Quick Dual channel DDR performance question

[ryandinan]

Hi guys,

I finally got another stick of Corsair XMS3200 (1024MB total now) to allow dual-channel on my P4P800 motherboard.
Now, since I only have a P2.4B (533MHz) running at 2.8GHz (620MHz), my motherboard only allows my memory to run on a 4:5 cpu/mem ratio - which translates to 387MHz DDR. In dual-channel mode, my theoretical bandwidth should be roughly twice that - 775MHz.
Now, my question is, since I'm not running on a 800MHz fsb, my theoretical memory bandwidth should be bottlenecked by my FSB (620MHz), correct?
Because I did some Sandra tests, and I'm only getting around 3898/3890MB/sec transfer rate. This is much better than a single stick of 3200 running at 400MHz, but I was expecting to see more. This performance limitation is due to my lower FSB, correct?

Also, another point to note, is that after the Sandra test, it reported only 79% efficiency for the memory. Is this also caused by the FSB bottleneck, or does this have to do with the dual-channel controller on my P4P800?

Thanks! I just want to make sure Im getting the most out of this rig!


-Ryan

[ewitte]

Originally posted by ryandinan

Now, my question is, since I'm not running on a 800MHz fsb, my theoretical memory bandwidth should be bottlenecked by my FSB (620MHz), correct?
Because I did some Sandra tests, and I'm only getting around 3898/3890MB/sec transfer rate. This is much better than a single stick of 3200 running at 400MHz, but I was expecting to see more. This performance limitation is due to my lower FSB, correct?

Seems kind of low. The theoretical max should be around 6.1GB/s. The lowest I've seen on my computer is 70% of theoretical. Which would be around 4200-4300MB/s. That is the lowest. With the CPU running faster than the memory I see more like 80% or 4800-4900MB/s if I were running at 387Mhz (actually about the same at 400Mhz). Is the MB pre 800Mhz or just the CPU? If the MB is pre 800Mhz that may be the problem.

Eric

[ryandinan]

The mobo can handle 800MHz FSB, but since I dont have a cpu that runs at 800MHzFSB, I can only run it as fast as my chip will overclock - which is 155MHz (x4 = 620MHz FSB).
Also, since running a 533MHz cpu (stock fsb), my mobo only allows cpu/mem ratios of 1:1 and 4:5. I'm obviously choosing to run at 4:5, as this gets me closer to my memory's rated 400MHz (386MHz, like I said).

I could see myself getting close to that 6.1GB/sec theoretical data rate only if I were running aa 800MHz cpu (because 400MHz DDR in dual channel fits perfectly at 800MHz).
Since my CPU can only take in data at 620MHz, it's bottlenecking my memory bandwidth accordingly... right?

I did some math - and if 6.1GB/sec is at 800MHz, I should expect to see 4.7GB/sec at my 620MHz fsb - BUT - Since my memory isn't running at a full 400MHz, I can probably knock that number down to 4.3B/sec or so. However, the lack of efficiency concerns me (78%), and may be the reason why my benchmark results were lower than expected (3898MB/sec).

Am I making any sense here?

-Ryan

[natgun]

yes your bandwidth is limited by the FSB of your chip, springdale and canterwood will only run memory at 333mhz with a 533mhz chip. for $175 why not pick up a p4 2.4c and use your memory to its full potential.

[ewitte]

Originally posted by ryandinan

I did some math - and if 6.1GB/sec is at 800MHz, I should expect to see 4.7GB/sec at my 620MHz fsb - BUT - Since my memory isn't running at a full 400MHz, I can probably knock that number down to 4.3B/sec or so. However, the lack of efficiency concerns me (78%), and may be the reason why my benchmark results were lower than expected (3898MB/sec).

Am I making any sense here?

-Ryan

Oops. I was completely only looking at the memory and not the CPU. While the memory can support a theoretical max of 6.1GB/s running dual channel at 387Mhz the CPU can not (full 6.4GB/s @ 400Mhz *2). The theoretical max at 620Mhz for the CPU is 4.96GB/s. Between 70-80% is about normal in terms of efficiency for DC-DDR. The more your CPU *needs* the data the higher the efficiency.

Eric