to interleave or not to interleave...

[Ken Gunderson]

On Thu, 5 May 2005 12:24:55 -0700
"David O'Brien" <obrien at freebsd.org> wrote:

> On Thu, May 05, 2005 at 12:08:21PM -0600, Ken Gunderson wrote:
> > Searching for enlightenment and I'm not a memory guru nor kernel hacker
> > here so could use some help understanding this. System is a Tyan
> > B2882T26 w/ a pair of 246's.
> 
> This uses the Tyan s2882 motherboard, which is really what the question
> centers around.

Yes;-)
 
> > I've two 1GB sticks of Corsair ECC
> > Registered memory onhand.  Question is whether it would be "best" to: 
> > 
> > 1) put both cpu0 dimm slots 1 & 2
> > 2) put 1 in dimm slot 1 of each cpu
> > If I understand correctly, config 1 results in 128 bit interleaved while
> > config 2 results in 64 bit non-interleaved.  (config 1 leaves cpu 0 as
> > memory "controller" for cup 1?).
> 
> Correct.

Thanks.  Glad to see I got at least that part of it right...;-P
 
> Using both of your DIMMs in one CPU means you do 128-bit (dual-channel)
> memory accesses, but your 2nd memory controller is idle.  And you are
> guarenteed the 2nd CPU will always go across the Hypertransport link for
> memory accesses.
> Putting one DIMM in each CPU means you only do 64-bit (single-channel)
> memory accesses, but you are able to have both memory controllers active.
> 
> 
> > I've read some stuff where
> > interleaved results in faster access but also can have "miss"
> > penalaties.  Any explanations on where one strategy is better than
> > other appreciated.
> 
> This is a hard question to answer.
> The real answer is to get another 2 DIMM's.  

It wasn't in the budget though and wanted to maintain good future
expandability, so went with the 2 x 1GB config (system was already
over budget and I was concerned about adding another $350 to the
price tag). Maybe my bad though...

I figured the 128 bit interleaved must be superior, else they wouldn't
have the option.  But reading some stuff here (that I don't completely
understand) <http://www.cs.swan.ac.uk/~csneal/HPM/memory.html>, as well
as a couple other places, talk about downsides to the strategy.

It seems in my situation is a six of one and half dozen of the
other?  If it's any help, the server will do web, email, and
some application server duties (e.g. Zope, JBoss, Tomcat, etc.  Not
sure which yet).  Envision some light db ala PostgreSQL feeding the app
server.  Either way I don't think this puppy is going to see enough
load to even begin to stress it.  I'd just like to do the best I can
with options at hand.

Thanks David.  Much appreciated.

-- 
Best regards,

Ken Gunderson
GPG Key-- 9F5179FD

"Freedom begins between the ears."	-- Edward Abbey