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 - Friday, October 2, 2009 - link
Yeah, of course, 3 million T core, and it doesn't need much bandiwth, and it won't be used, perhaps, or probably, because the GPU designers haven't a clue, and of course, you do.---
Another amazing clown with a red nose. You people really should stop typing stupid stuff like that.
Calin - Sunday, October 4, 2009 - link
There was no performance improvement from increasing the bandwidth of the Athlon64 processors due to the move to DDR2 memory (a theoretical doubling of performance, with about one and a half more measured bandwidth in the first generation of the processor).I might not have a clue, but do you?
SiliconDoc - Wednesday, September 30, 2009 - link
Very interesting of course, and so with your theory, it could also be LOWER, with slower ddr5, but the fact REMAINS, 240 has ALREADY BEEN LEAKED.so we know what it is, and Anand KNOWS IT'S MORE, BUT NOT DOUBLE AND SAYS SO !
---
SO WHAT WE HAVE IS A CONVENIENT COVER UP. PERIOD!
--
JUST NOT AS STUPID AS YOU, THAT'S ALL.
dragunover - Thursday, October 1, 2009 - link
"SO WHAT WE HAVE IS A CONVENIENT COVER UP. PERIOD!--
JUST NOT AS STUPID AS YOU, THAT'S ALL. "
I disregarded anything you said before and after that.
SiliconDoc - Friday, October 2, 2009 - link
I'm certain you attempted it, as you no doubt love to wallow in blissful ignorance and denial just by mere unchangeable habit.But having read it, you have nowhere to go. Your mind is already irreparably altered. Congratulations.
jonGhast - Wednesday, September 30, 2009 - link
True, you seem to be a whole new level of stupid.mapesdhs - Wednesday, September 30, 2009 - link
I have a theory on that one: faulty keyboard. Every time he hits Shift
to get upper case, his keyboard is zapping out brain cells with EMF bursts. :D
Best to just ignore & not reply IMO.
Ian.
SiliconDoc - Thursday, October 1, 2009 - link
Well you can't ignore the discussion, and as far as that goes, you want everyone else to do your will, as you beg for it, with your empty advice, which is by the way, all you provided in the last thread.So you quack around telling others not to participate. That's your whole gig mary.
I find the stupidity level amazing, as most of you can only spew in and beg eachother not to comment, and by the quality of the comments that actually try a counter, I certainly cannot blame you, for begging others to surrender ahead of time.
You notice how many new names are here ? lol
It's clear who and what it is that responds, and what level of conduct they are all about.
Now let's see the Sybillic mapes sign off on his/her own glorious advice, which she/he failed to follow already.
Silverel - Thursday, October 1, 2009 - link
Hey buddy. Take a pill and relax.Your product isn't on the market yet. There's no recommendation to be heeded. It doesn't matter how fast it is, you can't have one. No one can. If someone wants to get their next-gen performance on now, ATI is what you'll buy.
It's okay though. Not everyone is going to run out and grab an ATI card. There's plenty of people that will wait for hell to freeze over for nVidia to release a new card. Personally, I'll take the Q1 2010 release date as semi-fact, but hell freezing over is my fallback date.
So feel free to continue ranting like you've run out of meds. Insult anyone who has made a comment contrary to your own. It's not really doing you any favors, but that's okay. I'm sure it's having an effect on the opinions of people sitting on the fence. Ah, and just to dig the finger in the wound, you can consider me an ATI shill, just for good measure.
SiliconDoc - Thursday, October 1, 2009 - link
Gosh you';re a sweetheart, too, and wrong ! WRONG ! WRONG !the red bloviator> " . If someone wants to get their next-gen performance on now, ATI is what you'll buy. "
Gee, did you lose it that easily, was your heart beating so hard, were you sweaty, and upset, and out of control, and decided it was great to tell me to settle down, when your brain was on FART ?
Please see the GTX295 that BEATS the 5870. The 5870 is PRIOR GEN performance - as ati's own 4870x2/or CF is equal.
Golly, what a deal, another red rooster massive ruse.