Why 1GB DIMMs?

In past memory reviews, we have said a lot about the impact of the 2T Command Rate on memory performance. We have speculated with others that the Rev. E AMD processor would remove the 2T requirement for 4 DIMMs, but in fact, the release Rev. E still required a 2T Command Rate with 4 DIMMs on the new on-CPU memory controller. So, what is the impact of going from 1T with 2 DIMMs to 2T with 4 DIMMs?

Here, you see tests of 2 vs. 4 DIMMs of our standard OCZ PC3200 Platinum Rev. 2 based on Samsung TCCD memory chips. In all tests, the timings were 2-2-2-7 and the only difference was the 1T Command Rate with 2 DIMMs and 2T with 4 DIMMs:

Quake3 fps Sandra UNBuffered Sandra Standard Buffered Super PI 2M places
(time in sec)
Wolfenstein - Radar - Enemy Territory fps
2x512MB
2-2-2-7
2.6V 1T
550.2 INT 2621
FLT 2738
INT 5984
FLT 5938
80 121.1
4x512MB
2-2-2-7
2.6V 2T
529.6 INT 2276
FLT 2362
INT 4925
FLT 5938
83 116.9
1T Performance Increase 3.9% 15.5% 21.1% 3.8% 3.6%

For best comparison, we reran all benchmarks with the exact same settings for 2 DIMMs and 4 DIMMs, the only difference being 1T or 2T timings. We could have achieved faster timings (and a bigger performance difference) by tweaking each set of tests for best bandwidth. Therefore, these should be considered best case (lowest) performance differences.

As you can clearly see, the memory-only Sandra tests show a 21% drop in memory bandwidth in the standard buffered test when reducing to a 2T Command Rate. The more real-world Unbuffered Sandra test still shows a 15.5% bandwidth drop. Games and Super Pi, where memory is just one small component of the overall result, show about a 4% performance reduction. These represent the impact of this memory change on the overall system performance, where the Sandra scores measure impact on memory alone.

The second area where 4 DIMMs have a significant impact compared to 2 DIMMs is overclocking. This won't matter to those who run their systems at stock speeds, but it will be very important to those who try to squeeze the best performance from their memory. Here, we compared overclocking with our stock OCZ PC3200 Platinum Rev. 2 with 2 and 4 DIMMs. Again, we kept timings and settings exactly the same.

Highest Overclock at 9X Ratio
(4000+ Rev. E)
2x512MB
2.5-4-3-7 1T
318 (DDR636)
4x512MB
2.5-4-3-7 2T
240 (529.6)
2 DIMM Performance Increase 295%

Using the same DIMMs and same timings, we could only reach a 240 Processor Clock or +20% with 4 DIMMs. Two DIMMs allowed an overclock to 318 or +59%. With two DIMMs, we could overclock almost 3 times higher than with 4 DIMMs. You can likely overclock a bit higher with 4 DIMMs by further tweaking memory timings, but 4 DIMM overclocking will always fall far short of 2 DIMMs on current Athlon 64 memory controllers.

Higher performance at 1T vs. 2T and higher overclocking potential are certainly strong motivations to use 1GB DIMMs instead of 512Mb DIMMs when you want to populate an Athlon 64 board with 2GB. However, the gain of 1T in the past has often been more than offset by the slow memory timings available on 1GB DIMMs. The Athlon 64 controller is much lower latency than current Intel solutions, and thus, is much more bandwidth-sensitive to memory timings than a comparable Intel solution. We have shown in past memory reviews that slower memory timings can more than offset the 3.6% to 3.9% real-world performance loss that results in 4 DIMMs at 2T Command Rate. However, the availability of DDR400 1GB DIMMs rated as fast as 2-3-2 at DDR433 certainly has the potential to change this picture.

Index Impact of Athlon 64 Memory Controllers on 1GB DIMM Performance
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  • Slaimus - Tuesday, October 11, 2005 - link

    Does 4 single sided 512MB sticks behave the same as 2 double sided 1GB sticks? Reply
  • eastvillager - Tuesday, October 11, 2005 - link

    Why would you buy these when the 2-3-2-5 sticks are readily available? Reply
  • Wesley Fink - Tuesday, October 11, 2005 - link

    If you read the review you will see that ALL THREE of the 1GB dimms ran at 2-3-2 at DDR400 to DDR440 or so. They will all run 2-3-2-5, but we have shown in previous tests the the nForce 4 is fastest running a tRAS of 6 or 7. We ran 2-3-2-7 because it is faster than 2-3-2-5 on the nForce4. Try it for yourslef with memtest86 and differnet tRAS. Reply
  • Sunrise089 - Tuesday, October 11, 2005 - link

    Possible minor typos aside, this article is a great change of pace from some of the recent technical write-ups here on Anandtech(cough:R520:cough). The quality of writing as well as the attempt to put the parts in perspective and give the big picture is much appreciated. With so many sites out there, I can go anywhere for simple RAM benchmarks, but for me it is much harder to find informed discussions about why the part being reviewed is a good idea/choice or not. I really felt this side of the story was lacking in the X1800 reviews and am glad to see it here. Reply
  • Houdani - Tuesday, October 11, 2005 - link

    Page 3:
    quote:

    our overclocking clock frequency went up to DDR500 - 30 points higher.

    I think you meant DDR550.

    Page 4:
    In your table of memory, you list the 3 new sticks as 2x512. I think you meant 2x1024.
    Reply
  • Wesley Fink - Tuesday, October 11, 2005 - link

    Corrected. Thanks for bringing these to us. It's funny that they looked just fine at 3AM :-) Reply
  • Doormat - Tuesday, October 11, 2005 - link

    Whether its worth it or not to invest heavily (these pieces arent cheap) in DDR1 tech if you've already got a pair of fast running 2x512MB sticks. You'll just have to buy DDR2 sticks in a year if you want the fastest stuff (an A64 M2-socket based chip). Reply
  • emilyek - Tuesday, October 11, 2005 - link

    Lame. Why not a big review on the many available 2 and 2.5 cas DDR 400 sticks? The Geil, Patriot, OCZ, Gskill, and Corsair already top out at about 1k FSB when loosed up, and the timings on these RAMS sucks anyway. Reply
  • DrMrLordX - Tuesday, October 11, 2005 - link

    They've said it before, and I'll say it again: you just can't add every available DIMM variety to RAM tests. There's too much on the market, and many of the budget RAM types have wildly variable quality and performance. Reply
  • RockSolid - Tuesday, October 11, 2005 - link

    The RamGuy link on Page 5 is incorrect. Reply

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