RDRAM vs. DDR SDRAM

RickC · 2003-03-15, 01:53

I have a P4 with PC1066 RDRAM. My benchmarks are below. Judging by the Mersenne benchmark page, high bandwidth RDRAM doesn't really have any advantage over DDR SDRAM for Prime95. Does anyone know the memory bandwidth that Prime95 uses for 18M exponents?

Interesting, it looks like Intel is going away from RDRAM in favor of Dual Channel DDR400 SDRAM with ECC to match their new 800Mhz bus.
http://www.extremetech.com/article2/0,3973,907061,00.asp

My system:
Bus Speed: 533Mhz
L2 cache speed: Full
Memory Type: PC1066 RDRAM
OS: Windows 2000 SP3
Intel(R) Pentium(R) 4 CPU 2.40GHz
CPU speed: 2386.69 MHz
CPU features: RDTSC, CMOV, PREFETCH, MMX, SSE, SSE2
L1 cache size: 8 KB
L2 cache size: 512 KB
L1 cache line size: 64 bytes
L2 cache line size: 64 bytes
TLBS: 64
Prime95 version 22.12, RdtscTiming=1
Best time for 256K FFT length: 9.813 ms.
Best time for 320K FFT length: 13.089 ms.
Best time for 384K FFT length: 15.910 ms.
Best time for 448K FFT length: 19.076 ms.
Best time for 512K FFT length: 21.482 ms.
Best time for 640K FFT length: 27.875 ms.
Best time for 768K FFT length: 34.025 ms.
Best time for 896K FFT length: 41.684 ms.
Best time for 1024K FFT length: 45.211 ms.
Best time for 1280K FFT length: 63.859 ms.
Best time for 1536K FFT length: 79.060 ms.
Best time for 1792K FFT length: 98.161 ms.

xtreme2k · 2003-03-15, 05:35

RDRAM PC1066 will give you fastest performance at a given clock speed. If you run DDR at stock 266/333 it is quite a bit slower than RDRAM 1066. Only if you start running DDR at faster clock or at dual channel that it come close. Basically if you don't overclock, RDRAM will give you best performance, but at a high cost.

However, the problem with RDRAM is that they dont tend to overclock well. For example, my Kingston RDRAM PC1066 ECC dont even like to be overclock from its over 142FSB. That is a very poor overclock of only 6.7%. On the other hand, if you get a DDR board with DDR333/400 you will have NO PROBLEM running at 166FSB+ as long as your CPU allows. Now, if the CPU is running at such a higher speed RDRAM has no chance.

outlnder · 2003-03-15, 07:02

Rick, one thing I would suggest is to get the Prime95 ver. 23.2.

Here are my benchmarks running DDR400.

Intel(R) Pentium(R) 4 CPU 2.40GHz
CPU speed: 2400.80 MHz
CPU features: RDTSC, CMOV, PREFETCH, MMX, SSE, SSE2
L1 cache size: 8 KB
L2 cache size: 512 KB
L1 cache line size: 64 bytes
L2 cache line size: 64 bytes
TLBS: 64
Prime95 version 23.2, RdtscTiming=1
Best time for 384K FFT length: 15.480 ms.
Best time for 448K FFT length: 18.248 ms.
Best time for 512K FFT length: 21.118 ms.
Best time for 640K FFT length: 26.871 ms.
Best time for 768K FFT length: 33.090 ms.
Best time for 896K FFT length: 39.989 ms.
Best time for 1024K FFT length: 43.713 ms.
Best time for 1280K FFT length: 58.318 ms.
Best time for 1536K FFT length: 71.319 ms.
Best time for 1792K FFT length: 86.908 ms.
Best time for 2048K FFT length: 95.344 ms.

Notice mine start at the 384 FFT level while yours starts at the 256 FFT level.

Your system seems to be running rather slowly. Update the version and post your new benchmarks. My system should not be that much faster than yours.

xtreme2k · 2003-03-15, 08:00

His system should kick yours ass ;) ;)

arjanscholl · 2003-03-15, 10:56

Isn't it true that DDR memory has lower latencys as RDRAM? Maybe that causes a slowdown too, i don't whats more important with Prime95, bandwidth or latency.

xtreme2k · 2003-03-15, 12:04

Only very slightly lower. But RDRAM really excel at lots of random read+write.

RickC · 2003-03-15, 13:59

outlnder, I did take the lead over your 2.4 when I upgraded Prime95. I have a stock Dell 8250. It's interesting that my 2.4 is a few Mhz short of 2400.

Intel(R) Pentium(R) 4 CPU 2.40GHz
CPU speed: 2386.70 MHz
CPU features: RDTSC, CMOV, PREFETCH, MMX, SSE, SSE2
L1 cache size: 8 KB
L2 cache size: 512 KB
L1 cache line size: 64 bytes
L2 cache line size: 64 bytes
TLBS: 64
Prime95 version 23.2, RdtscTiming=1
Best time for 384K FFT length: 15.179 ms.
Best time for 448K FFT length: 17.883 ms.
Best time for 512K FFT length: 20.774 ms.
Best time for 640K FFT length: 26.421 ms.
Best time for 768K FFT length: 32.468 ms.
Best time for 896K FFT length: 39.261 ms.
Best time for 1024K FFT length: 43.023 ms.
Best time for 1280K FFT length: 57.438 ms.
Best time for 1536K FFT length: 70.477 ms.
Best time for 1792K FFT length: 86.128 ms.
Best time for 2048K FFT length: 94.622 ms.

RickC · 2003-03-15, 14:39

My RDRAM has a maximum transfer rate of 4.2GB/s. I find it hard to believe that Prime95 would use that amount. I think the bottleneck on my system would be doing the number crunching. I'm thinking the reason that RDRAM performs better is a matter of efficiency, like that the CPU has to do less work to get data in a out of memory so it can spend more cycles on math.
Any thoughts? I could be wrong.

RickC · 2003-03-15, 14:50

I have 4 RDRAM slots configured like this:

1. 256MB module
2. 256MB module
3. continuity module
4. continuity module

Something I will try if the price goes down is fill the other two slots and see the impact on performace not due to total MB but due to more modules in use.

Xyzzy · 2003-03-16, 04:13

Quote:

Originally Posted by RickC

Something I will try if the price goes down is fill the other two slots and see the impact on performace not due to total MB but due to more modules in use.

More devices/modules in a RDRAM chain means more latency...

http://arstechnica.com/paedia/r/ram_guide/ram_guide.part3-1.html

xtreme2k · 2003-03-19, 20:55

I tend to agree more chips means higher latency.

But I remember reading from some reputable sites that more sticks does offer MORE performance, which is weird. I think it was aceshardware.com but I really cant be sure.

2003-03-15, 01:53	#1
RickC Mar 2003 3²·5 Posts	RDRAM vs. DDR SDRAM I have a P4 with PC1066 RDRAM. My benchmarks are below. Judging by the Mersenne benchmark page, high bandwidth RDRAM doesn't really have any advantage over DDR SDRAM for Prime95. Does anyone know the memory bandwidth that Prime95 uses for 18M exponents? Interesting, it looks like Intel is going away from RDRAM in favor of Dual Channel DDR400 SDRAM with ECC to match their new 800Mhz bus. http://www.extremetech.com/article2/0,3973,907061,00.asp My system: Bus Speed: 533Mhz L2 cache speed: Full Memory Type: PC1066 RDRAM OS: Windows 2000 SP3 Intel(R) Pentium(R) 4 CPU 2.40GHz CPU speed: 2386.69 MHz CPU features: RDTSC, CMOV, PREFETCH, MMX, SSE, SSE2 L1 cache size: 8 KB L2 cache size: 512 KB L1 cache line size: 64 bytes L2 cache line size: 64 bytes TLBS: 64 Prime95 version 22.12, RdtscTiming=1 Best time for 256K FFT length: 9.813 ms. Best time for 320K FFT length: 13.089 ms. Best time for 384K FFT length: 15.910 ms. Best time for 448K FFT length: 19.076 ms. Best time for 512K FFT length: 21.482 ms. Best time for 640K FFT length: 27.875 ms. Best time for 768K FFT length: 34.025 ms. Best time for 896K FFT length: 41.684 ms. Best time for 1024K FFT length: 45.211 ms. Best time for 1280K FFT length: 63.859 ms. Best time for 1536K FFT length: 79.060 ms. Best time for 1792K FFT length: 98.161 ms.

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2003-03-15, 05:35	#2
xtreme2k Aug 2002 2·3·29 Posts	RDRAM PC1066 will give you fastest performance at a given clock speed. If you run DDR at stock 266/333 it is quite a bit slower than RDRAM 1066. Only if you start running DDR at faster clock or at dual channel that it come close. Basically if you don't overclock, RDRAM will give you best performance, but at a high cost. However, the problem with RDRAM is that they dont tend to overclock well. For example, my Kingston RDRAM PC1066 ECC dont even like to be overclock from its over 142FSB. That is a very poor overclock of only 6.7%. On the other hand, if you get a DDR board with DDR333/400 you will have NO PROBLEM running at 166FSB+ as long as your CPU allows. Now, if the CPU is running at such a higher speed RDRAM has no chance.

2003-03-15, 07:02	#3
outlnder Aug 2002 2·3·53 Posts	Rick, one thing I would suggest is to get the Prime95 ver. 23.2. Here are my benchmarks running DDR400. Intel(R) Pentium(R) 4 CPU 2.40GHz CPU speed: 2400.80 MHz CPU features: RDTSC, CMOV, PREFETCH, MMX, SSE, SSE2 L1 cache size: 8 KB L2 cache size: 512 KB L1 cache line size: 64 bytes L2 cache line size: 64 bytes TLBS: 64 Prime95 version 23.2, RdtscTiming=1 Best time for 384K FFT length: 15.480 ms. Best time for 448K FFT length: 18.248 ms. Best time for 512K FFT length: 21.118 ms. Best time for 640K FFT length: 26.871 ms. Best time for 768K FFT length: 33.090 ms. Best time for 896K FFT length: 39.989 ms. Best time for 1024K FFT length: 43.713 ms. Best time for 1280K FFT length: 58.318 ms. Best time for 1536K FFT length: 71.319 ms. Best time for 1792K FFT length: 86.908 ms. Best time for 2048K FFT length: 95.344 ms. Notice mine start at the 384 FFT level while yours starts at the 256 FFT level. Your system seems to be running rather slowly. Update the version and post your new benchmarks. My system should not be that much faster than yours.

2003-03-15, 08:00	#4
xtreme2k Aug 2002 10101110₂ Posts	His system should kick yours ass ;) ;)

2003-03-15, 10:56	#5
arjanscholl Dec 2002 2·3 Posts	Isn't it true that DDR memory has lower latencys as RDRAM? Maybe that causes a slowdown too, i don't whats more important with Prime95, bandwidth or latency.

2003-03-15, 12:04	#6
xtreme2k Aug 2002 2·3·29 Posts	Only very slightly lower. But RDRAM really excel at lots of random read+write.

2003-03-15, 13:59	#7
RickC Mar 2003 3²·5 Posts	outlnder, I did take the lead over your 2.4 when I upgraded Prime95. I have a stock Dell 8250. It's interesting that my 2.4 is a few Mhz short of 2400. Intel(R) Pentium(R) 4 CPU 2.40GHz CPU speed: 2386.70 MHz CPU features: RDTSC, CMOV, PREFETCH, MMX, SSE, SSE2 L1 cache size: 8 KB L2 cache size: 512 KB L1 cache line size: 64 bytes L2 cache line size: 64 bytes TLBS: 64 Prime95 version 23.2, RdtscTiming=1 Best time for 384K FFT length: 15.179 ms. Best time for 448K FFT length: 17.883 ms. Best time for 512K FFT length: 20.774 ms. Best time for 640K FFT length: 26.421 ms. Best time for 768K FFT length: 32.468 ms. Best time for 896K FFT length: 39.261 ms. Best time for 1024K FFT length: 43.023 ms. Best time for 1280K FFT length: 57.438 ms. Best time for 1536K FFT length: 70.477 ms. Best time for 1792K FFT length: 86.128 ms. Best time for 2048K FFT length: 94.622 ms.

2003-03-15, 14:39	#8
RickC Mar 2003 3²·5 Posts	My RDRAM has a maximum transfer rate of 4.2GB/s. I find it hard to believe that Prime95 would use that amount. I think the bottleneck on my system would be doing the number crunching. I'm thinking the reason that RDRAM performs better is a matter of efficiency, like that the CPU has to do less work to get data in a out of memory so it can spend more cycles on math. Any thoughts? I could be wrong.

2003-03-15, 14:50	#9
RickC Mar 2003 3²×5 Posts	I have 4 RDRAM slots configured like this: 1. 256MB module 2. 256MB module 3. continuity module 4. continuity module Something I will try if the price goes down is fill the other two slots and see the impact on performace not due to total MB but due to more modules in use.

2003-03-19, 20:55	#11
xtreme2k Aug 2002 2·3·29 Posts	I tend to agree more chips means higher latency. But I remember reading from some reputable sites that more sticks does offer MORE performance, which is weird. I think it was aceshardware.com but I really cant be sure.