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Allan Walsh March 29, 2004 16:38

Fluent and RAM Speed
We are thinking about replacing a desktop CFD computer with a laptop. One of the laptops we are looking at has DDR RAM at 400 MHZ. On the desktop we are currently using, I believe that we have a RAMbus memory at 800 MHZ. A while ago, we had a system at which the RAM ran at 133 MHZ or close, and it seemed very slow.

Anyone have any experience with these issues and Fluent (running under Windows)?

David March 30, 2004 08:47

Re: Fluent and RAM Speed
you can use models with 100 MB RAM which contain up to roughly 100 000 cells.

If you have 1 GB of RAM, you can use 1e6 cells etc.

If you can and need to use big and complex geometries, you better use parallel processing

hope this helps


Otilia March 30, 2004 18:46

Re: Fluent and RAM Speed
Limitation in laptops is usually memory.

As David said, the rule of thumb is 1Gb for 1 million cells. Laptops do not usually have more than 512 Mb and some solvers (i.e. coupled implicit) use more memory (twice as much as segregated).

Allan Walsh March 31, 2004 12:42

Re: Fluent and RAM Speed
Guys, thanks for the reply but you missed the point. I wasn't asking about how much RAM but rather RAM speed and type. As an aside, in our first CFD modeling efforts in the mid-1980's we could get 50,000 cells into a VAX II with 16 megs of RAM.

But back to the alphabet soup of RAM: SDRAM, DDR, RAMbus, 400 mHz FSB, 800 mHz. Does anyone really know what the effect is on FLUENT run times?

I did get a reply back from a woman at FLUENT support who seems to know what she is talking about. She says DDR is good so I'll try it.

Nico April 7, 2004 08:01

Re: Fluent and RAM Speed
We have several systems running different kinds of RAM and yes the clock speed matters a lot (2 Ghz xeon with 266 DDR is slower than 1.5 Ghz equipped with RDRAM), fastest so far seems to be 400 Mhz DDR (800 Mhz FSB)

chris April 17, 2004 11:47

Re: Fluent and RAM Speed
Hi Allen,

You might have bought your computer by now, but I looked into types of RAM for use with Fluent recently too so I thought I'd tell you what I found.

The short answer is that DDR400 RAM is currently the fastest mainstream RAM for PCs and I found it easily represented the best speed for the money. The 133MHz RAM in you old system was of the equivalent status a few years ago. Rambus is fairly obsolete for PCs at the moment.

The long answer is as follows: It IS possible to get faster RAM than this but in general you pay a premium and it is more normally used by tweakers, gamers, etc.

It is important that your motherboard and you RAM can "keep up" with one another. e.g. to take advantage of DDR400 RAM, you will need a motherboard with an 800MHz FSB (front side bus - the bus that communicates with the RAM). Note, 800MHz for DDR400; DDR400 is dual channel 400MHz RAM, hence it requires an 800MHz FSB.

So if you use faster RAM, you need a faster FSB etc. As I mentioned, at present DDR400 with 800MHz FSB is well established for current fast PCs (and you can get it on more expensive laptops, as you've seen). If you want to go faster than this, I recommend you take a look at

As far as desktop vs laptop, PCs still (and will continue to) outperform laptops on a speed per cost basis. I would say that if you're buying a dedicated CFD computer, you should consider a desktop.

The amount of RAM, as other users have pointed out, is also an important consideration. Basically, you should always make sure you have enough RAM such that the results of computations can be stored entirely in RAM whilst iterating. Otherwise, your computer will resort to swapping data between RAM and hard drive (MUCH slower) and all that speed of your expensive DDR400 RAM will go to waste!

In terms of use for Fluent specifically, it's really the raw computing speed you need to consider and the type of RAM is really just how it achieves this speed so you don't really need to know about the architecture - just get the fastest you can. (again, if you're interested in that sort of thing, tomshardware has some interesting benchmarking tests etc with different setups)

To summarise, my choice: A desktop with a decent motherboard, fast CPU, 800MHz FSB, and as much DDR400 RAM as you need for the grids you want to run computations on. If you must have a laptop, yeah, try and get one with DDR400 RAM.

Hope you find this useful (and not too boring!)



Allan Walsh April 21, 2004 16:08

Re: Fluent and RAM Speed
Thanks for the info. We ordered the laptop but haven't received it yet to try it out.

Like most everyone else, we have been through the swap from RAM to hard drive years ago (especially with Win95) but this is not a problem anymore.

Where I was surprised was a few years ago when we upgraded from 300 MHz Pentium to a 1 GHz AMD Athalon. With small problems, speed up was by about 3 times but for large problems (which still fit comfortably in RAM) there was little or no speed increase.

After many conversations with Fluent and no solution, we replaced the AMD box with an Intel-based machine. In retrospect, I'm guessing it was the RAM that was the "bottleneck", but at the time no one pointed to it as a problem.

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