen when GPU2 starts rending the next frame when GPU1 is halfway done with the present frame.
- Timing issues occur.
- Sometimes one frame takes longer to render than other making timing hard to predict.
- Sometimes the next frame depends on information from the previous frame
- These timing issues cause "microstuttering".
- Microstuttering can be smoothed out by holding onto frames longer.
- Proividng a buffer time allows more consistent frame latencies.
- Providing a buffer time will result in "input lag".
- The effect is similar to enabling vsync.
- Giving the user control.
- Crossfire users will be able to select two modes of operation.
- Mode 1: Traditional mode, frame latencies are reduced, no input lag. Microstuttering occurs.
- Mode 2: Smooth mode, a buffer is utilized to smooth out microstuttering, input lag occurs.
The Bottom Line
- AMD admits to finding latency issues.
- All titles are already being screened for latency issues and optimized. (all games from here on out should be as stutter free as possible)
- Optimizations have increased performance on AMD GPUs.
- Micostuttering will be addressed for Crossfire setups.
- Crossfire users will gain new driver options to control microstuttering in July.