[Beowulf] dual-core benefits?

Michael Will mwill at penguincomputing.com
Thu Sep 22 14:35:10 EDT 2005

Possible. The quad-dual core chassis starts at around $7k, with
the 865 that is about $12k, and if you add the 32G of RAM
you look at something just below $20k with 8 cores that fits into 3U.

Joel Jaeggli wrote:

> On Thu, 22 Sep 2005, Michael Will wrote:
>> Tahir wrote that they need 4G per node now and 16G later.
>> If you have dual single core, you have 8G per core, but if
>> you have dual dual core, you only have 4G per core.
>> On the other hand the scalability issues Tahir mentioned below
>> sounds like that interprocess communication is the bottleneck
>> and then you want as many cores as you can get in the system.
>> You could then architect your solution with quad-dual-core opteron
>> systems which gives you 8 cores and 32G of RAM in one node.
> I suspect that once it's cost effective for you make the jump to 8xx 
> cpu's and a non-commidity 4-way mainboard that going all the way to 
> the 8way 16 core box is also cost effective.
> the iwill h8501 chassis is around $10500 and the opteron 865 are about 
> 1,300 ea which is a premium of about $500 over the 265. so probably 
> $30k for 16core box that fits in 5u.
>> Michael
>> Joe Landman wrote:
>>> Hi Tahir:
>>> Tahir Malas wrote:
>>>> Hi everyone,
>>>> I would like to take advice for the processor selection for the 
>>>> cluster that
>>>> we will configure soon. Comparing the sequential performance of our 
>>>> programs
>>>> on an Opteron 246 and a much more expensive machine with Itanium 
>>>> Processor,
>>>> we have decided to use opteron processors with Tyan mbs. However, 
>>>> we are in
>>>> a confusion to decide on the processor selection. Before posing my
>>>> questions, I'd better give some info about our application 
>>>> requirements:
>>>> 1. The scalability of our program is not so good, less then 20 for 
>>>> 32 nodes
>>>> (measured on a single node system). So we don't plan to go beyond 
>>>> 16 nodes.
>>>> (which makes 32 processors due to dual-node usage)
>>> Do you mean a single cpu per node, or single core per CPU, or a 
>>> large SMP? You might also wish to look at the iWill motherboards.
>>>> 2. Memory requirement is huge; we will use 4GB memory per node for 
>>>> the time
>>>> being and increase this to 16 GB later. So wee need fast CPUs and 
>>>> efficient
>>>> usage of memory.
>>> Ok, you are going to want the later model MB's that properly support 
>>> DDR/400 to single rank dimms.  2GB dimms are still not cheap (at 
>>> least the good ones).
>>>> 3. Due to budget limitations we will first configure 8-node system 
>>>> with 4GB
>>>> RAM per node and extend this to a 16-node system with 16-GB of RAM 
>>>> in 6
>>>> months.
>>>> We were thinking of AMD 250 processors, but now the benchmarks of 
>>>> dual-core
>>>> CPUs (on the web site of AMD) seems encouraging, and the cost of 
>>>> dual-core
>>>> AMD 275 seems to be less then twice of AMD 250. 
>>> http://enterprise2.amd.com/downloadables/Dual_Core_Performance.pdf 
>>> and other I presume. :)
>>>> Since the memory cost of our
>>>> system will dominate other costs, we can afford to pass to dual-core
>>>> technology. However, the questions that arise are follows.
>>>> 1. Will it worth? And can we gain any advantages over single-core 
>>>> with the
>>>> not-so-good scalability of our parallel programs? 
>>> It depends upon the code.  If your code requires very low latency, 
>>> the benefit of dual core nodes are that you have 4 interconnected 
>>> cores (think of them as individual processors) connected over a very 
>>> high speed low latency interface.  If this is well coupled to the 
>>> rest of the system through an external low latency interface 
>>> (Infinipath, IB, Myrinet, etc), and your code is latency sensitive, 
>>> then dual core could be a substantial win for you.  If your code 
>>> simply hammers on memory bandwidth, then it is possible in some 
>>> cases for it to be a liability relative to single core. Some cases 
>>> (weather codes) demonstrated something like this here in the recent 
>>> past.
>>>> 2. Another question is that is dual-core technology brings any 
>>>> advantages
>>>> for the efficient usage of high amount of memory that we will 
>>>> utilize? 3. 3.
>>> Not really advantage or disadvantage.  With single core, your 
>>> aggregate memory bandwidth is N(cores) * Bandwidth of one of the 
>>> memory busses. With dual core, it is (N(cores)/2) * Bandwidth of one 
>>> of the memory busses. This may or may not be an issue for your code.
>>>> 3. Finally there is something basic that I'm not sure: When we 
>>>> assign a job
>>>> to dual-core CPU, can it divide it between the core-CPUs 
>>>> automatically, or
>>>> should we think dual-core CPU the same as dual-node CPU? If the 
>>>> latter is
>>>> the case, what is the advantage of this technology over dual-node?
>>> Think of this as 2 physicallly independent CPUs (that just happen to 
>>> share the same space on the motherboard).  That means your dual CPU 
>>> nodes become 4-ways.  In terms of assinging a job to a CPU (or 
>>> core), you still need a threading library or an MPI library and 
>>> appropriate changes to the source code to make it scale.  But the 
>>> advantage for you would be less overall latency between CPUs for 
>>> messaging using MPI, and large SMP nodes for OpenMP.  The potential 
>>> disadvantage is loss of effective memory bandwidth. If you look at 
>>> the above URL for the paper, you will see that the bandwidth issue 
>>> wasn't a factor for the tests we ran.  It could be for your code, 
>>> and that the important part.  You need to test to be sure.
>>>> If anyone has info and/or experiences about these, I will be very 
>>>> glad to
>>>> know.
>>>> Thanks in advance,
>>>> Tahir Malas
>>>> Bilkent University Electrical and Electronics Engineering Department
>>>> Phone: +90 312 290 1385
>>>> _______________________________________________
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Michael Will
Penguin Computing Corp.
Sales Engineer
415-954-2899 fx
mwill at penguincomputing.com 

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