Hi Guys

Here's and easy query for you CFX-5.7 users.

What is the maximum amount of memory which can be used when CFX-5.7 is run in serial mode? i.e. on one PC running windows XP Pro.

The user manual says that 2GB is the maximum. Is this still the case and does this mean that having a 4GB machine would be a waste of resources since only 2GB could be used for any single run?

Regards James

Windows can only allocate 2Gb to a single process. You can put 4Gb in the machine, but you'll have to run in parallel (i.e. 2 x 2GB processes) in order to fully utilize it.

Note, however, there are other issues that prevent the 4Gb from being seen and used, mostly to do with the Video Drivers taking up space under the 4GB memory address limit.

There's supposed to be a 64-bit Windows coming out, which would alleviate this limit on 64-bit Itanium chips, but last I heard it was SEVERELY delayed.

Jeff

2GB is maximum for 32bit machines(even on linux...). But under windows if you have something like 3Gb - you will have more chances to get memory closer to 2Gb (due to memory fragmentation reasons). But 4Gb is waste of resources for single run. But you can run job in parallel (even on single CPU without slow down(I test it)) and can run larger than 2Gb jobs.

Not quite true....

2^32 is 4Gb. Linux will run up to a 4Gb process on a 4Gb machine (Yes, I've done it). Windows, however, uses one less bit in it's memory addressing which drops the actual addressible memory for a single process by 1/2, down to 2Gb.

Jeff

Cheers everyone for the advice. I guess i'll have to go parallel if i want to use more than 2Gb of memory.

Regards James

Jeff, can you tell me how you can ran 4Gb job in serial on linux. What version of linux you used and so on. Because I can not run job bigger than 2Gb on linux.

deLuther

I use CFX57 and Linux RedHat 7.3 with native kernel (2.4) and observe 2GB limitation during serial run. What You do for 4GB process activation?

Hi,

A 2GB maximum job size is inherent in Linux and Windows. I cannot see how James ran a job bigger than 2GB on a 32 bit Linux machine - but if he did I would like to know how!

Genn Horrocks

Sadly i didn't run a 4GB serial job but Jeff believes he has on a Linux machine.

Regards James

Hi Jeff,

Are you sure you have run a job larger than 2GB single processor on Linux? My understanding is the 2GB limit is inherent in the 32 bit hardware, and what operating system you use can never go beyond that.

Glenn Horrocks

As I understand there is big memory kernel for linux which can adress 4Gb and even more...This kernel uses 36bit adressing(of course by coast of decreasing speed-purely software trick). This all what I know, never dealt with this.

It has nothing to do with the number of bits used for addressing.

It has to do with signed and unsigned pointers.

All pointers in Linux/Windows are 32 bits long. A signed pointer though can only address 2^31 - 1 spots. Hence a 2GB theoretical limit on both operating systems.

The "large memory" versions of linux allow for unsigned 32 bit pointers to be used which gives you theoretical access to all 4 GB.

Problem is that the OS, device drivers, services, etc... take up space and may or may not be able to be swapped out to the swap space. So your jobs will never get to these theoretical limits.

Practical limits for CFX-5 on Windows are maybe 1.5 GB in serial, if you are very lucky and larger (up to 2GB) on Linux because it seems to be more tolerant of swapping other processes out to make space.

Jeff's claim of 4 GB on Linux is just not possible with CFX-5. He may have done that with some other software.

Neale

Hi Glenn,

See my other post if you want.

There is no way this "Jeff" did this with CFX-5.

He may have done it with something else.

Neale

Hey All,

Sorry, I've been away and just got back to this flurry of discussion and doubt. I'm actually starting to doubt it myself, but I'm almost sure we ran above the 2Gb limit on a Dell Precision with 4Gb of memory. I live in an Itanium 64-bit world now (new job) and do not suffer from these issues.

This >2Gb run "may" have been w/ CFX-4. I will double check with my cohorts and post back shortly.

Regards, Jeff

Hi Jeff,

I think you will find CFX4 can't use over 2GB on 32 bit machines either.

Glenn Horrocks

Hi All,

This topic is discussed in a bit of detail in an FAQ on the CFX Community Site. Please see http://www-waterloo.ansys.com/cfxcom...ail.asp?id=664 for more details.

Regards,

Steve.