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Even a laptop can run RAM externally thanks to CXL

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Re: Even a laptop can run RAM externally thanks to CXL

#41

Post by simcop2387 »

e63f67dd-065b wrote: post #22

CXL is useful for many more things beyond just memory expansion (which is a "type 3 device"). The purpose of CXL is to allow for memory coherency between different CXL devices. To quote the spec on type 2 devices: > CXL Type 2 devices, in addition to fully coherent cache, also have memory, for example DDR, High-Bandwidth Memory (HBM), etc., attached to the device. These devices execute against memory, but their perfo…

I've got to imagine that at some point in the future when it's been common on server stuff for a while it'll end up as part of a next-gen "thunderbolt" style connection, simply because the silicon by then will be a commodity and it'll be cheaper to use the same thing everywhere. I'm imagining a docking station that doubles the ram, and gives it a powerful GPU/NPU/TPU for a workstation. Since the ram would be close to the accelerator over CXL they can talk directly and make full bandwidth use of it even if the laptop uses the remote ram as a "very very very fast swap" or something else to prioritize things. I'm not so sure it'd be useful for persistent storage at that level but it'd make for some really cool options for a portable workstation since you wouldn't need everything inside the device.

Re: Even a laptop can run RAM externally thanks to CXL

#42

Post by brucethemoose2 »

xhrpost wrote: post #12

The slowest clock speed for DDR5 is 2,000Mhz or 0.5ns per cycle. At 2/3c, electricity should be able to travel about 10cm max in one cycle. I suppose 4Ghz RAM would be only 5cm. Is having an external unit with much greater than 10cm of copper travel ultimately a limiting factor here with modern day RAM speeds?

This is not exactly the reason, but trace length/loss is a major problem.

Laptop SODIMMs are already severely limiting DDR5 bandwidth, server/desktop DIMMs are getting close.

Users aren't gonna like it, but unswappable packaged RAM is coming to most CPUs.

Re: Even a laptop can run RAM externally thanks to CXL

#43

Post by e63f67dd-065b »

gavinray wrote: post #36
e63f67dd-065b wrote: post #22

CXL is useful for many more things beyond just memory expansion (which is a "type 3 device"). The purpose of CXL is to allow for memory coherency between different CXL devices. To quote the spec on type 2 devices: > CXL Type 2 devices, in addition to fully coherent cache, also have memory, for example DDR, High-Bandwidth Memory (HBM), etc., attached to the device. These devices execute against memory, but their perfo…

CXL for Persistent Memory, as the Optane successor (or rather, sole-survivor if you will) is what excites me. Persistent Memory databases are such a neat idea, it's a shame that the hardware for it isn't commonplace.

Yeah optane seemed like it had so much potential -- with the right abstractions, it neatly dealt with the eternal problem of the separation between RAM and disk. Much of DB and storage system engineering comes down to decisions around when to persist what, and optane was very promising in allowing for new architectures that had much better performance at lower complexity.

Alas, optane's dead now. I do know people actively working on resurrecting a lot of pmem work on low-latency flash, however, and it seems like this is one area with a low of momentum behind it.

Re: Even a laptop can run RAM externally thanks to CXL

#44

Post by bloopernova »

tambourine_man wrote: post #40

Apple's dream come true. Sell one model with everything soldered and unupgradable. One universal computing core. Want extra? Plug it in. From an operations point of view (Cook's forte), it makes a lot of sense. Jony Ive would be proud as well.

Honestly I think I'd love this. Being able to augment an existing MacBook pro with a bunch of RAM or a good GPU would be very useful to me.

Re: Even a laptop can run RAM externally thanks to CXL

#45

Post by mbreese »

bloopernova wrote: post #44
tambourine_man wrote: post #40

Apple's dream come true. Sell one model with everything soldered and unupgradable. One universal computing core. Want extra? Plug it in. From an operations point of view (Cook's forte), it makes a lot of sense. Jony Ive would be proud as well.

Honestly I think I'd love this. Being able to augment an existing MacBook pro with a bunch of RAM or a good GPU would be very useful to me.

External GPUs working over thunderbolt is a thing. So, you can at least get part of this today.

Re: Even a laptop can run RAM externally thanks to CXL

#46

Post by vinay_ys »

jnwatson wrote: post #34
e63f67dd-065b wrote: post #22

CXL is useful for many more things beyond just memory expansion (which is a "type 3 device"). The purpose of CXL is to allow for memory coherency between different CXL devices. To quote the spec on type 2 devices: > CXL Type 2 devices, in addition to fully coherent cache, also have memory, for example DDR, High-Bandwidth Memory (HBM), etc., attached to the device. These devices execute against memory, but their perfo…

CXL for CPU access to DRAM is overkill. Plenty of server mobos allow you to directly attach 2 TiB of DRAM.

Rack level disaggregated compute-memory-storage-accelerator architectures allow for dynamic partitioning/aggregation of hardware to suite concurrent workloads and their evolution over time and make it easier to achieve both cost efficiency as well as incremental, continuous and non-disruptive hardware upgrades.

Re: Even a laptop can run RAM externally thanks to CXL

#47

Post by xhrpost »

wtallis wrote: post #38
xhrpost wrote: post #12

The slowest clock speed for DDR5 is 2,000Mhz or 0.5ns per cycle. At 2/3c, electricity should be able to travel about 10cm max in one cycle. I suppose 4Ghz RAM would be only 5cm. Is having an external unit with much greater than 10cm of copper travel ultimately a limiting factor here with modern day RAM speeds?

DRAM latency is tens of clock cycles. There's a minimum of about 10ns of latency happening within the DRAM chip itself, so a slightly longer bus doesn't affect total latency by much.

Wikipedia is showing a cycle time for CAS of less than 1ns for modern RAM

https://en.wikipedia.org/wiki/CAS_latency#Memory_timing_exam...

Re: Even a laptop can run RAM externally thanks to CXL

#48

Post by bloopernova »

mbreese wrote: post #45
bloopernova wrote: post #44

Earlier quoted context omitted.

Honestly I think I'd love this. Being able to augment an existing MacBook pro with a bunch of RAM or a good GPU would be very useful to me.

External GPUs working over thunderbolt is a thing. So, you can at least get part of this today.

Unfortunately not with the M series of apple CPUs :(

Re: Even a laptop can run RAM externally thanks to CXL

#49

Post by l1k »

momojo wrote: post #30

Who's using this? What company doesn't already have a rack they can add a few more sticks of RAM in for their workloads?

If you need a lot of RAM, usually you need to buy servers with multiple CPUs to which you can attach the memory. Because the amount of DRAM you can attach to one CPU is limited.

If you don't have the need for all the extra CPUs, just being able to attach more memory to a single CPU through CXL may be cheaper.

Re: Even a laptop can run RAM externally thanks to CXL

#50

Post by paulmd »

vinay_ys wrote: post #46
jnwatson wrote: post #34

Earlier quoted context omitted.

CXL for CPU access to DRAM is overkill. Plenty of server mobos allow you to directly attach 2 TiB of DRAM.

Rack level disaggregated compute-memory-storage-accelerator architectures allow for dynamic partitioning/aggregation of hardware to suite concurrent workloads and their evolution over time and make it easier to achieve both cost efficiency as well as incremental, continuous and non-disruptive hardware upgrades.

Wendell Level1techs has been talking about his Liqid PCIe Fabric stuff lately, sounds pretty sick, this mention inspired me to go search for his other videos on the topic

https://www.youtube.com/watch?v=x1NWdoXmVsE

https://www.youtube.com/watch?v=Iwep9D_q4e0

https://www.youtube.com/watch?v=zXlWeVrsk14

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