Which memory stick is "faster", and are the two "compatible"?

I recently received a brand new Lenovo Thinkpad P70, ordered with the basic 8GB of memory. I also ordered an additional 8GB of memory from Crucial, purportedly compatible with the P70. The P70 has four SODIMM memory sockets and can support up to 4x16GB=64GB total non-ECC DDR4 2133MHz storage with its 6th-generation i7-6700HQ/6820HQ Intel CPUs. It also can come with Xeon E3-1505M processor, which can handle 4x16GB=64GB of ECC memory.

I installed the 8GB of Crucial memory into one of the three remaining available memory sockets, not seeing anything in the documentation that it must be in a specific "paired" slot for best performance. In other words, the Lenovo memory came installed in what was slot #3 (located under the keyboard), with what I'm guessing is actually "paired" adjacent slot #1 next to it (also located under the keyboard). But I installed my additional 8GB of memory in what turned out to be slot #4, in an instantly accessible location on the underside of the laptop gotten to immediately after removing the bottom cover. The second adjacent "paired" (to #4) slot on the underside I assume would have been slot #2.

So I ended up with 8GB of memory in each of [what I believe to be] UN-PAIRED slots #3 and #4.

The net result is performance behavior shown by CPUZ as "single" rather than "dual" as I would have expected and certainly wanted to obtain.

The question is why? Is the issue that I didn't add the second 8GB to the corresponding "paired" slot #1 (which would have required me to remove the keyboard to get to), to go along with the Lenovo 8GB of memory which was delivered in slot #3? Or is the issue that the two memory sticks have slightly different timings and therefore the BIOS rejected them for "dual" operation?

To eliminate any possibility that is the different timing values for the two memory sticks which is at fault, I've gone ahead and ordered a second 8GB of Crucial memory, to match the first 8GB I'd already bought. I plan to replace the 8GB of Samsung memory with this second 8GB of Crucial memory, thus having an identical matched pair.

I also plan to move the Crucial memory currently installed in slot #4 (underside of laptop) to instead by in slot #1 (under the keyboard). And I will replace the Samsung memory currently installed in slot #3 (also underside of laptop) with the second stick of Crucial memory. This will then produce a matched pair of memory sticks with identical timing numbers, in adjacent "paired" slots #1 and #3.

Surely that should produce "dual".

But in the meantime I really do have questions as I've posed above.

(a) Is it the choice of the wrong slot that I installed the second 8GB into which is causing the "single" behavior?

(b) Or is it the somewhat different timing values of the two memory cards which is the culprit?

(c) And for my own knowledge, which of the cards (Samsung or Crucial) is the "faster"? Do lower timing numbers imply faster performance, in which case it seems the Samsung memory is faster? Or do higher timing numbers imply faster performance, in which case it seems the Crucial memory is faster?


Here are the outputs from CPUZ. Notice 16GB total, but running "single". And then there are the timing values for the Samsung (slot #3) and Crucial (slot #4) memory cards.

Ok. I've done some searching to answer my own question. There is a good detailed explanation of these numbers in this article on RAM timings.

And certainly with mostly similar memory sticks (as these are, in that they are both 1.20v and they are both DDR4-2133mhz) the larger the numbers the slower the memory. So it would appear that the Crucial memory is slightly slower than the Samsung memory.

Here are the relevant definitions from that article:

The operations that these numbers indicate are the following: CL-tRCD-tRP-tRAS-CMD. To understand them, bear in mind that the memory is internally organized as a matrix, where the data are stored at the intersection of the lines and columns.

• CL: CAS Latency. The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and then returning it.
• tRCD: RAS to CAS Delay. The time it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.
• tRP: RAS Precharge. The time it takes between disabling the access to a line of data and the beginning of the access to another line of data.
• tRAS: Active to Precharge Delay. How long the memory has to wait until the next access to the memory can be initiated.
• CMD: Command Rate. The time it takes between the memory chip having been activated and when the first command may be sent to the memory. Sometimes this value is not announced. It usually is T1 (1 clock cycle) or T2 (2 clock cycles).

As far as why my two memory sticks are behaving in "single" mode, I'm still of the opinion it is the result of using non-adjacent slots (#1 and #3, or #2 and #4) rather than the slight timing differences. Certainly seems like the more plausible explanation.

Again, there is a good discussion of single-channel vs. dual-channel in this thread on another forum. The essence of the discussion there is that dual channel is faster, can be up to 10-15% on Intel. Single channel runs the DRAM as the single 64bit device that it is. In dual channel mode the memory controller sees all the DRAM as a single 128bit device and runs it accordingly.

There is no mention in the discussion about use of "paired slots" being required for "dual", so either it just wasn't brought up or perhaps may no longer be critical with modern BIOS and motherboard architecture. I don't know.


A second discussion on TomsHardware has another perspective on "single" vs."dual".

With a dual channel system, one channel is always available for loading, refreshing, and housekeeping while the other channel is being read. The channels are read alternately, one being read into the buss while the other is being prepared for use, or being written to. What exactly happens is the pervue of the memory controller and it's firmware.

With single channel operation there is always something "waiting in line" while the memory is busy. This limits what operations can be performed, and those generating a "write" instruction have to wait their turn. This makes the buffer (onboard memory) more critical and works it harder.


And another point raised in the TomsHardware article pertains to motherboard/CPU dual-channel memory controller vs. quad-channel memory controller. This article describes the architecture of the i7-6700HQ CPU in my P70.

The Intel Core i7-6700HQ is a quad-core processor based on the Skylake architecture, that has been launched in September 2015. In addition to four CPU cores with Hyper-Threading clocked at 2.6 - 3.5 GHz (4 cores: max. 3.1 GHz, 2 cores: max. 3.3 GHz), the chip also integrates an HD Graphics 530 GPU and a dual-channel DDR4-2133/DDR3L-1600 memory controller. The CPU is manufactured using a 14 nm process with FinFET transistors.

So given that the memory controller is dual-channel, the TomsHardware article says if you had a motherboard/CPU with a quad-channel memory controller, you'd want 4x4GB=16GB RAM instead of 2x8GB=16GB. But if you have dual-channel like most modern-ish PCs, 2x8GB makes more sense.


Bottom line based on what I've found:

(1) with a dual-channel memory controller and given any target total memory size, if that can be accomplished with a 2x memory kit that will be superior to reaching that same memory size using a 4x memory setup.

(2) dual-mode is typically 10-20% faster than single-mode.

(3) for any given memory type (e.g. all DDR4-2133), larger timing values in CPUZ means slower, lower values means faster.

Mellon Head said:

dsperber said:

But in the meantime I really do have questions as I've posed above.

(a) Is it the choice of the wrong slot that I installed the second 8GB into which is causing the "single" behavior?

I'm not 100% sure on this one, but yes, it may be the slot choice that is affecting the dual/single state. It also may require two matched pairs to run in dual channel. The third module might be placing it into single channel mode. If you added a fourth module, it might work as dual channel. That's how my motherboard works anyway. Not sure about Lenovo laptops.

I may not have expressed what I did clearly enough.

I don't have three memory sticks installed. I only installed a single additional 8GB of Crucial memory in slot #4, to go along with the 8GB factory Samsung memory in slot #3. That is what is giving me 16GB running in "single" mode, most likely due to the choice of slots but possibly due to the slight mismatch in timings.

I've used Samsung+Crucial memory in a number of Lenovo M93p desktop machines and it works perfectly in "dual" mode, but in those machine the Crucial memory provided IDENTICAL timing values. Of course in those machines I also used the "paired DIMM slots" as well.


Now, when the second piece of Crucial memory arrives next week I will REPLACE the Samsung memory in slot #3 with the newly arrived Crucial memory. And I will also MOVE the currently installed first piece of Crucial memory (currently in slot #4) into the "matched pair" location slot #1 adjacent to slot #3 (both under the keyboard).

I am not looking for 3x8GB=24GB. I am simply wanting 2x8GB=16GB but running in "dual mode". And I'm sure I will fijnally achieve that using (a) "paired slots" #1 and #3, and (b) identical matching Crucial memory, albeit they are slightly slower than the Samsung memory provided by Lenovo (at what was about triple the price of memory from Crucial if I'd just ordered all 16GB from Lenovo for the P70 in the first place).

This is the first time I've had any problem with Crucial memory not being an EXACT TIMING VALUES MATCH for the existing installed memory in the computer I was upgrading. I will note this in my feedback to them.

Layback Bear said:

Your motherboard has control of Single, Dual, Quad channel according what slots you install the ram.

Per that article describing the architecture of the i7-6700HQ CPU, the memory controller is a dual-channel memory controller.

For dual channel you will need 2, 4, or 6 slots filled.

Correct. The P70 provides 4 slots for memory.

Install in pairs or you will end up with single channel.

Correct. But are all slots the same, so that any 2 can be used as a "pair"? Or are specific slots "paired" and must be used that way, e.g. #1 and #3, or #2 and #4?

If you don't use a specific pair of 2 slots for the 2 sticks of memory (in other words like my #3 and #4) do you lose "dual" capability and force "single"? That's what happens in a desktop motherboard I believe, so why not in a laptop motherboard as well?

When ever their is a question on what slots to use confer with the motherboard manual.

Well really that is the reason I'm having this problem in the first place! The Lenovo manual itself SAYS NOTHING about any considerations regarding memory expansion.

It not only doesn't even show the slot numbers in a graphic or diagram, but it doesn't provide any instructions on which particular 2 or 4 slots to use if you want to use 2 or 4 sticks of memory. It also does not distinguish any conceptual difference or specific "pairing" consideration regarding any of the four memory slots grouped into the two separate "memory slot bays" of two-each bay.

So I'm just proceeding based on experience here, and also from the empirical results from what I've done and from what CPUZ says.


[I personally don't mix and match ram and don't recommend it. Matched ram are tested together to assure they play well with each other.[/quote]Absolutely, 100%. For a new home-built machine I'd obviously by 2x or 4x kits of memory, so as to obtain matched identical pairs.

But in all the years I've been buying memory from Crucial, this is the very first time that the memory they sent me (chosen specifically to be compatible with a specific vendor/make/model) was not ABSOLUTELY IDENTICAL IN ALL TIMING VALUES with the vendor-supplied memory that came in that machine. I will phone Crucial again to discuss this, but I suspect it is due to the brand newness of the P-series Thinkpads that they got this wrong.

Yes, their DDR4-2133 memory "works", but apparently they didn't know Lenovo was going to be using Samsung memory that was slightly faster. Perhaps Lenovo changed design, or Samsung provided slightly faster memory cards, or who knows what.

However this happened it did. I bought the memory that the Crucial site said was the correct upgrade for the P70, and it turns out to be a mismatch with the Lenovo-provided Samsung memory. First time for everything I guess, but it was unexpected and forced me to now buy a second matching 8GB stick from Crucial in order to end up with two identical matching memory sticks.

Always install ram as per the motherboard manual to get the best results.

Sure... if only there had been relevant information in the documentation.