Mixing ram sizes? Whats the problems?

I've just had a call from brother in law...

He has a dell (i beleive 1501... i tihnk)... Hes currently got 1x1gb DDR2 667 installed.... he wanted to know if we was able to mix different size modules and STILL benefit from Dual Channel Memory.... I wasnt to sure on hte answer, and his question got me thinking the same thing as im going to be doing the same thing on my T61....

sooooo my question is (im becoming a bit of post whore today sorry!)

"can i mix differnet size modules, such as 1gig and 2gig to make 3gig and STILL benefit from dual channel memory, on both a T61 and a 1501"

Thanks again guys!!
Will

 

Dual channel will work with mis-matched memory modules, however, it will not be "true" dual channel. Instead, it will run in assymetric mode, which gives quite alot less bandwidth than standard dual channel. However, bear in mind, that dual channel is not something that increase performance massively. The effect it has on the majority of applications is very small, and it would only be benficial in very heavy memory applications.

 

in my experience the speed penalty of asymmetric dual channel is around 10%.

See the results that I posted in this thread.

John

 

See this thread:
http://forum.notebookreview.com/showthread.php?t=157819&highlight=dual+channel

 

ah right - thanks for that......

I know its a little wastefull, but to benefit FULLY from dual channel, shouldi replace the original 1gig stick with another 2gig stick, effectively giving me 4gig system ram.... i know Windows Vista 32 will not be able to see this (unless i decided to move to vista 64....) but it will potencially maximise the memory available to Vista 32 and also maximise the dual channel effect...

Does that make sense or am i talking rubbish?

 

I would get a 2GB module and put into the empty slot. That will make a big difference to the performance partly through higher bandwidth but mainly because of less need for disk accessing.

Replacing the existing 1GB with a further 2GB stick will only be worthwhile if you need to squeeze the last drop of performance out of the system. (OK, doing this to my computer is in my plans, but only when I can get a 2GB PC6400 module at a nice price, ready to move over to the next computer).

John

 

One thing that needs to be considered here is I believe we are talking about AMD not Intel. That makes a very big difference. Intel single vs dual appears to be 10%, not that much of a diff. On my AMD running with single vs dual is 30%, much more substantial. From what I have seen an Intel running an asymmetric RAM array (interleaved mode) consisting of 1X1GB + 1X2GB is 3% less bandwidth than running a symmetric RAM array. Those numbers make some sense, 3GB, 2 of them can match up and 1 is the odd “man” out. 1 of 3 is 1/3 which is about 3% of the whole of 10%. I cannot test my AMD in a asymmetric RAM array because I do not have diff size sticks but if I can extrapolate from the Intel numbers the same 3GB set up would be 10% slower because 1/3 of 30% is 10%.

Those are the differences as I understand them Intel takes much less of a hit than AMD in single channel or asymmetric RAM array.

And as Zero correctly stated real world most applications don't require the kind of bandwidths we are talking about so real world likely will not see a difference in any of the configurations, but they are measurable so therefore laid out above as best I could. And remember the computer geek credo: bigger,better,faster!