Downgraded memory speeds...

[usna92]

I have my MacPro 5,1 12 core/2.66 (Dual Xeon X5650) which should be running memory at 1333. When I got the machine it had two 1 GB sticks of 1066 running in slots 4 and 8. I have since replace them with 1333 4GB in those slots. All of the other memory is 1333 4GB stick. I should be at 32 GBs running at 1333, but I am running at 1066. Any thoughts on how to reset the memory? I have done the Command Option P R thing, but did not get a different result. Is it possible one of the sticks is bad or mislabeled?

 

[owbp]

I hope that you'll find all info you need in this topic
https://forums.macrumors.com/threads/using-all-slots-in-ram-4-1-to-5-1-firmware-update.1746981/

 

[usna92]

Thanks, covered most of it. So what I am to determine based on the thread is that if you populate all 8 banks you get the slower speed regardless of the speed of your actual RAM?

 

[Slash-2CPU]

Pretty much what I told you in the other thread: Fastest RAM performance will be with the three outermost slots on each side occupied, and the innermost one slot open.

By the 1066 down clock, it appears you're running at least one quad-ranked stick. All 4 slots occupied with any quad-rank sticks present, speed will drop to 1066. 3 slots with quad-rank will run 1333.

By adding the 1066 down clock to the equation, you're going from triple-1333 to dual-1066 by adding that 4th stick. Probably cutting your effective RAM throughput in half. Unless you desperately need the extra RAM, don't do it.

RAM caching won't offset app performance penalty, especially since you're already running a 256GB Fusion cache.

 

[usna92]

Thanks, I see now what you mean now that I have put into action (at least temporarily) as far as the drive goes. I didn't know how the change would manifest itself. You can see the dip in the benchmarks, but I am going to have to see how handbrake, etc. perform in real world application to determine if I am actually getting CPU bound or not.

 

[h9826790]

I also quite sure it's about memory rank, any nothing you can do about it except remove the 4th / 8th stick.

 

[MacUser2525]

By adding the 1066 down clock to the equation, you're going from triple-1333 to dual-1066 by adding that 4th stick. Probably cutting your effective RAM throughput in half. Unless you desperately need the extra RAM, don't do it.

What foolishness it is few percentage points and that is on benchmarks testing not real world performance.

"Do not confuse memory bandwidth with real-world application performance.

The difference between dual channel (8 modules) and triple channel (6 modules) generally amounts to no difference at all. Vastly more important is having enough memory to eliminate the need for disk access, and to allow caching."

https://macperformanceguide.com/Reviews-MacProNehalem-Tests-Memory.html

 

[Slash-2CPU]

Ok, so if 6x4GB is enough, is there any upside to more at a lower speed? Nope.