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Topic Title: "Opteron FSB runs at the speed of the processor"?
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Created On: 12/20/2004 08:47 PM
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 12/20/2004 08:47 PM
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MajorBloodnok
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AMD state that PC3200 memory should be used for highest bandwidth between DIMMs and Opterons:

http://www.amd.com/us-en/Proce...oduc...96_8805,00.html' ">http://www.amd.com/us-en/Proce..._118_8796_8805,00.html

No problem there but elsewhere:

http://www.amd.com/us-en/Proce...oduc...ml?redir=CPOS12' ">http://www.amd.com/us-en/Proce...9,00.html?redir=CPOS12

they state, "The front side bus (interface to memory) of the AMD Opteron™ processor runs at the speed of the processor".

Given that PC3200 runs at 400 MHz and dual channel makes that 800 MHz, that doesn't fit with Opteron frequencies of 1.4 - 2.4 GHz.

Obviously I'm missing something here and would be grateful if someone could explain.

 12/21/2004 01:37 AM
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Black No.1
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Allrighty... here we go...
First off, you need to know how DDR works. The communications bus to the RAM is operating at 200 Mhz, (what?!?.. it's true.) The way you get 400 MHz is the same way you get AGP x 2. Instead of sending 1 bit of information on the enire cycle of a wave... Information is passed at the rising AND falling edge. This makes it SEEM like 400 MHz, but you are still at 200 MHz operating at twice the efficiency. Here is a good link for details. How DDR works' ">http://www.ocfaq.com/article.php/overclocking/62
Now DDR2 (like AGP 4x) transfers data at the 3/4 rise, peak, 3/4 fall, 1/4 fall, trough (bottom) and 1/4 rise. This RAM is STILL at 200 MHz, but it quadruples the efficiency. Dual channel is like making a RAID 0 for the RAM.
Ok, so now that we know this RAM works at 200 MHZ, now to answer your question.

The Hammer's, like all processors, are made up of more then just a Processing Unit. You have a hyper transport interface, Northbridge, Control Logic, JTAG Debug and some other options. This is why you have a Clock multiplier!!!
The only reason the CPU core and the L1/L2 cache can operate in the 2+ GHz range is because they are so close together and quality/uniformity can be controlled by processor manufacturers to push the CPU cores to the edge.

Here is a link to AMD's site on there 940 layouts. (The 939's have the same Core). Look on Page 9.
Core Layout. (Go to page 9)' ">http://www.amd.com/us-en/asset...nd_tech_docs/31412.pdf

Plus the data paths are so small on the CPU that internal radio frequency interfearance isn't an issue. You try to push that frequency through circuit paths on a motherboard that are allmost 1000000 times wider, the RFI would be so bad you could probably cook your neighbors potatoes. And these are the data paths that the CPU must TALK to. A non uniform, big fat and lazy pipeline connecting PCI, AGP, IDE and a whole bunch of other abbreviated name components together..

Lets pretend frequency is water pressure, and bandwith is how wide your hose is.
In both instances you want to get something from point A to point B. You can get the same ammount of water at 100 PSI through a 1/2" hose as you can with a 50 PSI 1" hose. The memory controller dosn't need to run greater then 200 MHz because it uses bandwith, not frequency to pipe data.

In a nut shell, you are not missing something. About 1/2 of the processor operates at 200 MHz, and the rest of the processor operates at whatever it's rated to.. ie 1.4-2.6 GHz.


-------------------------
Dual AMD MP 2800+ OC'd to 3.08 (140 x 22) Dual Vapochill
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1024 MB Kingston DDR Reg ECC 3200
Radeon 9800 128 Vram
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 12/21/2004 10:55 AM
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MajorBloodnok
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QUOTE (Black No.1 @ Dec 20 2004, 10:37 PM)Allrighty...  here we go...

Wow, 8080, thanks a lot for taking the time to provide such an informative response...unfortunately, I still don't get it!

I can see from page 14 of the data sheet that a 13 x clock multiplier can match 200 MHz DDR to 2.6 GHz processor core but I cannot reconcile this with bandwidth in my little brain!

Presumably, DDR400 effectively pumps data over 128-line bus at 400 MHz, i.e. the equivalent of 800 MHz over a 64-line bus. Since Opterons are 64-bit processors then the 800 MHz rating is the one that matches the chip's proclaimed bit-size.

So, for that data rate to feed into a 2.6 GHz core we would have to see the width of the data path shrink more that three-fold. That's the bit I don't fully appreciate. You cannot jump from 800 MHz to 2.6 GHz and maintain the same bandwidth so it seems erroneous (or downright misleading) for AMD to claim that, "The front side bus (interface to memory) of the AMD Opteron™ processor runs at the speed of the processor".

Isn't this effectively like squishing the nominal 64-line bus down to about 20-line?

I'm sure I'm missing something!
 12/21/2004 04:58 PM
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Black No.1
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Well, you are getting the info a little mixed here....

What used to be the front side bus in no longer what we think of. The FSB is on the CPU now. But most people refer to the FSB meaning the speed that the RAM operates at.
True you have a 64 bit processor, but you have a 128 bit memory bandwidth. (Check page 9 again and look at the DDR SDRAM INTERFACE.)
If you look at URL=http://www.amd.com/us-en/Proce..._118_8796_8799,00.html]AMD Proces mem spec[/URL] it says at the bottom "+The front side bus (interface to memory) of the AMD Opteron™ processor runs at the speed of the processor." Means that the 800 MHZ pumping in @ 128 bit multiplied by the Core (2.4 GHz) gives you your 19.2 GB/s x-fer rate.

Now the 2 processor system can access at 12.8 GB/s because again, the processor has it's own FSB/Memory controller. If you are looking at Intel's Procs, even there very $$$ ones, the don't have this. The memory controller is still on the mobo. What this means is no biggie when you have 1 proc, but when you have 8 processors on one board pumping 51.2 GB/s bandwidth while an 8 proc Intel motherboard would
still be doing only 6.4 GB/s.

You are correct meaning that it can't keep the same bandwidth! NO PROCESSOR CAN!, at least not since the 486 DX 40 MHz. Memory has not spead up as quickly as processors have. This is why we need gobs of bandwidth to satisfy a hungry processor.

In a nut shell, this is not false advertizing, you just have to re-think where the FSB is. Cause it ain't on the mobo no mo fo AMD!

BTW, my handel is Black No.1, not 8088. (Honest misteak.)

-------------------------
Dual AMD MP 2800+ OC'd to 3.08 (140 x 22) Dual Vapochill
Gigabyte GA-7DPXDW+
1024 MB Kingston DDR Reg ECC 3200
Radeon 9800 128 Vram
2 x Maxtor 80 Gig ATA 133 8 M cache RAID 0
2 x Seagate 'cudas 80 Gig ATA 133 8 M cache RAID 0 (Swap + Junk)
1 Seagate 'cuda 80 Gig file storage
 12/21/2004 05:32 PM
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MajorBloodnok
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QUOTE (Black No.1 @ Dec 21 2004, 01:58 PM)the 800 MHZ pumping in @ 128 bit multiplied by the Core (2.4 GHz) gives you your 19.2 GB/s x-fer rate

Hi Black No.1 (sorry about that 80xx thang!

I saw the 128-bit memory bus on page 9 and I realize that the multi-Xeon architecture is poor (if AMD could ramp up Opteron production they would surely demolish Intel--shares in AMD fell today after the Prudential said that AMD wouldn't be able to produce enough chips to sell in 2005.). I also know that the Opteron connects directly to memory due to its on-chip memory management logic.

I still feel like a twit though!

I can see where the 6.4 Giga bytes per second figure comes from (I think):
DDR400 at (an effective) 400 MHz over a 128-bit bus transfers:

400,000,000 x 128 bits per second

i.e. 51.2 Giga bits per second or 6.4 Giga bytes per second.

I don't see the justification for multiplying by 2.4 GHz comes in unless you are saying that in 2.4 GHz Opteron systems DDR400 runs at 2.4 GHz which I find hard to believe (I don't see DDR400 all of a sudden being capable of running at 2.4 GHz).


 12/21/2004 05:51 PM
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Black No.1
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The justification like I said has no real value on a 1 cpu system. Don't forget, the Hammer's were made to be Dual core server chips. If AMD didn't design the FSB on the core, and you have more then 2 procs on one board, your system would choke. Look at the advantage, instead of running 8 blade servers to satisfy 54 GB /s bandwidth (About $30,000+), you can get 1 mobo with 8 procs serving 54 GB /s for about $15,000.
I'm telling you, AMD really did a good job thinking ahead on these chips.

That's why my next system will be a dual Opteron as soon as I finish this little minor detail. Check it out.

My Opteron unlock' ">http://forums.amd.com/index.ph...3427&st=0&#entry293395

-------------------------
Dual AMD MP 2800+ OC'd to 3.08 (140 x 22) Dual Vapochill
Gigabyte GA-7DPXDW+
1024 MB Kingston DDR Reg ECC 3200
Radeon 9800 128 Vram
2 x Maxtor 80 Gig ATA 133 8 M cache RAID 0
2 x Seagate 'cudas 80 Gig ATA 133 8 M cache RAID 0 (Swap + Junk)
1 Seagate 'cuda 80 Gig file storage
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