titus12 said:
What does this mean concerning DDR2 Memory...4-4-4-12 Dim and so on? Please
explain the difference between dual channel and single channel. What is the
best memory to buy for a 1333 fsb motherboard?
Thank you;
David
The numbers are timing numbers, in units of cycles.
The first number is CAS or Column Address Strobe
and is the most important. The other three to the right, are less important.
The first number CAS, represents the delay from requesting data, until the first
data is available. Thus, a lower first number, means a lower "latency" for the
first memory requested. Memory requests tend to be handled in bursts, and the
memory subsystem is designed to handle a cache line from or to the processor.
(I don't know if single words are transferred any more to the memory subsystem
or not. Since datasheets for computer components no longer go into details,
it is hard to get info like that.)
Dual channel, means two 64 bits busses are accessed simultaneously,
giving 128 bits per transfer cycle, instead of just 64. So a couple
channels function in parallel, for better performance from a
bandwidth perspective. Latency remains the same.
The best memory to buy, is the memory you can afford. I.e. It should be
cost effective. Motherboards accept a wide range of memory speeds and
timing values. Since the price rises exponentially as you enter the
"enthusiast memory domain", and the performance gained is tiny, spending
hundreds of dollars for memory doesn't make a lot of sense.
In terms of basic bandwidth balance, two 64 bit busses at DDR2-667 is
enough to balance an FSB which is 64 bits wide and working at FSB1333.
To look at benchmarks, I can take some numbers from here.
http://www.anandtech.com/showdoc.aspx?i=2820&p=7
They use an FSB1066 processor, so DDR2-533 would be the balance point.
Take the WINRAR benchmark. DDR2-533 3-3-3-9 benches in 5m 26sec.
DDR2-667 3-4-3-9 benches in 5m 23sec, about 1% better.
DDR2-800 4-4-4-12 benches in 5m 19sec, about 2% better.
Faster memory comes in handy, if you wanted to maintain the balance point,
while the FSB is higher. But I really wonder if there is a point to that.
Being at the balance point or better, makes a slight difference to
the SuperPI 2M results at the bottom of this page. This bench was
with a FSB1066 processor, so dual channel DDR2-533 is the balance point.
You have to go quite a bit above DDR2-533 to see more gains.
http://www.anandtech.com/memory/showdoc.aspx?i=2916&p=5
SuperPI 2M DDR2-400 45.02sec
DDR2-533 43.09sec (4% better than base)
DDR2-667 42.66sec (5% better than base)
DDR2-800 41.95sec (7% better than base)
DDR2-1066 41.48sec (7.8% better than base
Actually, different architectures have different sensitivity to RAM
characteristics. So comparing these results on an Intel chipset,
might be a bit different than say, DDR2 on Athlon64 X2 processors.
I think Intel isn't quite as sensitive to the RAM setup, and Intel
went to a good deal of trouble to achieve that. After all, since
Intel doesn't have the RAM interface on the processor, they have
to be clever to compete with what AMD can do. With AMD, there is
one fewer chips in the way, between memory and the core.
This article is for Athlon64 X2. The timing differences here (the
last digit) are not significant. Cranking up the memory speed
seems to help more here.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2741&p=3
SuperPI 2M DDR2-400 3-3-3-6 104 sec
DDR2-533 3-3-3-8 99 ( 5% better than base)
DDR2-667 3-3-3-9 95 ( 9% better than base)
DDR2-800 3-3-3-10 93 (11% better than base)
As long as the memory price does not rise dramatically, then
a little faster memory will help.
Say, as an example, I was buying a Q6600 with FSB1066, and
know I'm overclocking to FSB1333 to get 3GHz. Then DDR2-667
would be the slowest memory I would use. The leisurely incremental
improvement I'd get with anything over DDR2-800, just wouldn't
interest me. (I'm a cheapskate after all.) If I had the DDR2-800,
I could use it for either the Intel or the AMD system. But
anything more than that, is purely for bragging rights, because
it costs too much.
Paul
