3 gigs of ram = bad idea, I know.... But Why??

[Kunaak]

I've always heard that odd ram configs is a bad idea.
like 1.5 gigs of ram, 3 gigs, and 6 gigs...
cause then your running mismatched sets of ram.
something about mismatched sets of ram greatly reduce that rams effectiveness or... say 1.5 gigs of ram.... its really 1 gig of ram, cause if you had say 2 x 512 + 2 x 256... those 256's make the 512's run like 256's?
I dont remember the specifics...
I just remember that odd ram configs are a bad idea. just not why.

[alexio]

Dual-channel doesn't work with 3 DIMMs, is that what you mean?
In single-channel mode you can mix all sizes without a problem I think. Chip density can be a problem though, but it either works or doesn't work. I don't think there's a performance penalty from running 512mb + 2*256mb in single-channel compared to 2*512mb in single-channel. Double sided memory has a slight advantage over single sided memory. But let's say the 512mb and 256mb have the same density (so the 512mb is double-sided). The difference between 2*512mb and 512mb + 2*256mb is non existent. It's either 2 + 2 = 4 (2 times double-sided, 2*512mb) or 2 + 1 + 1 = 4 (1 time double sided, 2 times single-sided). Other than the ability to run dual-channel the 512mb*2 setup has no advantages.

[Kunaak]

not talking about single channel...

[EnJoY]

not talking about single channel...

Well back in the day it was bad due to being forced to run 2T on the AMD setups. However, now, I suppose the only downfall to running 3GB over 4GB would be that you wouldn't get the benefit of the better interleaving of the 4GB setup. And of course the single channel...

[irev210]

not sure it matters if they are installed in pairs

Dell used to ship lots of computers with 3GB ram, but now that they figured out people pay more for 4GB for the x86 OS, they only offer 4GB for most of there desktops, lol.

[-n7-]

P965 can do asymmetric dual channel (e.g., 3x512 MB or 3x1 GB, etc.).

[Kunaak]

hmm, none of this is sounding familiar at all...
this is all pretty obvious information so far.
there was more to this, then the obvious stuff like 2T and such...
just wish I could remember where I read this.

[STEvil]

It was a big no-no around the time of nForce2 because people assumed mixing and matching density as well as chip type was bad.

Its only bad when it causes you problems. Otherwise its all good.

[oohms]

If you run 2x1G and 2x512mb it should run perfect, unless your mem controller requires looser timings or running at 2T.

I think all the fuss back in those days were people that were running 3 sticks who had issues with that

[ozzimark]

Its only bad when it causes you problems. Otherwise its all good.

i just did some benching on am2, 2x1gb vs 2x1gb + 2x512mb...


2gb 3gb
read 6597 6457
write 3704 2940
latency 39.9 41.6
pifast 38.67 39.28


and ran 1M
http://www.eclipseoc.com/image/benching/2gb-1m.png
http://www.eclipseoc.com/image/benching/3gb-1m.png


edit: for kicks, just ran 4x1gb, with the same exact settings, i get 26.422s in 1M ;)

[RyderOCZ]

I think you may be thinking of the effects of Rank Interleave Kunaak.

Interleave is based on the module's density, now I am probably not going to get this exactly right, I am sure you can find the info if you start looking, but:

2 x 1GB and 2 x 512 in DDR2 would be 2 DS and 2 SS modules, so the Interleave would only "see" 4 SS modules because of the 2 x 512, thus reducing the effectiveness of the Interleave, which will reduce performance...just as Ozzimark has shown.

That is the general idea, but I probably have some of the mechanics messed up, meaning that it might not be related to DS / SS but more to density of the IC's, but I don't think so.

IMO its not a bad idea if you use memory hogging programs and don't want to go 64-bit OS to be able to use all 4GB. Its definitely a no no for benching though, as memory performance will be affected.

[Plester]

running 3gb in a DS3 rev 3.3 at ddr1000 - 2x1gb gskill GBNQ (promos) and 2x512mb ddr667 ballistix (probably micron). working great.

[Hutch]

I run 3x512mb most of the time unless benching. Had my 4th stick die on me awhile ago and haven't had the money for new ram. Back on topic, I believe it has to do with the refresh rates of the ram? Usually with 512mb sticks they run at 7,8uS and 1gig sticks run at 15,6uS. The problem is that it can't set individual refresh rates for each stick, it will default to the refresh rate of the first stick installed. If you use a 1GB stick in the first slot it will use 15,6uS and 512mb sticks have no problem running 15,6uS. It's the 1GB sticks that can have trouble running 7,8uS. Also, when the board reads the SPD timings, it only reads the timings for the first stick installed, and sets them accordingly.

[_damien_]

I think you may be thinking of the effects of Rank Interleave Kunaak.Indeed so. Any IC consists of at least 4 internal banks, so the Interleave in question regards chip select Ranks on modules.


Interleave is based on the module's densityNot necessarily. There is no *direct* relation between module/IC density and the number of module ranks/banks. Keep in mind that a lot of IC designs come in different versions - even those of similar density. The column width is the key factor here - we'd need 16 x4 ICs but only 8 x8 ICs in order to form one 64-bit wide chip select rank. Halving the column width also doubles the row length and halves the number of ranks/internal banks (when comparing two modules of the same size - a DS module using x8 chips and a SS module using x4 chips). I am not sure which one is better, but my guess would be that having twice the number of short rows (DS DIMM) allow for more fine-grained memory management.


2 x 1GB and 2 x 512 in DDR2 would be 2 DS and 2 SS modulesThat may be the norm but it's certainly no absolute truth. What I said about column width applies here as well.


the Interleave would only "see" 4 SS modules because of the 2 x 512, thus reducing the effectiveness of the Interleave, which will reduce performance...Yes, that's probably the explanation. That fact that 2 DS DIMMs outperform 2 DS + 2 SS strongly suggests that the SS DIMMs somehow prevent the controller from taking full advantage of DS DIMMs.

[EnJoY]

I think you may be thinking of the effects of Rank Interleave Kunaak.

Interleave is based on the module's density, now I am probably not going to get this exactly right, I am sure you can find the info if you start looking, but:

2 x 1GB and 2 x 512 in DDR2 would be 2 DS and 2 SS modules, so the Interleave would only "see" 4 SS modules because of the 2 x 512, thus reducing the effectiveness of the Interleave, which will reduce performance...just as Ozzimark has shown.

That is the general idea, but I probably have some of the mechanics messed up, meaning that it might not be related to DS / SS but more to density of the IC's, but I don't think so.

IMO its not a bad idea if you use memory hogging programs and don't want to go 64-bit OS to be able to use all 4GB. Its definitely a no no for benching though, as memory performance will be affected.


Didn't I say the same thing in less detail in my post above? :confused:

[tvdang7]

P965 can do asymmetric dual channel (e.g., 3x512 MB or 3x1 GB, etc.).

i didnt know that. do they have to be matched sticks?

[Viss]

Ran 3x1gb on P5B for a while. Cannot compare to 2x1 or 4x1 now because on air and temps are too high so rather useless pic. Stock X1950XTX and lod+3 for nature.

http://img.photobucket.com/albums/v636/loc_oc/3GB2001small.jpg

[nugzo]

I'm testing 6GB atm. 2x2, 2x1. I havent seen any difference in performance. I will be starting a new thread once i get all my numbers together. It may be a day or 2 though.

[xlink]

I run 3x512mb most of the time unless benching. Had my 4th stick die on me awhile ago and haven't had the money for new ram. Back on topic, I believe it has to do with the refresh rates of the ram? Usually with 512mb sticks they run at 7,8uS and 1gig sticks run at 15,6uS. The problem is that it can't set individual refresh rates for each stick, it will default to the refresh rate of the first stick installed. If you use a 1GB stick in the first slot it will use 15,6uS and 512mb sticks have no problem running 15,6uS. It's the 1GB sticks that can have trouble running 7,8uS. Also, when the board reads the SPD timings, it only reads the timings for the first stick installed, and sets them accordingly.

my 1gb sticks run at 7.8uS when set to auto in bios so i doubt thats an issue w/ most newer memory.

[xlink]

I'm testing 6GB atm. 2x2, 2x1. I havent seen any difference in performance. I will be starting a new thread once i get all my numbers together. It may be a day or 2 though.

disable pagefiling and set 50 programs to load simultaneously...

[STEvil]

Or just go to the search function, search *.*, select all, and hit enter..