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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Dobs - Thursday, October 1, 2009 - link
I'm with the zorro - will be setting this up for my son pretty soon - he is an extreme gamer who has mentioned multiple monitors to me a few times over the last few months. Up until now I only had a vague idea on how I could accommodate his desire.... that has all changed since the introduction of Eyefinity.Finally - Thursday, October 1, 2009 - link
..pussy-whipped by your son?the zorro - Thursday, October 1, 2009 - link
moron, i am going to buy two more monitors and then... eyefinity.chizow - Wednesday, September 30, 2009 - link
Nvidia didn't mention anything about multi-monitor support, but today's presentation wasn't really focused on the 3D gaming market and GeForce. They did spend a LOT of time on 3D Vision though, even integrating it into their presentation. They also made mention of the movie industry's heavy interest in 3D, so if I had to bet, they would go in the direction of 3D support before multi-monitor gaming.It wouldn't be hard for them to implement it though if they wanted to or were compelled to. Its most likely just a simple driver block or code they need to port to their desktop products. They already have multi-monitor 3D on their Quadro parts and have supported it for years, its nothing new really, just new on the desktop space with Eyefinity. It then becomes a question if they're willing to cannibilize their lucrative Quadro sales to compete with AMD on this relatively low-demand segment. My guess is no, but hopefully I'm wrong.
Dobs - Thursday, October 1, 2009 - link
I think Nvidia are underestimating the desire and affordability for multi-monitor gaming. Have you seen monitor prices lately? Have you seen the Eyefinity reviews?By not making any mention of it is a big mistake in my book. Sure they can do it, but it will reduce there margins even further since they obviously hadn't planned spending the extra dollar$ this way.
I do like the sound of the whole 3D thing in the keynote though... and everyone wearing 3D glasses...(not so much). But it will be cool once the Sony vs Panasonic vs etc.? 3D format war is finished, (although it's barely started) so us mainstream general consumers know which 3D product to buy. Just hope that James Cameron Avatar film is good :)
chizow - Thursday, October 1, 2009 - link
Yeah I've seen the reviews and none seemed very compelling tbh, the 3-way portrait views seemed to be the best implementation. 6-way is a complete joke, unless you enjoy playing World of Bezelcraft? There's also quite a few problems with its implementation as you alluded to, the requirement of an active DP adapter was just a short-sighted half-assed implementation by AMD.As Yacoub mentioned, the market segment for people interested or willing to invest in this technology is so ridiculously small, 0.1% is probably pretty close to accurate given multi-GPU technology is estimated to only be ~1% of the GPU market. Surely those interested in multi-monitor is below that by a significant degree.
Still for a free feature its definitely welcome, even in the 2D productivity sense, or perhaps for a day trader or broker....or anyone who wanted to play 20 flops simultaneously in online poker.
Dobs - Thursday, October 1, 2009 - link
Lol @ 20 flops simultaneously in online poker. I struggle with 4 :)Agree with 6 monitor bezelcraft - Cross hair is the bezel :)
I guess I'm lucky that my son is due for a screen upgrade anyhow so all 3 monitors will be new. Which one will be the problem - I hear Samsung are bringing out small-bezel monitors specifically for this, but I probably can't wait that long. (Samsung LED looks awesome though) I might end up opting for 3 of Dell's old (2008) 2408WFP's (my work monitor) as I know I can get a fair discount for these and I think they have DisplayPort. I'm not sure if my son will like Landscape or Portrait better but I want him to have the option... and yeah apparently the portrait drivers are limited (read crap) atm.
Appreciate your feedback as well as your comments on the 5850 article... I actually expected the GT prices to be $600+ not the $500-$550 you mentioned. Oops... rambling now. Cheers
chizow - Thursday, October 1, 2009 - link
Heheh I've heard of people playing more than 20 flops at a time....madness.Anyways, I'm in a similar holding pattern on the LCD. While I'm not interested in multi-monitor as of now, I'm holding out for LED 120Hz panels at 24+" and 1920. Tbh, I'd probably check out 3D Vision before Eyefinity/multi-monitor at this point, but even without 3D Vision you'd get the additonal FPS from a 120Hz panel along with increased response times from LED.
If you're looking to buy now for quality panels with native DP support, you should check out the Dell U2410. Ryan Smith's 5870 review used 3 of them I think in portrait and it looked pretty good. They're a bit pricey though, $600ish but they were on sale for $480 or so with a 20% coupon. If you called Dell Sm. Biz and said you wanted 3 you could probably get that price without coupon.
As for GTX 380 price, was just a guess, Anand's article also hints Nvidia doesn't want to get caught again with a similar pricing situation as with GT200 but at the same time, relative performance ultimately dictates price. Anyways, enjoyed the convo, hope the multi-mon set-up works out! Sounds like it'll be great (especially if you like sims or racing games)!
RadnorHarkonnen - Thursday, October 1, 2009 - link
Eyefinity is screaming for DIY.Bezel Craft can be easly avoided. Just tear you monitor apart. A stand for 3 monitors is easly ordered/DIY made. Ussually the bezel is way thicker than it need to be.
Unfortunely i alreayd have a 4850 CF that i will keep for more a year or two and let the tecnology mature for now.
wifiwolf - Wednesday, September 30, 2009 - link
Can we have a new feature in the comments please?I just get tired of reading a few comments and get bugged by some SiliconDoc interference.
Can we have a noise filter so comments area gets normal again.
Every graphics related article gets this noise.
Just a button to switch the filter on. Thanks.