Does AM2's Performance Make Sense?
Assuming for a moment that the performance we're seeing here today is representative of what AMD will show off in 2 months, does it make sense? AMD has effectively doubled their memory bandwidth but they've seen virtually no increase in performance, other than in some very isolated situations.
If you'll remember back to the introduction of AMD's Revision E core, we did an article about how the new core brought support four new memory dividers allowing you to run at speeds up to DDR500 without overclocking your CPU or the rest of your system. In that article we looked at the overall performance benefit of DDR-500 over DDR-400 on a Socket-939 platform in a variety of situations. A recap of our performance results is below:
| Benchmark | Socket-939 (DDR-400) | Socket-939 (DDR-480) | % Advantage (DDR-480) |
| Multimedia Winstone 2004 | 41.9 | 42.7 | 2% |
| 3dsmax 6 | 2.78 | 2.80 | 1% |
| DivX 6.0 | 50.6 fps | 53.2 fps | 5% |
| WME9 | 4.22 fps | 4.28 fps | 1% |
| Quake 3 (10x7) | 121.9 fps | 127.2 fps | 4% |
| ScienceMark 2.0 (Bandwidth)* | 5378 MB/s | 5851 MB/s | 9% |
As you can see, given almost a 9% increase in memory bandwidth, we saw similarly small increases in overall performance. It would seem that the Athlon 64, at its current clock speeds, just simply isn't starved enough for memory bandwidth to benefit from an increase in bandwidth. You'll also see that the areas where faster DDR memory helped back then are pretty much the areas where DDR2-800 is showing gains today.
Based on our results from back then, if a 9% increase in memory bandwidth doesn't increase performance tremendously, then the 35% increase in bandwidth we see with DDR2-800 on AM2 shouldn't yield any more of a performance increase. Or simply put, yes, our AM2 performance numbers make sense.
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Calin - Monday, April 10, 2006 - link
DDR2-800 at 4-4-4 should be equivalent to DDR-400 at 2-2-2. Also, DDR2-800 at 6-6-6 would be the same (latency-wise) as DDR-400 at 3-3-3.Furen - Monday, April 10, 2006 - link
Not quite, DDR2-800 at 4-4-4 is the equivalent of DDR-400 at 4-4-4 because the memory cells run at 200MHz on both modules. Like I said above, though, module latency is not the only factor affecting the total latency, so perhaps DDR2 memory controllers help mitigate this huge latency hit. One of the main reasons why DRAM manufacturers love DDR2 is because their yields are much higher than they are on higher-clocked, aggresively-timed DDR1 due to the higher prefetch (lower operating clock) and the looser timings.defter - Monday, April 10, 2006 - link
That's not true. "Cas latency" values are relative to the 400MHz clock instead of 200MHz base clock that DDR2-800 has.
Furen - Monday, April 10, 2006 - link
The 400MHz clock is the clock the IO operates at, while the memory arrays operate at half the IO clock, so 200MHz in this case (so yes, DDR2 ram operates at a sort of quad data rate). Since a Column Access Strobe is a memory array operation it is, naturally, measured in memory array clocks. The "base clock" for DDR2 is actually 400MHz because it is the external clock.menting - Monday, April 10, 2006 - link
defter is correct,time delay on memory is calculated by the clk speed that the memory takes in * latency
think of it as a black box operation.
MrKaz - Monday, April 10, 2006 - link
OK. Didnt know that.I always tought that DDR1 2-2-2 was always better than higher DDR2 numbers...
Furen - Monday, April 10, 2006 - link
It is, the main factor affecting latency is the memory cell clock, which runs at the same clock on both modules. So you can do a straight comparison between the two latencies to see which will yield you a better MODULE latency. Of course, module latency is just one part of the whole latency equation, the memory controller being the other big part. Perhaps AMD made the controller more latency-friendly by sacrificing maximum bandwidth, which would explain the abnormally-low usable bandwidth.ozzimark - Monday, April 10, 2006 - link
just something to keep in mind. same 1.8ghz cpu clock:200mhz at 2-2-2 = 51.5ns
300mhz at 3-3-3 = 43.8ns
mhz wins over timings when it comes to comparing absolute latency
Furen - Monday, April 10, 2006 - link
That only applies when comparing the same type of memory.DDR2 memory cells run at 1/4 the "Effective clock," so DDR2 800 runs at 200MHz, which is the same as DDR 400.
ozzimark - Monday, April 10, 2006 - link
true, but you notice the latency that is in the review. seems that what i say holds true to an extentbtw, the timings are in signal clocks last i checked, not cell clocks, which runs at 1/2 the speed of the double data rate signal. kinda confusing, but oh well. point of the matter is that ddr400 at 2-3-2 is higher latency than ddr2-800 at 4-5-4