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I am considering purchasing a new laptop, specifically the Lenovo ThinkPad t580, which offers multiple memory configurations up to dual channel. Two of the many memory options (on the UK website) are:

16GB (8+8) DDR4 2400MHz SODIMM

24GB (8+16) DDR4 2400MHz SODIMM

When less than 16GB is in use, can the larger mixed 24GB(8+16) configuration ever perform worse than the matched 16GB(8+8) configuration? Or will the 24GB(8+16) behave exactly like 16GB(8+8) for the first 16GB of memory used?

There have been plenty of questions about mixing different sizes, speeds and brands of RAM, but in this case I would expect that the only difference between the 2 sticks of RAM would be the size. Also, I am only asking about the first 16GB of memory used, not the region from 16GB to 24GB, where I expect the larger, but mixed configuration would have the advantage for page fault reasons.

This laptop is not for gaming, but will potentially be hosting multiple databases (SQL Server, Oracle, SAP HANA) and virtual systems.

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    You really think that mixed size memory could have any disadvantages worse than using a laptop as a database and/or virtualization server? – Esa Jokinen May 15 '18 at 11:11
  • Esa Jokinen, good point, but these databases would get relatively light use for my own education, minor developments etc. – nubbers May 15 '18 at 12:05
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You will not be able to achieve dual channel performance with modules that are not identical.

What you get is called by intel FLEX mode.

That means, if the MB is good enough, you still get faster-than-single channel operation, but not entirely dual-channel operational speed. Motherboards that support FLEX mode will list it as "dual channel" regardless, so that's not an accurate indicator.

You can see FLEX here.

Basically, the common dual channel area will work in dual channel, while the difference of the larger module will be slower, will work on single channel.

in your example, yes, the 24GB(8+16) will behave very close like 16GB(8+8) for the first 16GB of memory used.

For things like HANA you should definitely use identical modules.

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    It helps a lot to know to search for FLEX mode, but it seems to be a not so well known topic. – nubbers May 15 '18 at 13:05
  • Also read about memory rank en.m.wikipedia.org/wiki/Memory_rank – Mikhail Khirgiy May 15 '18 at 15:45
  • FLEX is just a name given by intel. We're actually talking about partial or limited dual channel. That would be universally appropriate. – Overmind May 16 '18 at 6:27
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So Far, I used 16 + 8 Gb RAM on Dell Precision 5510 and Desktop with Ryzen 2400G, They both run at dual channel with mixed size RAM. It is faster than single module and in real usage no difference with 8+8 or 16+16. Only in benchmark the number will be 150% of single channel with one module. The reason is simple, it runs dual channel up to 16 Gb and single channel on the rest of 8 Gb. Sisoftsandra values about 18 Gbps memory bandwith. With dual channel it reach 25 Gbps, single module 13 Gbps. So just upgrade to biggest single module you can get and keep the old RAM. 24 Gb total RAM is better than 16 Gb.

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