Originally posted by hitechjb1
For A64,
- Can run any speed (ASYNC) using the CPU_memory_divider, to match memory module speed.
- Slow or faster memory can be used to get whatever bandwidth allowed in the memory modules.
- ASYNC has minimum impact on memory bandwidth efficiency
- 754 efficiency 95%+, 939 efficiency 86%+
...
Memory modules (for 754 and 939 platforms)
Socket 940 Opteron and FX platforms require the use of registered memory modules, in ECC mode for mission-critical systems.
A nice thing about 754 and 939 CPU's is that they support, for non-mission critical system, existing unbuffered, DDR 400/500 (PC 3200/4000) memory modules that most of us have.
Both 754 and 939 support either ECC or non-ECC memory modules.
754 supports up to 3 dimms. 939 supports up to 4 dimms.
PC3200 or DDR 400 is rated at 200 MHz, max bandwidth = 200 x 2 x 8 = 3200 MB/s
PC3500 or DDR 438 is rated at 219 MHz, max bandwidth = 219 x 2 x 8 = 3500 MB/s
PC3700 or DDR 462 is rated at 231 MHz, max bandwidth = 231 x 2 x 8 = 3700 MB/s
PC4000 or DDR 500 is rated at 250 MHz, max bandwidth = 250 x 2 x 8 = 4000 MB/s
PC4400 or DDR 550 is rated at 275 MHz, max bandwidth = 275 x 2 x 8 = 4400 MB/s
For A64 system, the memory bus frequency can be flexibly set at various (SYNC/ASYNC) ratio to CPU frequency and FSB setting (see post on overclocking setting) with minimal impact on memory bandwidth efficiency.
Existing DDR400 (PC3200) modules would work fine with A64 754 and 939 system.
If getting new memory modules, DDR 500 memory module, or even DDR 550 are preferred, even with higher tRCD and CAS latencies. With the DDR500+ (PC4000+) memory modules, memory bus can be overclocked to the level 250 - 300+ MHz. Memory bus at such 250 - 300+ MHz level delivers significantly higher raw memory bandwidth than that at the low 200 - 230 MHz even with lower latancy of x-2-2-2 by most PC3200 modules. Though some existing programs, drivers may not be able to take full advantage of the high raw bandwidth, have to see whether future programs, OS can benefit such excess bandwidth.
Some low latency modules with DRAM chips such as BH5 (new memory modules with BH5 are hard to get currently (2004)) may be able to overclock to 230-250 MHz with tight timing x-2-2-2 and enough high Vdimm (if choosing to use 3.0 - 3.3 V).
There are modules with
- Micron 256Mb DRAM chips (-5BC), found in some Buffalo PC3700, OCZ EB
- Hynix chips (BT-D43), found in some Kingston PC3200, to the 250-275+ MHz with 2.x V
- HyperX PC4000 (SamSung TCCC chips) rated 2.6 V to at least 250 MHz
- Samsung TCCD chips (4ns) to 250 MHz level with CAS 2.5
- A-DATA Vitesta modules to 250-300 MHz
Running at 2.x V would lessen some concerns about high Vdimm on the CPU memory controller interface.
OCZ EB series, e.g. PC3700, overclock very well to high bus frequency (DDR550 speed) with low equivalent latency.
There are tradeoffs in overall performance (not just raw memory bandwidth) between low latency x-2-2-2 at 230-250 MHz with 1T (command rate), and higher latency between x-3-3-2.5 and x-4-4-3 at 270-300 MHz with 2T (command rate), test out these two cases for specific applications and benchmarks.
For 939 system, as the dual channel 128-bit memory bus can already provide 80+% bandwidth over the 754, a tight x-2-2-2 running 240 - 250 MHz may provide sufficient memory bandwidth for the whole system using overclocking memory such as with BH-5 chips.
I've seen 939 system achieving 90%+ memory bandwidth efficiency.
Assuming 90% bandwidth efficiency, with good memory modules for high bus frequency (probably at relaxed timing),
at 250 MHz, the effective bandwidth would be 7200 MB/s,
at 275 MHz, the effective bandwidth would be 7920 MB/s,
at 300 MHz, the effective bandwidth would be 8640 MB/s (doable for 4400 module).
Based on my reading of the Anandtech 939 run-up test using Mushkin PC3500 Level 2 and OCZ PC 3500 Platinum (both use BH5 DRAM chips), with 2 dimms (not clear 256MBx2 or 512MBx2), at 200 MHz memory bus, 10-2-2-2 1T timing, the Sandra (SP2) memory banwdith test obtained was 6100 MB/s (95.3% efficiency) for MSI K8N neo2, 6000 MB/s (93.8% efficiency) for ABIT AV8 and became 5000 MB/s (78.1% efficiency) for 2T.
For a typical 754 system, the effective memory bandwidth is around 3000-4000 MB/s.
Towards year end of 2005, A64 platforms may move to DDR2 memory module (discussed in other posts in this thread) which would require new motherboards, maybe new CPU socket (not clear as of May 04).
