Faster to have all 1333mhz memory or mix and match 1333 and 1066mhz for a higher GB?

Hi All,

Upgrading a machine with 4 GB 1066mhz memory, adding 8 GB 1333mhz.  There is enough DIMM slots to run all 12 GB.  Is there a rule of thumb as to if it will run better with 12 GB (but presumably it will auto-set them all down to 1066mhz) or if it would run better with just the 8 GB at 1333mhz?

We could also find 8 GB 1066mhz but that seems to have little point if it can just underclock automatically.

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mklpConnect With a Mentor Commented:
Hello Jsmply,

As a rule of thumb, the simple answer is that additional RAM will improve performance more than increased clock speed of the RAM. That said, the fun begins and you might also consider the following:

1) Yes, the BIOS will cripple the 1333 stuff back to 1066 because of the 4 GB, but what is the FSB (Front Side Bus) speed of the system? Memory management is handled by the Chipset  If the FSB is 533 MHz anyhow (1066 RAM) then no harm no foul concerning overall system performance.

2) When memory (the silicone) is fabricated, the chips are tested to see how many clock cycles that particular chip can handle. Although it may be engineered to come out at 1333, if it has errors at that speed but runs fine at 1066 then, voila, you've got a brand new shiny 1066 chip. Thus if 1333 is running at 1066 it will simply last forever and not be prone to errors. Kind of like running a 100 watt lightbulb at 60 watts on a dimmer. The tungsten will last forever.

3) Concerning using only the 8 GB of 1333 vs all 12 GB mixed (thus running at 1066 because of the first 4 Gigs) in general the more RAM the merrier as mentioned above. BUT, what is the machine used for and how much RAM is generally required for the applications? If the 4 GB left you short and thus paging to the disk a lot, then any addition will help but possibly not as much as you would like it to. If it's video editing, then as much as possible is probabaly best, but if it's office applications, browsing, and games (presuming the 8 GB is enough to satisfy all of what you may have open at the same time) then, (again presuming the system supports 1333) 8 GB at 1333 MHz may show better performance. If you've got multiple GB databases you're trying to crunch, then more is merrier no matter what the clock speed.

4) You should check the motherboard, processor, and chipset specs to make sure that you don't need to be running your RAM in matched pairs (or "sets"). Let's say you have 4 slots and two are now occupied with 2 x 2 GB sticks. You're going to add 2 x 4 GB sticks to the other 2 slots to make your 12 GB. That's fine. However if your system wants "matched pairs" then you want to know some specific stuff. Are slots 1 and 3 along with slots 2 and 4 the "pairs" or is the motherboard configured so that slots 1 and 2 are a pair along with slots 3 and 4 for the other pair? In any case, if "matched pairs" are required for optimal perfomance, mixing the speed (and even the brand) can cause obscure headaches. It's always best to use the same brand and clock speed when pairs are required, at minimum in their respective "pair" slots, but ideally throughout. In other words, don't mix a Corsair 1333 with a Micron 1333 and call it a "pair". And NEVER put different clock speeds or sizes together in the same "pair" slots no matter what the brand.

5) You're not likely to disable the system if you don't follow 4 above because the BIOS and Chipset will likely handle it for you, but your shiny new 8 GB might just make things run slower (and maybe funkier) than when you had only 4 GB configured properly.

6) I would be inclined, if you really need (or just want) 12 GB, and want it to perform, to grab another 4 GB of 1333 to replace the 1066. This again presumes the system will handle it. Check first to make sure that the BIOS will recognize it, and then check the motherboard and Chipset to see if the 1333 is even going to help it. The system won't reject the 1333 because of the lightbulb analogy above, but if the SYSTEM is going to only run it at 1066, then putting 1333 in it will never make it "1333" because the system won't handle it.

7) This should really have been number 1 in my answer because it might have saved you a lot of reading. Please tell me you're running a 64 bit OS of some flavor. If you're on 32 bit XP, then all of the above is for naught. You can shove as much as you want into the slots and a 32 bit OS is only ever going to use "about" 3 GB. And I do mean "about". There is no hard rule because it's dependent on factors other than just the OS. 64 bit XP or Win 7 should be happy with as much as you can throw at them, one last time, dependent on the system (hardware). :-)

Hope this helps,
Gary CaseConnect With a Mentor RetiredCommented:
A couple of thoughts ...

(1)  The system can't run RAM at differing speeds, so as you've already surmised, it will run ALL of the RAM at 1066 if you mix modules of the two speeds you've indicated.

(2)  It MAY run at the slower speed anyway -- even if you install all 1333 modules.    This depends on what the motherboard does to compensate for the high bus loading with 6 installed modules.     I'm assuming from your comments that you're using a motherboard that (a) uses unbuffered RAM, and (b) has 6 memory slots.     If this is the case, it's NOT a good idea to run with 6 installed modules.     If you're using a board that supports dual channel RAM, I'd limit the installed modules to 4;   if it's a triple channel board, I'd limit it to 3 installed modules.    Your memory will be FAR more reliable with this configuration.    If you want a large amount of RAM, use 4GB modules.
SemperWiFiConnect With a Mentor Commented:
@ Jsmply - As you can see, answers on this subject can be quite lengthy. There is certainly some good information posted above. But the one rather important item and seems to be currently missing from this thread. This is a bit of information from you concerning what is the primary use for this system?

The basics behind how RAM works requires all of the modules to be match in operation. So, yes, since it makes most sense to think faster modules can more easily run slower than the reverse, all BIOS coding will relegate all modules to the lowest denominator. This not only includes speed, but latency settings as well. The question is, does this really matter to you?

Some processes perform best with a lot of memory while many don't use very much in terms of memory resources and therefore benefit more greatly by having more speed (or faster memory). Habits can also play a factor; do you use a lot of open applications at a single time ensuing a collective saturation of resources?

The simple way for you to sort out which is best for you. Start with the smaller amount of faster memory. Run the processes which are most intensive on your machine.... is the your memory saturated? If yes, is it sustained or just a quick blip? If the memory does not get saturated for a sustained period of time then you are not running processes which are memory intensive and will therefore benefit more greatly by an increase in memory speed.

On the other hand, if you find that your system memory is pegged about 85% utilization while running your system as you normally do then it would be a performance increase to add more RAM to the setup. Of course this is best done by using matched modules at the highest speed supported by your platform. However, 1333 vs 1066... the honest answer, you'll never feel that behind the mouse. The difference will only become apparent in benchmarks or when running sustained processed which are time and/or action sensitive.
Craig BeckConnect With a Mentor Commented:
Never mix memory speeds - end of!

You will be better using the 8GB at 1333Mhz and having a stable system.
JsmplyAuthor Commented:
Thx all!
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