P4 Memory Speed

[Chumpman]

My friend has an Athlon 1800+ processor, which shows no increase whether the RAM is running at 133 MHz or 166 Mhz, due to the 64 bit pathway. He also believes that the Intel Pentium 4 suffers from this same flaw, where it cannot take advantage of 133 Mhz memory if the chip is running at 100 Mhz FSB. Would anyone with a Pentium 4 be willing to run a couple of benchmarks to confirm/disprove my argument? A game would be preferable (obviously synthetic benchmarks are worthless in this type of test). Hopefully you can try to run one with the RAM and the processor at the default speed of the processor, with the RAM in sync, and then run a test with the RAM up to 166 MHz (if the processor is a NWood B) or 133 MHz (if its an A). Thanks for your time.

 

[Duvie]

Your friend obviously knows little about amd and intel processors...

The reason the amd cannot gain much performance is that the fsb is the bottleneck....At 266 (or 133fsb) the chip has a theoretical bandwidth capability of 2.1gb/s, which is pc2100 or 266mhz ddr (133x2)....At 166x2 for 333mhz ddr the bandwidth is 2.7gb/s....So unless he raises the fsb of the processor to a matching 166 (333ddr) the cpu will not have enough throughput....Hence why you always see much amd systems run memory in synch mode and not asynch....



The reason for the Intel p4 is that it has a whopping 400,533 or 800mhz fsb for a much larger theoretical bandwidth....In almost all single channel DDR systems the memory is the clear bottleneck....

Ie 400fsb p4 has a max bandwidth of 3.2gb/s....therefore this system wants to run 400mhz ddr (pc3200) but in most instances the early i845d mobos only had support for 266mhz ddr or pc2100 with i845pe mobos offering official support for pc2700...But once again the memory is still below what the cpu can handle therefore with a 400fsb p4 or 100 system fsb the memory needs a 2:1 memory ratio which I don't think even existed on sis mobos....

Ie 533fsb p4 has 4.2 gb/s...no amount of single channel ddr will make the cpu the bottleneck. It needs dual channel pc2100 to saturate it and even then I don't think it does it completely....

Ie 800fsb p4 has 6.4gb/s and needs dual 400mhz ddr (2xpc3200)....

see the math...

For your friends example a 400fsb p4 produces 3.2gb/s but 133mhz memory or 266ddr only delivers 2.1gb/s...therefore he is wrong as using mem multipliers on sis mobo like sis645 and sis645dx shows the p4 takes advantage of running 400fsb processor with 333mhz ddr (pc2700) with a 3:5 mem ratio....


P4's have more memory bandwidth headroom due to the quad pumped fsb and thus standrad ddr cannot keep up with it...Only in Dual channel ddr moeds are we seeing it starting to meet its needs...

 

[Chumpman]

Ahhh, I see, thanks for the help Duvie.