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Meta reuses old RAM in new servers with custom bridge chip

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Re: Meta reuses old RAM in new servers with custom bridge chip

#41

Post by kjs3 »

malfist wrote: post #32
joha4270 wrote: post #21

Earlier quoted context omitted.

You're not the first person to say so (and I don't mean to dispute it), but I have never been able to find a clear answer for /why/ those processes are incompatible. Is it built in different silicon, is it physical steps that's incompatible (ie its actually incompatible), is it different physical preparations that needs to be made (making it economically infeasible to combine) I cannot help but wonder, even if the an…

[flagged]

Or, you could look at your peer answers where people very much do provide a non hand-wave answers. Chip fabrication isn't undergrad philosophy; there are well understood reasons for why things are done if you care to find and understand them. And there are stakes in the millions or billions of US$ for getting them wrong.

Re: Meta reuses old RAM in new servers with custom bridge chip

#42

Post by NooneAtAll3 »

torginus wrote: post #10

With regards to RAM price I never understood the following: A 16GB RAM stick has 16*8=128 billion bits, with 1 transistor per bit, thats still 128B, yet its supposed to cost like $60 before the price hikes? In contrast, a 5090 GPU was $2000 (true it has RAM, but you're paying for the GPU ASIC really, I guess the rest of the GPU was less than $500), it had 93B transistors. GPU transistors are smaller due to the more a…

RAM is literally copy-paste of the same circuit over and over, you're trying to compare cost to produce million AK-47 with 1 carrier

and cost per transistor stopped decreasing at ~20-30nm, now small nodes are targetting energy efficiency (and thus performance, since heat is the main limiter)

Re: Meta reuses old RAM in new servers with custom bridge chip

#43

Post by dlenski »

joha4270 wrote: post #21
rcxdude wrote: post #16

Earlier quoted context omitted.

Chip fabrication processes are not fungible: GPUs and CPUs might be made on roughly the same process, but DRAM is not (flash is a different process again, as is power electronics, analog electronics, MEMS, etc. And even within those broader categories there are different variations). While there are some overlaps in machines and techniques, a fab set up for one cannot generally switch to the other, and the economics…

You're not the first person to say so (and I don't mean to dispute it), but I have never been able to find a clear answer for /why/ those processes are incompatible. Is it built in different silicon, is it physical steps that's incompatible (ie its actually incompatible), is it different physical preparations that needs to be made (making it economically infeasible to combine) I cannot help but wonder, even if the an…

To add to the other comments…

At a very abstract level, when you're manufacturing DRAM you need to manufacture a lot of circuit elements that have HIGH capacitance, since a DRAM cell is basically a capacitor and the higher its capacitance the less frequently it needs to be refreshed.

On the other hand, when manufacturing logic (CPU/GPU/ASIC) you want to minimize the capacitance of almost all circuit elements, since capacitance introduces delay and switching energy cost.

Nearly everything about the manufacturing processes for DRAM and logic is optimized around this fundamentally incompatible figure of merit.

I worked on the development of Intel's eDRAM process, which was used to integrate DRAM into the CPU/GPU die for Iris Pro embedded graphics from 2013-23. https://ieeexplore.ieee.org/document/6576667/

Re: Meta reuses old RAM in new servers with custom bridge chip

#44

Post by kjs3 »

I have always wondered why there was never a big market[1] for "cheap PCI/PCI-X/PCI-e card you can stick a boatload of your old/surplus/n-generation old simms/dimms on and use as swap/slow memory/ram disk/etc". It's rare you can populate a motherboard with a full address space full of 'new' memory, and you can teach kernels to prefer some memory to others because of speed[2], so it seems like a no-brainer.

I seem to remember the market for doing similar with flash got neutered over patent issues, but I can't recall the details. And flash cache did end up being a market, at least for bigger players. Maybe something similar happened here, or maybe it just hit a niche I cared about at the time?

[1] I know there were a handful of products in this space, but my impression is they never really took off. I could be wrong. [2] Definitely can in NetBSD; I've done it for archs like VMEbus where it's common to have a small, fast on board memory and much slower, often larger memory out on the bus. I assume this sort of thing is enabled in Linux by the work to support NUMA, but I've never looked into it.

Re: Meta reuses old RAM in new servers with custom bridge chip

#45

Post by Aurornis »

jzb wrote: post #8

It’d be nice if there were a consumer version of this. I have plenty of old RAM.

Adding the RAM to the system this way isn’t exactly like expanding the main system RAM. The RAM is connected over a PCIe type link so it doesn’t behave like the primary RAM.

It’s better for server farms where engineers can customize and tune for an architecture like this.

There have been some cards that use RAM as a storage device. They were never popular because having to set it up as a disk had very limited use cases.

Re: Meta reuses old RAM in new servers with custom bridge chip

#48

Post by edb_123 »

keanebean86 wrote: post #25
jzb wrote: post #8

It’d be nice if there were a consumer version of this. I have plenty of old RAM.

Gigabyte had a ram disk addin card years ago. Not exactly the same but since it's presented as a storage device you could use it as OS swap space. https://en.wikipedia.org/wiki/I-RAM

CXL Vistara reminds me a bit of the AST Rampage 286 memory expansion ISA card I had in my 286 back in the day, as a kid. Things go in circles, I guess.

Re: Meta reuses old RAM in new servers with custom bridge chip

#49

Post by pjc50 »

joha4270 wrote: post #21
rcxdude wrote: post #16

Earlier quoted context omitted.

Chip fabrication processes are not fungible: GPUs and CPUs might be made on roughly the same process, but DRAM is not (flash is a different process again, as is power electronics, analog electronics, MEMS, etc. And even within those broader categories there are different variations). While there are some overlaps in machines and techniques, a fab set up for one cannot generally switch to the other, and the economics…

You're not the first person to say so (and I don't mean to dispute it), but I have never been able to find a clear answer for /why/ those processes are incompatible. Is it built in different silicon, is it physical steps that's incompatible (ie its actually incompatible), is it different physical preparations that needs to be made (making it economically infeasible to combine) I cannot help but wonder, even if the an…

Medium ELI5 answer: each company and to a great extent each individual fab has a slightly different recipe, which is known as a "process node". This defines all the fabrication steps, every individual layer and its chemical/physical processing.

This in turn affects the electrical properties: parasitic resistance/capacitance, gate dielectric properties and so on. The dielectric in particular is critically different between DRAM and regular CMOS, because DRAM needs to minimise leakage (as that determines how long the memory lasts between refresh cycles).

Regular factories will retool somewhat between jobs. Because it is quite difficult to finetune a silicon process node, it is more common that a fab will set up for a particular node and then switch to "do not touch or change anything under any circumstances", as doing so may wreck yields.

("different substrate entirely" does exist: that's GaN, for power transistors in phone chargers, and SiC, for even higher power transistors.)

Re: Meta reuses old RAM in new servers with custom bridge chip

#50

Post by chadgpt3 »

kjs3 wrote: post #44

I have always wondered why there was never a big market[1] for "cheap PCI/PCI-X/PCI-e card you can stick a boatload of your old/surplus/n-generation old simms/dimms on and use as swap/slow memory/ram disk/etc". It's rare you can populate a motherboard with a full address space full of 'new' memory, and you can teach kernels to prefer some memory to others because of speed[2], so it seems like a no-brainer. I seem to…

They used to exist
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