[Beowulf] Nehalem memory configs

richard.walsh at comcast.net richard.walsh at comcast.net
Thu Apr 9 17:10:33 PDT 2009




> Kilian CAVALOTTI wrote: 
>> On Wednesday 08 April 2009 19:48:30 Joe Landman wrote: 
>>> As an FYI, Beowulf veteran Jeff Layton wrote up a nice article on 
>>> memory 
>>> configuration issues for Nehalem (I had seen some discussion on this 
>>> previously). 
>>> 
>>> Link is here: 
>>> http://www.delltechcenter.com/page/04-08-2009+-+Nehalem+and+Memory+Configur 
>>> ations 
>> 
>> That's a great write-up, gathering all the useful information in one 
>> place. A must read. Thanks Jeff! 


It is a good write up, very good ... but there looks to be an error under "Memory Bandwidth 

Performance" ... first, for socket-local-memory, I am not sure of the relevance of 

the QPI GT rate.  I would calculate the maximum rate to socket-local-memory 

with 1 DIMM per channel at: 



1.333 GHz  x 3 Channels x 8 bytes =  31.992 GBytes/sec 



not 35 GByters/sec.  Maybe I am doing something wrong.  Rates to socket-remote 

memory would be lower of course, bounded by the QPI 6.4 GT/sec transfer rate, which 

on this 2 byte duplex channel would be 25.6 GBytes/sec I believe. There would also 

be a performance drop due to the latency increase from 60 nanos to 100 nanos. 



Perhaps I have made an error somewhere ... if so, I would love have someone tell where I 

made my mistake. 



rbw
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