Running DDR3 slower than designed: Performance implications?

rpg · 25 Feb 2010 at 19:31

I'm looking to buy some aftermarket DDR3 RAM for a laptop. The laptop currently runs DDR3-1066 at the speed of 1066MHz. However I want to overclock the FSB and run the RAM at 1333MHz. The RAM currently in the machine is not capable of that high of an overclock.

So I've found some RAM that's suitable. It's 4GB of Kingston HyperX 1333MHz which will do the job nicely. However, I notice that for only £1 more, I could buy the 1600MHz version. Both kits have a CAS latency of CL7.

I won't be able to run the RAM at 1600MHz on my current laptop but when I sell it, the new RAM will come out (the old RAM will go back in) and maybe my new laptop will be able to run DDR3 at 1600MHz. So I'm seeing it as a possible future investment.

So, the question... With a rated FSB of 1333MHz, is there any performance difference between using CL7 DDR3-1600 and CL7 DDR3-1333?

rpg · 27 Feb 2010 at 17:03

I never like to bump threads, but I'd be very grateful for a response.

Obviously I understand the 1600MHz RAM will be downclocked to 1333MHz but my question is will downclocked 1333MHz RAM with the same CAS latency perform the same as RAM actually designed to run at 1333MHz?

Thanks,
Richard

gurusan · 27 Feb 2010 at 17:05

rpg said:
my question is will downclocked 1333MHz RAM with the same CAS latency perform the same as RAM actually designed to run at 1333MHz?

Yes

rpg · 27 Feb 2010 at 17:13

gurusan said:
Yes

Thanks gurusan, much appreciated.

rpg · 28 Feb 2010 at 15:12

Last question, I promise (I removed the previous question)...

For reference, I'm actually talking about a laptop here so I can't adjust the RAM timings, so I presume they'll just run at their pre-set JEDEC timings.

As I mentioned above, the RAM in the machine is DDR3-1066 and is running with standard timings of 7-7-7-20. But if I bought 1600MHz RAM and it downclocked itself to 1066MHz, would it run with 7-7-7-20 timings also or would they be higher?

On the manufacturer's website it claims this about the 1600MHz RAM. "The SPDs are programmed to JEDEC standard latency DDR3-1333MHz timing of 9-9-9 at 1.5V."

Marvin · 28 Feb 2010 at 18:35

They normally program more than one set of timings in, for users like yourself who cannot run it at the specified speeds. The timings should get better/lower when run slower.

rpg · 28 Feb 2010 at 20:51

Marvin said:
They normally program more than one set of timings in, for users like yourself who cannot run it at the specified speeds. The timings should get better/lower when run slower.

Thanks for the reply Marvin. That's good, hopefully at 1066MHz it'll run at less or equal timings to what I have now (7-7-7-20).

cmndr_andi · 28 Feb 2010 at 21:11

Is this upgrade purely for memory bandwidth, or are you upgrading the memory capacity at the same time (ie 2GB to 4GB)? If you are upgrading the capacity too, then the 1600MHz kingston stuff looks great. I imagine these sticks are just higher binned versions of the 1333Mhz ones - so they are effectively the same but performed better in initail tests so were rated to run at higher clockspeeds. Running these "1600MHz sticks" at 1333 is perfectly fine and will just put less stress on them.

However, if this upgrade is purely to increase memory bandwidth I would ask you to have another think. £96 is a lot of money to spend and the actual performance benefit you will see is marginal at best. Depending on your laptop type you may be able to replace the CPU yourself and get a much more tangible speed boost for similar money.

thingemajib · 28 Feb 2010 at 22:39

you can OC your laptop?

rpg · 1 Mar 2010 at 14:01

cmndr_andi said:
Is this upgrade purely for memory bandwidth, or are you upgrading the memory capacity at the same time (ie 2GB to 4GB)? If you are upgrading the capacity too, then the 1600MHz kingston stuff looks great. I imagine these sticks are just higher binned versions of the 1333Mhz ones - so they are effectively the same but performed better in initail tests so were rated to run at higher clockspeeds. Running these "1600MHz sticks" at 1333 is perfectly fine and will just put less stress on them.

However, if this upgrade is purely to increase memory bandwidth I would ask you to have another think. £96 is a lot of money to spend and the actual performance benefit you will see is marginal at best. Depending on your laptop type you may be able to replace the CPU yourself and get a much more tangible speed boost for similar money.

Thanks for your input cmndr_andi. I was wondering that, the datasheets for the 1333MHz and 1600MHz are pretty much identical, so thanks for clearing that up.

I am actually upgrading the capacity at the same time, the laptop currently has 2GB so I might see a small performance increase with 4GB depending on the app. Unfortunately I can't replace the CPU as it's a BGA CPU but if I manage the OC, I'll be happy!

thingemajib said:
you can OC your laptop?

Well I can, but the second method I might have to use is a bit extreme. I've contacted IDT for the datasheet on the clock generator used in the laptop, so if it allows OC through software, I might be able to get someone to program in the PLL to CPUCool or SetFSB. If this method works I'll be very pleased. The second method (which I don't really want to do as it's irreversible) is actually modifying the PLL input pins on the device itself.

To be honest I'm not too pushed whether I get it done or not, I just want to make sure that if I can actually do it, that I have RAM capable of running at the higher clocks.