Is 800 MHz FSB required to fully utilize dual channel DDR400 ?

[itachi]

this topic is kinda old.. oh well..

Originally posted by: user1234
Thank you ! now we're in agreement. My follow-up question is "does Athlon XP 3200+ have 800 MHz FSB, and if not isn't dual channel wasted on the Athlon XP ?" Anyone (who has a clue) ?

as was already stated, the athlon xp doesn't have an 800 ddr fsb.. the fsb is still 400. however, dual-channel isn't wasted..

to fully understand why, you'd have to know more about alpha's ev6 bus.. something that i dont' know much about..

memory can only support 1 operation at a time.. so, assume that all operations always take 20T. with single-channel, if you perform an operation.. the next operation you perform on the memory will take another 20 cycles.. with dual-channel, the operation you wanted done after the first one will be finished in 1 clock cycle.

pretty big generalization.. but u get the idea. if memory ran with absolutely no latency, a dual-channel config would be pointless for an athlon xp.

 

[user1234]

itachi, I don't really know what you are talking about. Dual channel really does double the bandwidth of the memory channel, but the Athlon XP can only read at half this bandwidth. But the extra bandwidth is not completely wasted because other components in the system, besides the CPU, may also access the memory directly (DMA) - for example disk drives. So the dual channel still helps a little.

So based on the above, my conclusion is that a dual channel athlon XP will not run slower even if slower memory is used, because the memory channel is twice the FSB bandwidth to begin with. Therefore, you could potentially use dual PC2100 sticks (DDR266) which run at 2/3 the speed of the FSB (400MHz), but suffer no slowdown because the memory channel bandwidth is still higher than the FSB's bandwidth (266 x 128bit > 400 x 64bit) !!!

If this is true, it's great news for people which still have old PC2100 modules lying around. I'm going to try this on my Athlon XP system by setting the memory clock to 2/3 of the FSB and measuring the performance to see if there is any significant decrease.

 

[batmanuel]

Originally posted by: user1234

itachi, I don't really know what you are talking about. Dual channel really does double the bandwidth of the memory channel, but the Athlon XP can only read at half this bandwidth. But the extra bandwidth is not completely wasted because other components in the system, besides the CPU, may also access the memory directly (DMA) - for example disk drives. So the dual channel still helps a little.

So based on the above, my conclusion is that a dual channel athlon XP will not run slower even if slower memory is used, because the memory channel is twice the FSB bandwidth to begin with. Therefore, you could potentially use dual PC2100 sticks (DDR266) which run at 2/3 the speed of the FSB (400MHz), but suffer no slowdown because the memory channel bandwidth is still higher than the FSB's bandwidth (266 x 128bit > 400 x 64bit) !!!

If this is true, it's great news for people which still have old PC2100 modules lying around. I'm going to try this on my Athlon XP system by setting the memory clock to 2/3 of the FSB and measuring the performance to see if there is any significant decrease.

Not sure if there there is a similar performance hit with the Via dual channel chipset, but I know that with the nForce 2 boards, there is a pretty decent performance hit when you run the RAM and FSB out of sync (caused by the extra latency incurred by running async, I believe), more than enough to make up for the bandwidth gained by running the PC2100 in DC mode.

 

[user1234]

On my Athlon XP system (Barton 2500+ on Shuttle AN35N Ultra mobo and 2x256MB dual channel PC3500) which is runnig at 210 MHz FSB x 11 = 2310 MHz, I wanted to test the theory that it may be possible to run the memory at lower speed without taking a performance hit since the bandwidth provided by the dual channel memory is anyway higher than the FSB's bandwidth. So I changed the memory to run at 66% the FSB which came to 140 MHz x 2 = DDR280, and then I ran a few benchmarks. In Aquamark the overall score has fallen by only 5.5%, and in 3dmark05 the score fell by 2%. So lowering the memory speed by 33% would only lower gaming performance by 5% or less, assuming you run in dual channel mode. I think that means that it may be cost effective for people which already own PC2100 memory to use them instead of buying new PC3200 modules.

 

[Pavica]

Ok guys, calm down. This is the way it works: In pre-Athlon 64 and Intel Motherboards the FSB connects the CPU to the Northbridge chipset. In these motherboards the Northbridge chipset also houses the memory controller. Keep in mind that the mentioned components, although they are 32-bit, have 64-bit host bus widths. Now, if the CPUs link to the Northbridge (FSB) is 400mhz (200mhz x 2) in the case of the Athlon XP, you can do the simple math to get the maximum theoretical memory bandwidth.

Follow very closely

200mhz + 2 bits per clock = 400mhz effective.
400mhz = 400 million clock cycles.
400 million clock cycles/second= 400,000,000hz
64 bits of data can be sent per hz.
400,000,000 x 64 = 25600000000 bits/second
There are 8 bits in a byte.
25600000000 bits divided by 8 = 3200000000 bytes/second
There are 1024 bytes in a Kilobyte.
Divide this by 1024 to get KB/sec.
3200000000 divided by 1024 = 3125000
There are 1024KB in a Megabyte.
Divide this by 1024 to get MB/sec.
3125000 divided by 1024 = 3051MB/sec.

Therefore, the maximum theoretic memory bandwidth of an Athlon XP CPU with 400FSB is 3051MB/sec.

DDR400 (PC3200) RAM works like this:
200mhz x 2 = 400mhz effective
Same math applied for the CPU because DDR is also 64 bits wide.
DDR400's max theoretical bandwidth is 3051MB/sec. (PC3200 is close )

What Dual channel DDR does is this:
The two 64-bit "Channels" of RAM each has its own 64-bit memory controller.
These controllers use a system of interleaving to work together.
So.....

200mhz x 2 = 400mhz effective --> Dual 64-bit memory --> 64-bit x 2 = 128-bit effective.
All of the math in the above is applied here, but 64 is doubled to 128.
This gives you a maximum theoretical bandwidth of 6102MB/sec.

Because this is higher than the maximum communication speed from an Athlon XP to the memory controller, theoreticly, you see no performance increase.
But you do! Madness you say? Impossible?
No, just a little thinking thats all.
Explained:

No memory controller is 100% efficient, hence the frequent usage of the word "theoretical speed".
Therefore 3051MB/sec RAM will only run at about 2600~2900MB/sec.
So, for Athlon XPs, dual channel memory increases the effeciency, and basicly brings the RAM up to normal speed.
So yes, there is a benifit.
And no, its not noticable.

 

[user1234]

Pavica, thanks for the detailed info, note that this is the same conclusion we arrived at in previous posts - that is, the dual channel memory bandwidth is 6.4 GB/s which is twice the FSB's bandwidth, so the dual channel config only helps to increase the utilization of the FSB by a little, and also accomodate DMA requests by other devices. But overall, it's a small benefit.