AMD Core Counts and Bulldozer: Preparing for an APU World
by Anand Lal Shimpi on November 30, 2009 12:00 AM EST- Posted in
- CPUs
Last week Johan posted his thoughts from an server/HPC standpoint on AMD's roadmap. Much of my analysis was limited to desktop/mobile, so if you're making million dollar server decisions then his article is better suited for your needs.
He also unveiled a couple of details about AMD's Bulldozer architecture that I thought I'd call out in greater detail. Johan has been working on a CMP vs. SMT article so I'll try to not step on his toes too much here.
It all started about two weeks ago when I got a request from AMD to have a quick conference call about Bulldozer. I get these sorts of calls for one of two reasons. Either:
1) I did something wrong, or
2) Intel did something wrong.
This time it was the former. I hate when it's the former.
It's called a Module
This is the Bulldozer building block, what AMD is calling a Bulldozer Module:
AMD refers to the module as being two tightly coupled cores, which starts the path of confusing terminology. A few of you wondered how AMD was going to be counting cores in the Bulldozer era; I took your question to AMD via email:
Also, just to confirm, when your roadmap refers to 4 bulldozer cores that is four of these cores:
http://images.anandtech.com/reviews/cpu/amd/FAD2009/2/bulldozer.jpg
Or does each one of those cores count as two? I think it's the former but I just wanted to confirm.
AMD responded:
Anand,
Think of each twin Integer core Bulldozer module as a single unit, so correct.
I took that to mean that my assumption was correct and 4 Bulldozer cores meant 4 Bulldozer modules. It turns out there was a miscommunication and I was wrong. Sorry about that :)
Inside the Bulldozer Module
There are two independent integer cores on a single Bulldozer module. Each one has its own L1 instruction and data cache (thanks Johan), as well as scheduling/reordering logic. AMD is also careful to mention that the integer throughput of one of these integer cores is greater than that of the Phenom II's integer units.
Intel's Core architecture uses a unified scheduler fielding all instructions, whether integer or floating point. AMD's architecture uses independent integer and floating point schedulers. While Bulldozer doubles up on the integer schedulers, there's only a single floating point scheduler in the design.
Behind the FP scheduler are two 128-bit wide FMACs. AMD says that each thread dispatched to the core can take one of the 128-bit FMACs or, if one thread is purely integer, the other can use all of the FP execution resources to itself.
AMD believes that 80%+ of all normal server workloads are purely integer operations. On top of that, the additional integer core on each Bulldozer module doesn't cost much die area. If you took a four module (eight core) Bulldozer CPU and stripped out the additional integer core from each module you would end up with a die that was 95% of the size of the original CPU. The combination of the two made AMD's design decision simple.AMD has come back to us with a clarification: the 5% figure was incorrect. AMD is now stating that the additional core in Bulldozer requires approximately an additional 50% die area. That's less than a complete doubling of die size for two cores, but still much more than something like Hyper Threading.
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Zool - Monday, November 30, 2009 - link
This is the K10 core from wikipedia with integer pipeline highlited (and other areas too) http://en.wikipedia.org/wiki/File:K10h.jpg">http://en.wikipedia.org/wiki/File:K10h.jpg .The 5% are is quite realistic if count in the shared L1 and L2 cache for one module.
psychobriggsy - Tuesday, December 1, 2009 - link
The L1 caches are duplicated however. Also the Load/Store units I presume, but maybe there is a way to share some resource there.What that diagram does show is that there are two 64-bit SIMDs (one of which can do x87) in K10 (not K10.5).
In Bulldozer there are two 128-bit SIMDS (that can also do FMA). I presume that they can each do x87 if they deign to lower themselves to the task.
That's why the FP performance has gone up. FMA counts as two operations when it comes to Linpack. :D FP is doubled compared to K10, even on a per-BDcore basis.
Will we refer to a Bulldozer module as K11?
GaiaHunter - Monday, November 30, 2009 - link
In the guy own wordshttp://forums.anandtech.com/showpost.php?p=2893509...">http://forums.anandtech.com/showpost.php?p=2893509...
[quote]I think the difference between 50% and 5% might be the difference between marketing and engineering. Engineers tend to be very literal.
If 2 cores get you 180% performance of 1, then in simple terms, that extra core is 50% that gets you the 80%.
What I asked the engineering team was "what is the actual die space of the dedicated integer portion of the module"? So, for instance, if I took an 8 core processor (with 4 modules) and removed one integer core from each module, how much die space would that save. The answer was ~5%.
Simply put, in each module, there are plenty of shared components. And there is a large cache in the processor. And a northbridge/memory controller. The dies themselves are small in relative terms.[/quote]
tatertot - Monday, November 30, 2009 - link
The guy is wrong, or his engineering team misunderstood.Moore (the lead designer) said about 50% increase to double the integer resources, L1D, etc. That sounds about right.
What I COULD believe is this:
Q: "If I took an 8 core processor (with 4 modules) and removed 1 integer core from ONE module, how much die space would that save?" A: 5%
In other words, if you removed them from all 4, you'd save 20%.
If you figure that the uncore takes up a bit more than half of the die, that would be totally consistent with Moore's 50% larger core figure.
For example (totally made up numbers):
Die size 300 mm2
uncore 160 mm2
4 BD modules 140mm^2
1 BD module 35 mm2
1 BD module without extra integer units: 23 mm2
(Savings from lopping 1 BD module: 12mm^2)
4 BD modules without extra integer units: 93 mm2
(Savings from lopping 4 BD modules: 47mm^2)
12/300 is 4%, which is what his engineers thought he was asking.
But really he was asking about 47/300 or ~16%.
So as stated, the 5% is wrong. It's the area cost of 1 of the module's extra int resources on a 4 module die. All 4 of them cost more.
And this would be consistent with Moore's estimate that relative to JUST the module, it is a 50% area increase.
psychobriggsy - Tuesday, December 1, 2009 - link
Thanks for doing the example maths.Yes, it looks like adding a single core to each module adds around 15%-20% to the die size of a dual-module/quad-core Bulldozer.
So 20% die space for 80% performance increase. Well, until you decide to make the L2 larger because there will be more contention for it.
Of course the 5% die area for SMT in Nehalem is negligable when you start factoring in the uncore portions as above...
GaiaHunter - Monday, November 30, 2009 - link
Meh, I miss-clicked and reported your post by mistake, sorry about that. :(Anyway, imagine a int core is 5% area of a total module and that a module size is 100 (size units not mm^2), so the int core size is 5. 4 modules will then be 400 and 4 int cores will be 20. 20 is 5% of 400, not 20%. Same for 8 modules.
You have to see, that they do need 50% more area to get 80% Int boost performance as they are using a second Int core to accomplish that. So the dedicated area of module to do Int operations is 2x the size of a regular Int core.
GaiaHunter - Monday, November 30, 2009 - link
Meh, I miss-clicked and reported your post by mistake, sorry about that. :(Anyway, imagine a int core is 5% area of a total module and that a module size is 100 (size units not mm^2), so the int core size is 5. 4 modules will then be 400 and 4 int cores will be 20. 20 is 5% of 400, not 20%. Same for 8 modules.
You have to see, that they do need 50% more area to get 80% Int boost performance as they are using a second Int core to accomplish that. So the dedicated area of module to do Int operations is 2x the size of a regular Int core.
HolKann - Monday, November 30, 2009 - link
Nah, you don't understand him. His assumptions are:1. one int core is about 5% of the whole die (including uncore).
2. one int core is about 50% of a module.
3. the uncore makes up about half of the core.
Put this in numbers:
Take a module as 100 size units. 4 modules means 400 size units, adding the uncore makes the size of the whole die 800. 5% of 800 is 40 size units. And tadaa, this makes an int core 40% of the size of a module ;) The number gets closer to 50% if one takes the uncore bigger.
If his assumptions are correct, a 25% total die increase (4*5% to 80%) results in 80% extra performance. This is about as good as Intel's 5% die increase for 15-20% extra performance (I know, this is a bold statement, a lot of unknown variables could alter this situation drastically).
GaiaHunter - Monday, November 30, 2009 - link
The data we have is: Removing 1 int core 5% from each module would result on 5% reduction of total die size. 1 int core = 50% of the total area dedicated to integer operations.So, for a total die of lets say 1000 units with 8 int cores, 4 int cores represent 5% of the total die size or 50 size units.
So each int core is 12,5 size units and the 8 int cores take 100 size units or 10% of the core.
Assuming sizes for total die size or what is the Bulldozer Module size relative to total size is pure speculation, as we don't have any numbers other than that JF affirmation.
To remember:
"What I asked the engineering team was "what is the actual die space of the dedicated integer portion of the module"? So, for instance, if I took an 8 core processor (with 4 modules) and removed one integer core from each module, how much die space would that save. The answer was ~5%. "
That was the affirmation.
In no way this contradicts the affirmation that AMD increased the Module area dedicated to integer operations by 50% to achieve 80% performance.
Main point is DIE SIZE ? BULLDOZER MODULE.
tatertot - Monday, November 30, 2009 - link
I am disputing the JF claim: " Removing 1 int core 5% from each module would result on 5% reduction of total die size. "I suspect that his engineers misunderstood his question, and it is actually the removal of the "extra core" from ONE BD module that would result in 5% overall die savings.
You can take it to the bank that Moore is correct that adding another integer execution unit group , L1D, etc to the core (thus making 2 cores, or a 'module') increased the size by 50%. Moore is the designer, not a marketing guy.
In order for Fruehe's claim to be correct, the uncore area would have to be VERY large:
Some more (different numbers):
Assume BD module is 30 mm2, (thus increased by 10 mm2, or 50% from 20 mm2 to add the second 'core', per Moore)
If 5% were actually the correct estimation of the area added for 4 BD modules (4 * 10 mm2 increase = 40 mm2 increase), then the overall die size would need to be... 800 mm^2.
This is nuts.
On the other hand, if "5% of the total die area" is the estimate of the space needed to add the integer resources to just 1 BD module, then the overall die can be 200 mm^2, so uncore 80 mm^2, 4 BD modules at 120 mm^2, and then Moore's numbers can be consistent with what JF heard back from the engineers.
So, my theory is that his engineers thought they were being asked how much of the total die (for a 4 BD module part) the increase in integer units to 1 BD module resulted in, while JF thought he was asking how much the increase to ALL 4 modules would be. This would be an easy misunderstanding to have, and I don't see another way to reconcile Moore's information (which I trust), with JF's claim.