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Comment by dvh | Hacker News Reader

What's wrong with DIMM?

The physical size of the socket and having the connections on the edge means you're forced to have much longer traces. Longer traces means slower signalling and more power loss due to higher resistance and parasitics.

This[1] Anandtech article from last year has a better look at how the LPCAMM module works. Especially note how the connectors are now densely packed directly under the memory chips, significantly reducing the trace length needed. Not just on the memory module itself but also on the motherboard due to the more compact memory module. It also allows for more pins to be connected, thus higher bandwidth (more bits per cycle).

[1]: https://www.anandtech.com/show/21069/modular-lpddr-becomes-a...

I'd wager for most consumers capacity is more important than bandwidth and the power losses are going to be small compared to the rest of the stack.

Bandwidth translates directly into better (igpu) performance

> power losses are going to be small compared to the rest of the stack

While certainly not the largest losses, they do not appear insignificant. In LPDDDR4 they introduced[1] a new low-voltage signalling, which I doubt they could have gotten working with SODIMMs due to the extra parasitics.

If you look at this[2] presentation you can see that at 3200MHz a DDR4 SODIMM would consume around 2 x 16 x 4 x 6.5mW x 3.2GHz = 2.6W for signalling going full tilt. Thanks to the new signalling LPDDR4 reduces this by 40% to around 1.6W.

Compare that to a low-power CPU having a TDP of 10W or less a full 1W reduction per SODIMM just due to signalling isn't insignificant.

To further put it into perspective, the recent Lenovo ThinkPad X1[3] uses around 4.15W average during normal usage, and that includes the screen.

Obviously the memory isn't going full tilt at normal load, but say average 0.25W x 2 sticks would reduce the X1's battery lifetime by 10%.

edit: yes I'm aware the presentation is about LPDDR4 yet the X1 uses LPDDR5, just trying add context using available sources.

[1]: https://www.jedec.org/news/pressreleases/jedec-releases-lpdd...

[2]: https://www.jedec.org/sites/default/files/JY_Choi_Mobile_For...

[3]: https://www.tomshardware.com/reviews/lenovo-thinkpad-x1-carb...

useful, thank you!

1 watt is exactly what I am saying is almost inconsequential. People leave charger bricks plugged in all day and the lights on.

And this line of thinking is exactly why we can't have M1 Macbook levels of battery life on Windows laptops. Believe it or not but a lot of people like to be able to have a light device they can just take without a charger and use for a solid day or 2 of work.

There are windows laptops that can do this already though with a sodimm slot no less

It requires too much power, according to the article. This allows using "LP" (Low Power) parts to be removable, they normally have to be soldered on board close to the CPU because of the low voltage tolerances.

One of the biggest problems is that edge connections don't give you enough density. Edge connections are great for serves where you stack 16 channels next to each other, but in a laptop form factor, your capacity is already limited, so you can get more wires coming out of the ram by connecting to the face rather than the edge.

There is literally an entire section explaining why LPDDR needs to be soldered down as close as possible to the memory controller.

Larger footprint, taller, longer traces and signal degradation in the connectors.

There's nothing _wrong_ with it, it performs according to spec, but it has limitations: trace length, power requirements, signal limitations, heat, etc.

The size, the sockets, the heat distribution, etc, etc, etc.

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