NVIDIA's Fermi: Architected for Tesla, 3 Billion Transistors in 2010
by Anand Lal Shimpi on September 30, 2009 12:00 AM EST- Posted in
- GPUs
A More Efficient Architecture
GPUs, like CPUs, work on streams of instructions called threads. While high end CPUs work on as many as 8 complicated threads at a time, GPUs handle many more threads in parallel.
The table below shows just how many threads each generation of NVIDIA GPU can have in flight at the same time:
| Fermi | GT200 | G80 | |
| Max Threads in Flight | 24576 | 30720 | 12288 |
Fermi can't actually support as many threads in parallel as GT200. NVIDIA found that the majority of compute cases were bound by shared memory size, not thread count in GT200. Thus thread count went down, and shared memory size went up in Fermi.
NVIDIA groups 32 threads into a unit called a warp (taken from the looming term warp, referring to a group of parallel threads). In GT200 and G80, half of a warp was issued to an SM every clock cycle. In other words, it takes two clocks to issue a full 32 threads to a single SM.
In previous architectures, the SM dispatch logic was closely coupled to the execution hardware. If you sent threads to the SFU, the entire SM couldn't issue new instructions until those instructions were done executing. If the only execution units in use were in your SFUs, the vast majority of your SM in GT200/G80 went unused. That's terrible for efficiency.
Fermi fixes this. There are two independent dispatch units at the front end of each SM in Fermi. These units are completely decoupled from the rest of the SM. Each dispatch unit can select and issue half of a warp every clock cycle. The threads can be from different warps in order to optimize the chance of finding independent operations.
There's a full crossbar between the dispatch units and the execution hardware in the SM. Each unit can dispatch threads to any group of units within the SM (with some limitations).
The inflexibility of NVIDIA's threading architecture is that every thread in the warp must be executing the same instruction at the same time. If they are, then you get full utilization of your resources. If they aren't, then some units go idle.
A single SM can execute:
| Fermi | FP32 | FP64 | INT | SFU | LD/ST |
| Ops per clock | 32 | 16 | 32 | 4 | 16 |
If you're executing FP64 instructions the entire SM can only run at 16 ops per clock. You can't dual issue FP64 and SFU operations.
The good news is that the SFU doesn't tie up the entire SM anymore. One dispatch unit can send 16 threads to the array of cores, while another can send 16 threads to the SFU. After two clocks, the dispatchers are free to send another pair of half-warps out again. As I mentioned before, in GT200/G80 the entire SM was tied up for a full 8 cycles after an SFU issue.
The flexibility is nice, or rather, the inflexibility of GT200/G80 was horrible for efficiency and Fermi fixes that.
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PorscheRacer - Thursday, October 1, 2009 - link
I have no clue what the red rooster thing implies, and I never understood why people called nVIDIA the green goblin. Until now. You, sir, have made it clear to me. They are called the green goblin, because that's where the trolls come from. Like wow. Your partisan and righteous thinking has no merit, no basis except conjecture and criticism. Save a keyboard, chill out and let's see if you can post anything in here without using the words, nVIDIA, ATI, red rooster, green goblin, and anything with ALL CAPS.It's fine to be passionate about something. But to exessive extents that push everyone else away and leave people ashamed, discouraged and embarrased; that's not how to win hearts and minds. I can already see you getting riled up over this post telling you to chill out....
SiliconDoc - Friday, October 2, 2009 - link
Hmmmm, that's very interesting. First you go into a pretend place where you assume green goblin is something "they call" nVIDIA, but just earlier, you'd never seen it in print before in your life.Along with that little fib problem, you make the rest of the paragraph a whining attack. One might think you need to settle down and take your own medicine.
And speaking of advice, your next paragraph talks about what you did in your first that you claim noone should, so I guess you're exempt in your own mind.
kirillian - Thursday, October 1, 2009 - link
Yall...seriously...leave the poor NVidia Fanboy alone. His head is probably throbbing with the fact that he found his first website (other than HardOCP) that isn't extremely NVidia biased.SiliconDoc - Friday, October 2, 2009 - link
Gee, I find that interesting that you know all about bias at other websites...So that says what again about here ?
silverblue - Thursday, October 1, 2009 - link
The 5870 is but one single GPU. The 295 is two and costs more. The 4870X2/CF is also a case of two GPUs. A 5870X2 would annihilate everything out there right now, and guess what? 5870 CF does just that. If money is no object, that would be the current option, or 5850s in CF to cut down on power usage and a fair amount of the cost without substantially decreasing performance.By stating "if someone wants to get their next-gen performance now", of course he's going to point in the direction of ATI as they are the only people with a DX11 part, and they currently hold the single GPU speed crown. This will not be the case in a few months, but for now, they do.
SiliconDoc - Friday, October 2, 2009 - link
I kinda doubt the 5870x2 blows away GTX295 quad, don't you ?--
Now you want to whine cost, too, but then excuse it for the 5870CF. LOL.
Another big fat riotous red rooster.
Really, you people love lies, and what's bad when it's nvidia, is good when it's ati, you just exactly said it !
ROFLMAO
--
Should I go get a 295 quad setup review and show you ?
--
How come you were wrong, whined I should settle down, then came back blowing lies again ?
There's no DX11 ready to speak of, so that's another pale and feckless attempt at the face save, after your excited, out of control, whipped up incorrect initial post, and this follow up fibber.
You need to settle down. "I want you banned"
Finally, you try to pretend you're not full of it, with your spewing caveat of prediction, "this will not be the case in a few months" - LOL
It's NOT the case NOW, but in a few months, it sure looks like it might BE THE CASE NO MATTER WHAT, unless of course ati launches the 5870x2 along with nvidia's SC GT300, which for all I know could happen.
So, even in that, you are NOT correct to any certainty, are you...
LOL
Calm down, and think FIRST, then start on your rampage without lying.
silverblue - Friday, October 2, 2009 - link
My GOD... you're a retard of the highest order.Why would I want to compare a dual GPU setup with an 8 GPU setup? What numpty would do that when it would logically be far faster? Even a quad 5870 setup wouldn't beat a quad 295 setup, and you know what? WE KNOW! 8 cores versus 4 is no contest. Core for core, RV870 is noticeably faster than the GT200 series, but you're the only person attempting to compare a single GPU card to a dual GPU card and saying the single GPU card sucks because it doesn't win.
And where did I say "I want you banned"? As someone once said, "lay off the crack".
SiliconDoc - Friday, October 2, 2009 - link
Aren't you the one who claimed only ati for the next gen performance ?Well, you really blew it, and no face save is possible. A single NVIDIA card beats the best single ati card. PERIOD.
It's true right now, and may or may not change within two months.
PERIOD.
silverblue - Friday, October 2, 2009 - link
No, I said that ATI currently has the single GPU crown. Not card - GPU. In a couple of months, ATI may have the 5870X2 out, and that WILL send the 295 the way of the dodo if it's priced correctly.No face saving necessary on my part.
Zaitsev - Wednesday, September 30, 2009 - link
^^LOL. I don't see what all the bickering is about. If you're willing to wait a few more months, then you can buy a faster card. If you want to buy now, there are also some nice options available. Currently there are 5 brands of 5870's and 1 5850 at the egg.