Friday, July 22, 2016

SLi, Everything you need to know

Good evening everyone, today again slightly different topic and kinda written in brief and it is about SLI. Hope you will enjoy it.

What is SLI?

I won't go much into details here with all those technical terms of how it works but to put simply this is a GPU technology developed and used by Nvidia to utilize more than one GPU in a single system to boost the performance. For two cards alternate frame rendering can be used where one GPU renders the odd frames other one renders the even frames thus increasing the overall throughput. Although SLI was first used by 3DFX on their Voodoo2 cards but that SLI meant Scan Line Interleave and SLI used by Nvidia means Scalable Link Interface.

3DFX Cards in SLI configuration


What Do you need? 

1. Multiple SLI capable GPU.

2. SLI enabled motherboard.

3. SLI Bridge.

4. SLI enabled operating system.

5. Proper drivers and applications.

Nvidia GTX980 2 Way SLI on an Asus X99 board.


Gains and advantages: 

1. Substantial performance increase.

2. It might be the case when you already have the best card and you want more performance out of your system you have no choice but to add in another card.

3. Chances are you are running a card from older generation and price of your card has dropped as with the release of new generation it always happens. You can either get a new card from new generation or add in a same card and get more performance out of it.

4. You can easily hit higher frame rates or better quality with SLI, there are games at this time that would even put the best card out there in good amount of stress.

5. Show off and bragging rights, don't deny it many people use SLI just for this. You will get huge scores in benchmarks.


Limitations and disadvantages:

1. Video RAM won't add up which means two 4GB card won't give you 8GB of video memory instead it will give you 4GB. The reason being each card has to store the same data so eventually the total effective memory stays the same.

2. Not all tittles support SLI and many of the games are optimized for taking full advantages of it. So many games will just run on a single card. For example Company of Heroes does not even support SLI, so when I played this game with my SLI setup only one card was essentially taken all the load.

3. Performance does not scale up that much, two cards do not necessarily guarantee 200% performance and three of them won't give you 300% and as more and more cards are added the scaling goes down even further and latest generation of 1000 series cards actually have a lot of things regarding SLI but it is ideal to use only two of them.

4. You will need a SLI capable motherboard that will be slightly costly and as you add in more cards PCI express link width will decrease in most cases. Let me remind you that most common AMD and Intel desktop processors only have 16 PCI Express lanes. For example the Core i7 6700k has 16 lanes. So for single card it will be 16X but if you add in another card the link speed will go down to 8X, adding a third card will decrease the link speed to 4X for two of them. For single moderately powerful card 8X bus speed is enough but for high end card you might loose a little performance for decrease of Bandwidth. However for 28/40 lane Intel Extreme processors you can add up multiple cards without loosing that much bandwidth.

5. A much beefier power supply is needed, preferably one with SLI certification. Personally I use one with a single 12V Rail that can provide good amount of current on it because that way I don't have to worry about balancing the load. To learn more about this you can check this post.

6. Only same Graphics Processing Card can be used together. For example you can use two GTX980 together but not one GTX980 with a GTX970. You can however use different manufacturer. I have used Gigabyte Card with MSI card, as long as they have the same Graphics processing unit you can use them together. DirectX 12 supposed to solve this issue but let's see how things turn out.

7. Cards with aftermarket cooler might make the inside of the case hotter as these cards tend to exert heat inside the case. Blower style reference cards are better in this regard as they blow heat out of the case. Moreover top card might run hotter in many cases as it will get less air due to the close proximity of another card.

8. In poorly optimized games it might result in stuttering which drastically decreases the overall gaming experience. You will see that the game is running at a very high frame rate but frame time will be so different that the game won't be enjoyable any more. For example I had this issue with Far Cry 4 and Call of Duty Advanced Warfare with my SLI setup.

9. Many CUDA enabled software won't be able to use both cards at the same time. In that case a single card would have been better.


Final thought:

So if you are out for shopping a new card buy the absolute best you can get for your budget. It is not wise to spend on two moderately powered cards over a powerful one. Think about it in this way, if you want to accommodate multiple cards you will be using SLI enabled motherboard and high end power supply unit rather you can just go with a single GPU and a slightly cheap board which will actually give you more performance for the money.

But like I said earlier, if you want the absolute best and even the single card in the market is way below your maximum capability, by all means go ahead and add another one and if you want to get a older generation card to pair with your existing one rather than buying a new generation card, you can do that too.

That's pretty much it, good day everyone.

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