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Memory Speed - FSB Speed question

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December 27, 2004 8:58:01 PM

Hello all, having read a few of the related THG articles, there is a few question I'd like to ask about the Memory speed and the CPU's FSB speed. (Hope these question are not too stupid to ask :) 

1.
Using the latest p4 as an example, where the FSB is operating at 1066MHz (4*266 approx), is it better to use DDR2-533Mhz memory, than say, DDR2-333 ones? Because 533*2 = 1066
Is this kind of logic correct when aiming for optimization?

2.
Just another variation of question 1, with a P4 that operates the FSB at 800MHz, is it better to use DDR2-400, than say, DDR2-533, even though DDR2-533 has a higher speed? Because of the same reason as above, that 2*400 = 800, matching the FSB speed.
Again, is this kind of logic correct when aiming for optimization?

And finally....

3.
With the Latest Athon chipsets supporting 1GHz HyperTransport, instead of having two variables like in the P4 examples, there is now three!
With Intel:
- CPU FSB speed
- Memory speed

With Athlon:
- CPU FSB speed - which at the moment is 400MHz (2*200)
- HyperTransport speed - which at the moment is 1GHz
- Memory speed

Assuming my logic for optimization from question 1 & 2 is correct (otherwise I'll just look real stupid :p ), what is the best memory speed for an Athlon FX-55? I was thinking:

Would it be DDR400? (to match the 400Mhz FSB)

Or would it be DDR500? (to match the 1GHz HyperTransport)


Thankyou for your time :) 

Note: question 1 & 2 assumes the usage of DDR2, where in question 3, the Athlon64 FX-55 could only work with DDR (at the moment).
December 27, 2004 9:11:18 PM

It's usually best to run FSB and memory in sync-> at the same frequency. Some appliciations might benefit from a memory speed that is higher than the FSB, others don't. It's best to test which setup delivers best performance for your needs but sync usually is the best.

For the Athlon 64, memory and Hypertransport (=FSB!) don't run in sync. The processor requests data form RAM at the core speed (because of integrated memory controller) and gets the data returned at memory speed (being DDR400). Hypertransport communicates with rest of the system, memory speed doesn't matter (there's no being "in sync", because of the integrated memory controler). For more infor, read the AMD-website.

As for your note: DDR2-533 hardly offers any advantages over DDR-400, that's for faster generations of DDR2.
December 27, 2004 9:39:11 PM

Thanks for your reply, having read what you said, does that mean my assumption for optimization for the P4s is correct?

To recap, I was assuming:

For a P4 with 800FSB, DDR2-400 is 'better' than DDR2-533 (because 400 and 800 is in 'sync'?)


As for the Athlon example, I can see now it doesn't have to 'sync' like in the P4 examples.
But I still need confirmations for the following assumptions:

With HyperTransport at 1GHz, and FSB 400MHz:

1. Even the performance gain is only very tiny, so tiny that it is not worth it, DDR-500 is still better than DDR-400.

2. Performance won't be affected as much in this Athlon setup when switching between DDR-400 and DDR-500.......

...than if you switch between DDR2-400 and DDR2-533 in a P4 1066FSB setup.


So in this reply, I need 3 confirmations in total.... hope I didn't make them sound too confusing, thanks again :) 
Related resources
December 27, 2004 10:09:31 PM

Well, as I said, the running in sync usually is better (faster). That means, it delivers better performance in most applications. However, applicitions that depend more on RAM-speed than CPU might benefit from the RAM running faster than the FSB. Test it, you know it. Of course there's a lot to do about this, running the memory at a lower frequency should let you choose lower latencies for the memory which improve performance while losing some speed (if MHz would say it all..). There really is no way to predict exactly what is best, test. It really depends on the chipset and memory type you use. Of course you can always use ddr2-533 and set it at 400 for future reasons.

On the Athlon: The faster the memory, the better the performance. The athlon64 really benefits from low latency and/or high speed memory modules. The relationship is quite logical in this case.
On the P4 with FSB 1066: I think DDR2-533 is the slowest kind of DDR2 available. The mainboard has to support this kind of memory. Recent benchmark didn't show much difference between DDR-400 and DDR2-533, however, I don't know if that was with a 1066 FSB, you'll have to look up those benchmarks yourself. DDR2 probably will be faster in the near future, at the moment the technology is young and far from being optimized. Right now DDR2 just is too expensive, for the same money you can buy a lot more speed with other parts than DDR2.

This also might get confusing, Tomshardware has some recent reviews on new Athlon and Intel systems which explain the architecture.
a b V Motherboard
a b } Memory
December 27, 2004 10:19:31 PM

OMG!
1.) There is no such thing as DDR2 333. 333MHz DDR2 is DDR2-667. DDR2-533 runs at 266MHz. Running faster memory doesn't have any affect on performance because you'll set the memory at the slower speed in BIOS.
2.) Same deal, DDR533 will run at DDR400 speed by default when using a QDR800 bus CPU.

The reason people buy faster RAM is for overclocking: It allows them to raise the RAM speed with the CPU bus speed while not overstressing the RAM.

3. DDR400 would be fine for the FX-55, but DDR500 would allow you room to overclock without reducing the memory speed ratio.

<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>
December 28, 2004 12:20:19 AM

Why did you stop that confusing thread??? It was quite amusing ..

-Always put the blame on you first, then on the hardware !!!
December 28, 2004 6:15:52 AM

ok I think I get it now, thanks everyone.
December 28, 2004 8:49:42 AM

I was just about thanking Crashman for shining some light cause I couldnt explain it anymore...

And I did never say ddr2-333.. Did I?
December 28, 2004 10:46:52 AM

DDR1 - JEDEC Specified up to PC3200(200Mhz*2), although up to PC4400 (275Mhz*2) is availiable esspecially for overclockers.

DDR2 - Availible in PC4300 and PC5400 forms, but due to lower latencies DDR2 performs a lot slower than DDR1 at similar clock speeds. According to anandtech PC4300 DDR2 will perform about the same as PC3400 DDR1.

Question 2 - For a P4 operating at 800Mhz FSB I would use DDR1 PC3200 in a dual channel configuration. There is no such thing as PC3200 DDR2. Even though you could use DDR2 at higher clock speeds (PC4300 for example) there wouldnt be a whole lot of point because the P4's 800Mhz bus would bottle neck the 1066Mhz memory bandwidth.

Question3 - The Athlon64 has an onboard memory controller that only supports PC3200 or below memory. It does not support DDR2 and cannot support higher speed DDR1 without being overclocked.
Quote:
CPU FSB speed - which at the moment is 400MHz (2*200)

Technically the Athlon64/fx does not have an FSB because it has no northbridge.
Instead it has a hypertransport link to the southbridge and a memory bus.
December 28, 2004 10:46:54 AM

DDR1 - JEDEC Specified up to PC3200(200Mhz*2), although up to PC4400 (275Mhz*2) is availiable esspecially for overclockers.

DDR2 - Availible in PC4300 and PC5400 forms, but due to lower latencies DDR2 performs a lot slower than DDR1 at similar clock speeds. According to anandtech PC4300 DDR2 will perform about the same as PC3400 DDR1.

Question 2 - For a P4 operating at 800Mhz FSB I would use DDR1 PC3200 in a dual channel configuration. There is no such thing as PC3200 DDR2. Even though you could use DDR2 at higher clock speeds (PC4300 for example) there wouldnt be a whole lot of point because the P4's 800Mhz bus would bottle neck the 1066Mhz memory bandwidth.

Question3 - The Athlon64 has an onboard memory controller that only supports PC3200 or below memory. It does not support DDR2 and cannot support higher speed DDR1 without being overclocked.
Quote:
CPU FSB speed - which at the moment is 400MHz (2*200)

Technically the Athlon64/fx does not have an FSB because it has no northbridge.
Instead it has a hypertransport link to the southbridge and a memory bus.
!