Quick question about ram speed

[Benjiih]

Hello all. 

 

I have a quick question concerning ram speeds. I while ago I began upgrading my pc, but I had a short economical setback so I had to put the rest of the parts (CPU and ram) on the backburner for a while. But now I have the cash again, and I am ready to get the last 2 parts I need. 

As far as I can read the 6700k I am going to get and put into my already purchased Asus z170 Pro Gaming motherboard supports up to 2166mhz memory. It says on the motherboard specifications that it can support up to Dual DDR4 3400 (OC) support.

What exactly does this mean? Does it mean that I shouldn't bother getting memory which speed exceeds 2166mhz? Or should I still get the highest speed possible within my budget?

 

Thank you all!

Sincerely, 

Benjamin

 

[WereCat]

It means that you are guaranteed that 2166MHz will work and anything else above that is OC... which may or may not work. That also largely depends on the memory controller.

[racer243l]

No guarantee that 3400 will run, since it´s an "overclock". My Mobo doesn´t even boot with 3200 unless I do a lot of tinkering and at least for my taste to high voltages

[Benjiih]

So I can "easily" get higher ram speeds. Like 3000 or something similar? The prices don't change that much from 2166 to 3000 from what I can see anyway.

[xentropa]

29 minutes ago, Benjiih said:

So I can "easily" get higher ram speeds. Like 3000 or something similar? The prices don't change that much from 2166 to 3000 from what I can see anyway.

 

There is kind of a trick to this.

 

 Depending on the RAM timings, ddr 3000 could actually be slower than ddr 2133.  The ddr 2133 or 3000 is more for synching with the motherboard.

 

Check the RAM timings.  It should be composed of 3 or 4 numbers, like 3-3-3-5 or 7-7-7-10.  These numbers indicate how many clock cycles it takes to perform a particular action.  If the ddr2133 ram ur looking at has the exact same ram timings as a ddr3000, then the ddr3000 will be faster.

[Benjiih]

Ok thanks a lot @xentropa I will look for that! :-)

[JoostinOnline]

1 hour ago, xentropa said:

Depending on the RAM timings, ddr 3000 could actually be slower than ddr 2133.  The ddr 2133 or 3000 is more for synching with the motherboard.

Is there a way to benchmark this?  I'm not really a fan of the "it feels slower" method.

[XanonDarkley]

Yes, Aida64 has a ram benchmark. but don't put to much thought or money in it. the benefits for normal everyday usage (incl. gaming) aren't that much different between 2133 and 3200. in synthetic benchmarks, sure, in Games not so much. If you have to choose between faster Ram and more ram, take more^^

 

PS: the 2133 mhz is the standard speed, wich Intel specifies for the CPU memory controller, so it's "garantied" to work. Faster Ram works most of the time without a problem. If in doubt, look up the compatible Ram list for your motherboard and try to get one of those kit or atleast get an idea for wich brand should work.

[xentropa]

1 hour ago, JoostinOnline said:

Is there a way to benchmark this?  I'm not really a fan of the "it feels slower" method.

 

Compare these two memories.  Ignore the different capacity, just focus on speed.

 

https://www.newegg.com/Product/Product.aspx?Item=N82E16820233998

CORSAIR Dominator Platinum 32GB (4 x 8GB) 288-Pin DDR4 SDRAM DDR4 2400

DDR4 2400 (PC4 19200)

Timing 10-12-12-28

Cas Latency 10

 

 

https://www.newegg.com/Product/Product.aspx?Item=9SIAA443S22485

APACER Commando 8GB (2 x 4GB) 288-Pin DDR4 SDRAM DDR4 3466

DDR4 3466 (PC4 27700)

Timing 18-18-18-42

Cas Latency 18

 

At a glance, it may seem like the APACER DDR4 3466 is faster than the CORSAIR DDR 2400; 3.46ghz (DDR frequency) vs 2.4 ghz.  But what I wanted to show you guys is the CAS (column address strobe) latency.  CAS latency is the time it takes for a memory row to be activated, and when sufficient charge has been sent to the sense amplifier to register as data (I think).  This delay time is represented by the number of CLOCK CYCLES.  In this case it is 10 clock cycles for the Corsair and 18 clock cycles for the APACER.   

 

So how can we determine the actual delay?  The formula is 2 * CAS latency/frequency (2 is because DDR).  

 

2*10/2.4E9 is 8.2 ns.  So the 10 clock cycles @ 2.4 DDR ghz it takes the Corsair memory to read data translates to 8.2 nanoseconds real time.

 

For the APACER, 2*18/3.466E9 is about 10.4 ns.  

 

You can see despite the lower clock speed, the read latency is lower for the corsair memory.  So when using the APACER memory, it will run at 3.466 ghz but during the 18 clock cycles, the CPU will just be receiving nothing (unless another memory channel happens to be running parallel).  The really bad situation is if you use that APACER memory on a motherboard that can't support the 3466 speed, it will automatically downclock and possible still be stuck on the 18 clock cycle CAS latency.

 

*Note that the CAS latency excludes any other delays in multiplexing data to the CPU or any other delays inherent to the circuitry.

[JoostinOnline]

6 minutes ago, xentropa said:

The formula is 2 * CAS latency/frequency (2 is because DDR).

That's what I was looking for.