New computer memory configuration

[Matthew ..]

Hi All,

 

We are about to buy a new machine which will mostly be used for FDS simulations.

 

Due to customer support issues we are likely getting a Lenovo C30 system with 2 x Xeon E5-2650 v2's.

 

I understand that the CPU is a key component that affects simulation speeds however memory and hard disk speed also have an impact.

 

Am I right in thinking that it would be better to provide 8 x 4 GB of memory (The CPUs have 4 memory channels each) rather than 2 x 16 GB? Most of the models that are run will use ~2GB to ~8GB of memory, with more than 20GB being a very rare event. 

 

Also, my understanding is that hard disk speed is not often a critical factor - particularly for medium to larger models where the outputs are written less often. Is that a fair assumption? Will there be much benefit from a raid-0 array?

 

Regards,

 

Matt

[dr_jfloyd]

If you want to take full advantage of the quad-channel architecture then yes 4x8 will be better than 2x16.

Disk speed has almost no impact. Even in a trivial case (~2000 grid cells) dump.f90 only takes a few percent of the run time and much of that is still processing data to get it ready to write to disk.

[Matthew ..]

Thanks very much for the info Dr J!

 

If I may ask a bit further info, on a qualitative level, will the 8x4 configuration be notably quicker than the 2x16?

 

Regards,

 

Matt

[Matthew ..]

Hi Again,

 

It seems like the IT department are insisting on 2 x 16GB instead of 8 x 4GB because it is cheaper and they don't believe there will be any performance gain.

 

I'm not a computer engineer, I cant really convince them that 8 x 4GB will be faster. Does anyone happen to have any links to studies or reports which might show that RAM configuration has an impact on FDS?

 

Regards,

 

Matt

[Matthew ..]

Hi All,

 

Just an update - I have spoken to Lenovo and they clearly said that less sticks will give you more speed - see the conversation below. I'm not entirely convinced though.

I've also been bounced around their sales and technical on the phone without much success on getting a convincing answer.

 

Maybe its just me being ignorant and I should take their word for it? 

Regards,

Matt


Online conversation with Lenovo:

Please wait while we connect you with a Lenovo Chat Representative.

Connected to Harathi Vemishetty
Harathi Vemishetty : Thank you for contacting Lenovo Sales Chat. My name is Harathi and my Rep ID is 2900711681. Happy to help you today. Hello, may I know your name please?



You : Hi, my name is Matt
You : I have a quite technical question
Harathi Vemishetty : I will try to answer Matt
Harathi Vemishetty : We have dedicated technical team for this
You : We are about to buy some new machines which will mostly be used for Computational Fluid Dynamics simulations using the Fire Dynamics Simulator program. The simulations are very memory intensive and are normally run using MPICH. We are likely getting Lenovo C30 systems with 2 x Xeon E5-2650 v2's. I understand each Xeon processor has 4 memory channels and there are therefore 8 memory channels in total. Am I right in thinking that it would be better (higher performance) to provide 8 x 4 GB of memory rather than 2 x 16 GB? The key issue I see is that if you only use 2 of the DIMM slots then you are only maximising two of the memory channels. In that case the memory transfer will be limited. If you use all 8 DIMM slots you will use all the memory channels and maximise throughput. Therefore 8 x 4GB will have more transfer bandwidth than 2 x 16GB. I also realise that 8 x 4GB will be more expensive and will limit upgrade options in the future.
You : Sorry, this tool doesn't like paragraphs.
Harathi Vemishetty : Let me read
Harathi Vemishetty : Have you customized C30?
You : That is the question, when we buy the new machines will customising it with 8 x 4GB RAM modules give a performance boost compared to 2 x 16GB?
Harathi Vemishetty : Let me check that for you.
Harathi Vemishetty : Please give me few minutes
You : Ok, no problem, thanks.
Harathi Vemishetty : Thank you
Harathi Vemishetty : Thank you for your time. 2 x 16GB will give better performance
Harathi Vemishetty : May I know if you are customizing it now to order?
You : Yes
Harathi Vemishetty : Okay
Harathi Vemishetty : I will be online while you place the order
You : Why will 2 x 16GB give better performance? Can you explain a little?
You : Why would not using all the 8 memory channels give better performance?
Harathi Vemishetty : I will explain you
Harathi Vemishetty : System will take some of the memory capacity from every RAM stick
Harathi Vemishetty : This is just an approximate figure. When a 8GB RAM is added, the actual RAM you get will be 7.5GB or so, that way you'll loose more capacity of the RAM when you add more sticks.
You : Ok, I can understand that. So 2x16GB will give you more total memory than 8x4GB.
You : However, capacity is not the main concern. I am concerned with speed. I want the fastest memory to CPU transfer available.
Matt, less # of memory sticks will give you more speed

You : Ok, that's interesting. Thanks.
Harathi Vemishetty : You are welcome

 

Regards,

 

Matt

[dr_jfloyd]

http://www.legitreviews.com/ddr3-memory-performance-analysis-on-intel-x79_1779/3

on this motherboard there is some performance gain with quad channel memory in going from 1 to 4 memory sticks.

[Matthew ..]

Hi,

 

Thanks very much!

 

I have also found the following quotes (sorry for the lazy formatting):

 

This white paper from HP states that “By interleaving memory access across multiple memory channels, the integrated memory controllers are able to increase memory throughput significantly. Optimal throughput and latency are achieved when all channels of each installed CPU are populated identically. As Figure 8 shows, adding a second DIMM to the system (and thus populating the second memory channel) essentially doubles system read throughput. Gains in throughput for each additional DIMM installed are almost linear until all eight memory channels are populated.” http://h20565.www2.hp.com/portal/site/hpsc/template.BINARYPORTLET/public/kb/docDisplay/resource.process/?spf_p.tpst=kbDocDisplay_ws_BI&spf_p.rid_kbDocDisplay=docDisplayResURL&javax.portlet.begCacheTok=com.vignette.cachetoken&spf_p.rst_kbDocDisplay=wsrp-resourceState%3DdocId%253Demr_na-c03293145-2%257CdocLocale%253D&javax.portlet.endCacheTok=com.vignette.cachetoken This is for the same CPU family, but different computer manufacturer.

 

This white paper from DELL states that “First and foremost, customers concerned about extracting all available performance out of the memory subsystem should strive to populate all memory channels evenly with the same rank, size, and speed of DIMMs. “ and “As Figure 31 indicates, the overall system memory bandwidth declines as memory channels are depopulated. This decline occurs in an almost linear fashion as fewer memory channels are contributing to overall bandwidth. This decline in memory bandwidth will have direct performance impacts on standard server workloads, especially scientific workloads that tend to be the most memory bandwidth sensitive. Anything less than 4 memory channels populated indicates an unbalanced memory configuration.”  http://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/12g-memory-performance-guide.pdf This is for the same CPU family, but different computer manufacturer.

 

 

“the bottleneck of computing is currently in data transfer [between CPU and memory] not in processing” http://www.cfd-online.com/Forums/hardware/95432-ram-speed-worth-fastest.html

 

“understand that it is vital to utilize all 4 memory channels for each CPU, as 2 channels will only have approx. half the transfer rate“. It then goes on to talk about memory “Ranks” which is above my head. http://www.cfd-online.com/Forums/hardware/124834-optimal-memory-configuration.html

 

This post talks about getting the fastest/best ram and the most memory channels and these being important.   http://www.cfd-online.com/Forums/hardware/111416-hardwares-workstation-more-important-do-cfd-analysis.html

 

“Neither CPU clock speed, nor cache size, nor even architecture, is as significant as the memory system, when it comes to performance in CFD”, and also when referring to whether 8 x 4GB or 4 x 8GB is better “8 channels, 8 slots, 8 modules.” http://www.cfd-online.com/Forums/hardware/113263-advice-technical-requirements-new-fluent-workstation.html

 

Regards