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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SiliconDoc - Thursday, October 1, 2009 - link
Yes that is quite disappointing, but the 3 million transistor count and ddr5 somewaht makes up for it, and the fact that we're told by the red roosters that even 153 bandwith is plenty or just a tiny bit shy, and with what it looks like the 384 bit ddr5 4000 or 4800 data GT300 will come in at 192 bandwith minimum, or more likely 240 bandwith, quite a lot higher than 153.6 for ati's 5870.---
So really, what should I believe now, 153 is really plenty except in a few rare instances, or what you say LOL @ Trolls should believe, that 192 or 240 is worse than 153 ?
--
You might LOL @ TRolls, but from my view, you just made an awful fool of yourself.
HINT: The ati 5870 is only 256bit, not 384, and not 512.
Now, look in the mirror and LOL.
sigmatau - Thursday, October 1, 2009 - link
I know the ATI cards have 256 bit connections dumb ass. I'm just using your logic (or lack of it.) ATI has been able to outperform Nvidia cards with their 256 bit connections so your point about bandwidth is meaningless idiot.Now go pull that G295 out your ass, ok?
SiliconDoc - Thursday, October 1, 2009 - link
Golly, that ddr5 has nothing to do with bandwith, right, you stupid idiot ?--
Talk to me about the 4850, you lame, near brain dead, doofus. It's got ddr3 on it.
---
See, that's the other problem for you idiotic true believers, NV is moving from 2248 data rate ram on up to 4800.
But you're so able to keep more than one tiny thought in your stupid gourd at once, you "knew that".
BTW, you're not doing what I'm doing, you're not capable of it.
Now that sourpussed last little whine by you, the 295, beats everything ati has, making your simpleton statement A BIG FAT JOKE.
moltentofu - Thursday, October 1, 2009 - link
My god silicondoc you aren't really succeeding here. At what purpose? To convince people not to buy ati cards? You are such a complete, massive ahole it makes me want to go out and buy ati cards in bulk just to spite you.I'm guessing that if nvidia PR ever watched you rant your all-caps rants they would politely request that you stop associating yourself with their product.
Go ahead everybody google "silicondoc" if you have a strong stomach. Talk about spreading yourself all over the tubes! This guy's fingerprint is unmistakeable. Looks like he got banned on the HondaSwap forums after 14 posts. Guess he sucks on every forum. Maybe anandtech could ban him?
SiliconDoc - Thursday, October 1, 2009 - link
I think every one of you, that instead of actually leaving me alone, or responding with a counter argument to my points, every one of you that merely got logged in, and ripped away at me with an insult ought to be BANNED.That's what really should happen. I make my complaints and arguments on article and cards and companies, and the lies I see about all those, and most, but not all of you, have no response other than a pure trolling, insulting put down.
Every single one of you that came in, and personally attacked me without posting a single comment about the article, YOU are the ones that need to be banned.
Your collective whining is pure personal attack, and instead of commenting on the article, your love or hate for it, you texted up and did one single thing, let loose a rant against me. Just because you could, just because you felt apparently, "it was taking the high road"... which is as ignorant as the lies I've pointed out.
Time for YOU PEOPLE to be banned.
(minus those of course that actually made counterpoints, wether or not they insulted me or complained when they did - because AT LEAST they actually were discussing points, and contributing to the knowledge and opinion groupings.
Like for instance Monkeypaw, who made a reply that wasn't a pure trolling hate filled diatribe like you just posted, having nothing to do with the article at all.
Take a look in the mirror then consider yourself fella.
bobvodka - Thursday, October 1, 2009 - link
In which case I request that YOU are banned for calling ME a liar when I did nothing beyond reply telling you how, on launch day, I ordered a HD5870 and had it the next day.SiliconDoc - Thursday, October 1, 2009 - link
Oh you're full of it again, you pretended your view is the world, and therefore lied your butt off, and in a smart aleck fashion, PERIOD, pretending everyone doesn't know a few trickled out, which if you had clue one, you'd KNOW I was the one who posted that very information on this site.Claiming anyting else, is plain stupid, smart alecky, and LYING.
Just because drunken bob got a card, he claims, on the morning, it shhipped that day, he had it immediately, and has been enojying it ever since, the whole world is satisfied with the paper launch, that "does not exist in bob's drunken vodka world" where who knows what day it is anyeway.
You know, you people are trash,and expecting anyone else to pretend you're not is asking for way too much.
shotage - Thursday, October 1, 2009 - link
SillyDuck - please tone it down. You're getting out of control again!tamalero - Thursday, October 1, 2009 - link
hu.. you'r the one insulting every people who doesnt share your opinion with your "RED ROOSTERS" and other stuff...you're really special Mr. Doc, but in the sad way.
SiliconDoc - Thursday, October 1, 2009 - link
What red roosters, there aren't any here I'm told. Just plain frank and honest people who tell the truth.So if I say red rooster, it cannot possibly mean anyone here, posting, lurking or otherwise, as I'm certain you absolutely know.
( not like coming down to your level takes any effort, there you are special, just for you, so you don't feeel so bad about yourself )