Scalable Link Interface is a brand name for a multi-GPU technology developed by NVIDIA
for linking two or more video cards together to produce a single output.
SLI is an algorithm of parallel processing for computer graphics, meant to increase the
processing power available for graphics.
The acronym SLI was first used by 3dfx for Scan-Line Interleave, which was introduced
to the consumer market in 1998 and used in the Voodoo2 line of video cards.
After buying out 3dfx, NVIDIA acquired the technology but did not use it.
NVIDIA later reintroduced the SLI name in 2004 and intended for it to be used in modern
computer systems based on the PCI Express bus; however, the technology behind the name
SLI has changed dramatically.
Implementation SLI allows two, three, or four graphics processing
units to share the workload when rendering real-time 3D computer graphics.
Ideally, identical GPUs are installed on the motherboard that contains enough PCI-Express
slots, set up in a master-slave configuration.
All graphics cards are given an equal workload to render, but the final output of each card
is sent to the master card via a connector called the SLI Bridge.
An example, in a two graphics card setup, the master works on the top half of the scene,
the slave the bottom half.
Once the slave is done, it sends its render to the master to combine into one image before
sending it to the monitor.
The SLI bridge is used to reduce bandwidth constraints and send data between both graphics
cards directly.
It is possible to run SLI without using the bridge connector on a pair of low-end to mid-range
graphics cards with NVIDIA's Forceware drivers 80.XX or later.
Since these graphics cards do not use as much bandwidth, data can be relayed through just
the chipsets on the motherboard.
However, if no SLI bridge is used on two high-end graphics cards, the performance suffers severely
as the chipset does not have enough bandwidth.
Configurations currently include: Two-Way, Three-Way, and Four-Way SLI.
Uses two, three, or four individual graphics cards respectively.
Two GPUs on one graphics card.
Examples include the GeForce GTX 590, GeForce GTX 690 and the GeForce GTX Titan Z.
This configuration has the advantage of implementing Two-Way SLI, while only occupying one PCI-Express
slot and two expansion I/O slots.
This also allows for Four-Way SLI using only two cards.
NVIDIA has created a set of custom video game profiles in cooperation with video game publishers
that will automatically enable SLI in the mode that gives the largest performance boost.
For more information on SLI-optimized games, visit NVIDIA's SLI Zone.
SLI Modes SLI allows for the following modes.
Split Frame Rendering This analyzes the rendered image in order
to split the workload 50/50 between the two GPUs.
To do this, the frame is split horizontally in varying ratios depending on geometry.
For example, in a scene where the top half of the frame is mostly empty sky, the dividing
line will lower, balancing geometry workload between the two GPUs.
This method does not scale geometry or work as well as AFR, however.
Alternate Frame Rendering Each GPU renders entire frames in sequence.
For example, in a Two-Way setup, one GPU renders the odd frames, the other the even frames,
one after the other.
Finished outputs are sent to the master for display.
Ideally, this would result in the rendering time being cut by the number of GPUs available.
In their advertising, NVIDIA claims up to 1.9x the performance of one card with the
Two-Way setup.
While AFR may produce higher overall framerates than SFR, it also exhibits the temporal artifact
known as Micro stuttering, which may affect frame rate perception and input response.
SLI Antialiasing This is a standalone rendering mode that offers
up to double the antialiasing performance by splitting the antialiasing workload between
the two graphics cards, offering superior image quality.
One GPU performs an antialiasing pattern which is slightly offset to the usual pattern, and
the second GPU uses a pattern offset by an equal amount in the opposite direction.
Compositing both the results gives higher image quality than is normally possible.
This mode is not intended for higher frame rates, and can actually lower performance,
but is instead intended for games which are not GPU-bound, offering a clearer image in
place of better performance.
When enabled, SLI Antialiasing offers advanced antialiasing options: SLI 8X, SLI 16X, and
SLI 32x.
A Quad SLI system is capable of up to SLI 64X antialiasing.
Hybrid SLI This is another mode that is not for performance
enhancing.
The setup consists of an IGP as well as a GPU on MXM module.
The IGP would assist the GPU to boost performance when the laptop is plugged to a power socket
while the MXM module would be shut down when the laptop was unplugged from power socket
to lower overall graphics power consumption.
Hybrid SLI is also available on desktop Motherboards and PCs with PCI-E discrete video cards.
NVIDIA claims that twice the performance can be achieved with a Hybrid SLI capable IGP
motherboard and a GeForce 8400 GS video card.
NVIDIA has a similar technology called Nvidia Optimus.
Caveats Not all motherboards with multiple PCI-Express
x16 slots support SLI.
Recent motherboards as of August 2014 that support it are Intel's Z and X series chipsets
and AMD's 990FX chipset.
Aside from a few exceptions, older motherboards needed the nForce series of chipsets to allow
for SLI.
In an SLI configuration, cards can be of mixed manufacturers, card model names, BIOS revisions
or clock speeds.
However, they must be of the same GPU series and GPU model name.
There are rare exceptions for "mixed SLI" configurations on some cards that only have
a matching core codename, but this is otherwise not possible, and only happens when two matched
cards differ only very slightly, an example being a differing amount of video memory,
stream processors, or clockspeed.
In this case, the slower/lesser card becomes dominant, and the other card matches.
Another exception is the GTS 250, which can SLI with the 9800 GTX+, as the GTS 250 GPU
is a rebadged 9800 GTX+ GPU.
In cases where two cards are not identical, the fastest card – or the card with more
memory - will run at the speed of the slower card or disable its additional memory.
SLI doesn't always give a performance benefit – in some extreme cases, it can lower the
frame rate due to the particulars of an application's coding.
This is also true for ATI's CrossFire, as the problem is inherent in multi-GPU systems.
This is often witnessed when running an application at low resolutions.
Vsync + Triple buffering is not supported in some cases in SLI AFR mode.
Users having a Hybrid SLI setup must manually change modes between HybridPower and GeForce
Boost, while automatically changing mode will not be available until future updates become
available.
Hybrid SLI currently supports only single link DVI at 1920×1200 screen resolution.
When using SLI with AFR, the subjective framerate can often be lower than the framerate reported
by benchmarking applications, and may even be poorer than the framerate of its single-GPU
equivalent.
This phenomenon is known as micro stuttering and also applies to CrossFire since it's inherent
to multi-GPU configurations.
See also Scan-Line Interleave by 3Dfx
AMD CrossFireX – AMD's competing solution MultiChrome
Micro stuttering Conventional PCI
References
External links Official website