Even a laptop can run RAM externally thanks to CXL • Hacker News Board

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

#81

Post by abound » Fri, Oct 20, 2023, 12:38 PM UTC

bogantech wrote: post #74

abound wrote: post #66

Earlier quoted context omitted.

I've spent a lot of time down this rabbit hole recently. I have a Dell M1000e blade server (16 blades, each with dual CPUs). A comical amount of compute for a home, sounds like a leaf blower. Hosted a couple dozen small apps on it. Power usage was 1.5kW (ouch), made less painful by large rooftop solar. Just today, I replaced the whole thing with a single mini PC with a laptop Ryzen chip (6900HX). Pulls like 60W at th…

> I have a Dell M1000e blade server (16 blades, each with dual CPUs). A comical amount of compute for a home, sounds like a leaf blower. That's pretty awesome! I would have loved that at one point but they're extremely loud and I remember them needing 16 amp IEC leads

Loud is an understatement, I wear ear protection when I'm physically in the room for more than a minute.

And indeed, the power supplies run on ~200V, my house serendipitously is wired with a dedicated 240V/50A circuit because the previous owner did glasswork and had a massive electric kiln.

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

#82

Post by simcop2387 » Fri, Oct 20, 2023, 1:45 PM UTC

jauntywundrkind wrote: post #76

simcop2387 wrote: post #41

Earlier quoted context omitted.

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…

CXL 3.0 is where it gets interesting, where you can start to have switched fabric where many hosts can talk to many devices. Having that pool of ram & GPUs be on demand usable by whomever has some attractive possibilities. Also, one can just imagine having some pools of data in CXL memory that a host can just attach to & read, which seems like a cool possibility. It does kind of upset me a bit that CXL 3.0 still seem…

I think a large part of that is probably because the needs for doing that kind of host to host thing aren't well understood yet for the general cases. Right now I think that's being fleshed out in the market using the whole DPU model still. I think sadly you're going to be right about how it turns out for a while and it won't be until something like CXL 6.0 or whatever comes after it that starts to really address that.

I'm wondering when we'll see the first CXL DPU style device that uses some of the fancy new 800Gbps or even 1.6Tbps networking stuff that's being developed right now. That'd be enough bandwidth to put things on the other end of a data-center with very little other than the latency penalty.

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

#83

Post by bzzzt » Fri, Oct 20, 2023, 2:53 PM UTC

userbinator wrote: post #67

Isn't this already possible with things like Thunderbolt, which is essentially "external PCIe"?

Yes, if you don't mind the latency and throughput restrictions of Thunderbolt. Eg. Apple's M1 has a memory throughput of 66GB/s, which is an order of magnitude more compared to Thunderbolt 40Gbit/s. So it's more like a really fast SSD instead of a local memory upgrade.

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

#84

Post by [deleted] » Sat, Oct 21, 2023, 5:55 AM UTC

dvas wrote: post #56

paulmd wrote: post #52

Earlier quoted context omitted.

I'm utterly unqualified to discuss using it in practice but it always seemed like PMEM would have required a fundamental idiom shift away from the DRAM concept. If you treat it like memory it's always going to be slow memory. If you treat it like storage, it can be very fast storage. If that makes sense. Software that "treats storage like memory" would end up looking a lot like javacard imo. Or something like Samsung…

Some support has been added to the Linux kernel like DAX [0] to avoid extra copy operations [0]. SNIA has some interesting talks regarding using non volatile memory and its applications (DBs). Where using pmem and DAX to directly to store logging operations [1]. [0] What is Direct-Access (DAX)? https://kb.pmem.io/faq/100000008-What-is-DAX/ [1] SDC2020: How can Persistent Memory Make Databases Faster, and How Could we…

[deleted]

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

#85

Post by wtallis » Tue, Oct 24, 2023, 5:14 AM UTC

xhrpost wrote: post #47

wtallis wrote: post #38

Earlier quoted context omitted.

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...

The figures under 1ns are not latency numbers but merely the duration of a clock cycle.