Memory latency vs. fsb speed

[Munky]

Hi,

I'm lookng to upgrade some RAM for my AthlonXP system, and also I'm planning to buy the Abit AN7. I've heard those can do 240fsb, so if I plan to overclock the system, which memory should I get? Is it better to run 200fsb at cas2, or 240fsb at cas2.5 or cas3? Any suggestions?

 

[century child]

The best way to determine what works best and gives the best performance for your individual system is to run benchmarks at each setting and see which one is the best.

 

[Navid]

It is very difficult to answer this question.
You are comparing the memory frequency increase (higher clock rate) with increasing the memory speed (reduced timing).
You are leaving out the CPU. Often, the CPU speed is not the same in those two scenarios!

If the CPU frequency was the same, you could calculate the overall speed in both cases. The FSB determines the clock rate. How many clocks are required to perform the same function depends on the memory timing. You have both of those parameters in each case.

But, as I said, when you change FSB, the CPU clock rate is affected too.
So, the overall computer speed becomes difficult to calculate.

 

[Munky]

Originally posted by: Navid
It is very difficult to answer this question.
You are comparing the memory frequency increase (higher clock rate) with increasing the memory speed (reduced timing).
You are leaving out the CPU. Often, the CPU speed is not the same in those two scenarios!

If the CPU frequency was the same, you could calculate the overall speed in both cases. The FSB determines the clock rate. How many clocks are required to perform the same function depends on the memory timing. You have both of those parameters in each case.

But, as I said, when you change FSB, the CPU clock rate is affected too.
So, the overall computer speed becomes difficult to calculate.

I have an unlocked CPU, and in either case I would overcklock it as far as my system can handle. I also know that you get best performance if CPU and memory run at the same fsb rate. So, it seems like it's better to run a higher fsb with possibly higher latency.

Since I can't seem to find memory that would run a 240fsb with cas2 latency, how much of a performance hit would cas2.5 latency make? And what about cas3? I always hear people losening their latency in order to get higher fsb, so is this trade-off worth it?

 

[ahurtt]

I think the important thing is that you get RAM that will run as fast as possible at a 1T command rate. If you get RAM that will run at 240 but you have to lower the command rate to 2T to do this, you are better off to run at say 230 but make the command rate 1T. And naturally of course if your RAM will also do 1T with a CAS Latency of 2, even better. But from my understanding, I think the 1T command rate is the key thing to keep in mind. I think I saw somewhere on this board that if you run 2T but want the same performance as 1T 200MHz, you would have to up the memory speed to like 290MHz. If you are going to overclock a lot, you might want to buy overrated RAM beyond what your system spec states it supports so that you can get max RAM efficiency. Plus you won't be taxing the RAM as much by overclocking if you buy higher rated RAM. So if your system says it supports DDR 400, and you buy RAM that says it will do DDR 400 @ 2 2 2 5 1T, that doesn't mean it will do DDR 480 at 2 2 2 5 1T. So you might be better off to buy DDR 500 RAM to handle this. Cuz if you can run @ DDR 480 2 2 2 5 1T, now you're cooking with gas. But you will pay for it.

 

[gobucks]

well, look at it this way - the latencies are the number of cycles required to perform a specific function. Therefore, as the cycles take less time, the memory is faster. For example, if you had DDR400 CAS2.0, and DDR600 CAS3.0, they would both take exactly the same time for a CAS instruction. That is why good low-latency DDR can overclock so well, but you have to reduce timings. Personally, I don't think the extra bandwidth is worth it for its own sake. Somwhere on Anandtech's site, there is a review of DDR on the A64 platform, in which they take DDR from 400MHz all the way to the max they can, ~590MHz. They relax timings as is necessary. CPU multipliers are lowered when possible to keep the CPU speed close to constant. Anyways, the general trend is that the overclocked memory can usually pull off better benchmarks than its 2-2-2-5 DDR400 competition, but not by a lot. Maybe 1FPS here or there, nothing worth bragging about. However, when you combine that with the overclocking potential of the CPU, it suddenly becomes well worth it. See for yourself:

http://www.anandtech.com/memory/showdoc.aspx?i=2226&p=9

 

[Munky]

Thanks for the info