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A64: Latancy or Bandwidth

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BBigJ

Member
Joined
Dec 28, 2001
Location
Bay Area, CA
Back when the A64 first came out, the everyone was saying that dual channel provided the cpu with way more bandwidth than it needed. This was most noticeable in AMD's initial decision to use 754 as the desktop socket and 940 for servers. Eventually AMD conceded that dual channel did help and they introduced socket 939. But since single channel was almost enough to feed the a64, conventional wisdom said that the best course of action was to run memory at the tightest timings possible.

I'm wondering how much this still applies. The frequency of a64 has increased almost 50%, and with that, I'm sure the bandwidth demands have increased. I also see all the emphasis on running 1:1 memory despite the prevailing wisdom that running a divider has (almost) no performance hit. So where is the sweet spot in bandwidth vs. latancy?

The reason I'm asking is that I am currently building two machines with Abit AN8 mobos (one is the SLI version, the other is the Ultra), Kingston HyperX pc3200 (2*512meg), and opteron 144's. The opterons are CAB2E which have been hitting 2.8 consistently at stock volts and 3.0-3.1 with extra volts. I want the machines to last a while, so I'm not willing to put (much) extra voltage through the cpus or RAM. I've decided that the sweet spot is to run both chips at 2.7 gigs (one will be on water, the other with an XP90c). This works out nicely at 300*9 with a 3x hypertransport multi. However, my options for the memory are somewhat limited. If I run a 5:6 divider the memory is running at DDR 500 speeds which is probably at about the limit. If I run the 2:3 divider, however, I can run the memory at stock ddr400 speeds, but with extremely tight timings. I like this second option a lot, but I want to make sure I'm not starving the machine for bandwidth. Another option is to run an 8x cpu multi with a 340ish htt. This would put my memory at ddr ~450 with the 2:3 divider (the Abit boards are supposedly able to go this high).

Outpost has the following chart for what my memory is able to do. I haven't been able to find this directly from Kingston (which only says "overclocking will void your warranty...blah".)

333MHz (PC2700) 2-2-2-5-1 (CAS Latency 2)
370MHz (PC3000) 2-2-2-6-1 (CAS Latency 2)
400MHz (PC3200) 2-3-2-6-1 (CAS Latency 2)
434MHz (PC3500) 2-3-3-7-1 (CAS Latency 2)
466MHz (PC3700) 3-4-4-8-1 (CAS Latency 3)
500MHz (PC4000) 3-4-4-8-1 (CAS Latency 3)

Oh yeah. One machine will be for gaming, with the occasional scientific calculation running. The other machine does lots of photoshop and will probably do some video editing. Both machines will also be folding for 32.
 
There is no "sweet spot" because different applications respond differently to higher frequency and lower latencies. I believe you need to make such an assessment based on your individual rig and the applications you plan to use.

Using my system as an example, I could run at 1:1 with 3-4-4-8 timings at 9x278, 11:12 with 2.5-3-3-6 timings at 9x282 or 5:6 with 2-3-3-6 timings at 9x282. The 1:1 setting yields the highest bandwidth, but the 11:12 divider has the best overall performance based on benchmarking and Super-Pi times. Despite this, I have recently had to switch to the 5:6 divider because of stability issues with the 183 divider on my board. BTW, a 2:3 divider and tighter timings is not a possibility on my system due to the inadequacies of my MSI board.

Looking back on my decision to buy this ram I can see now that it was a mistake to spend so much money for so little performance gain. In your case I suspect the 2:3 divider with tight timings will work out the best. The reason I say that is you will most likely have to give up too much in the way of latency to reach those higher clocks, (If you can do it without going to to CL3 then it may be a better option) but the only way to know for sure is to try it and benchmark it for yourself. Good luck with it.
 
Yeah, I suspect the 2:3 option is the best. I'm glad I ended up getting relatively inexpensive ram (<$100 gig). Otherwise I would feel obligated to push the frequency. With this stuff I can enjoy the fast latancy without feeling like I wasted money. Back when I was looking at ram, I had no idea the cpu would allow me to reach ddr400 speeds at a 2:3 divider.

I'm suprised this has only gotten one response so far. I would have thought that thread would have set off a "tastes great" vs. "less filling" style fight.
 
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