Cheap memory

[PlainName]

Just had to replace an old but otherwise reliable server - ML330G6 which, after a power cut, decided the ROM was now blank and needed reprogramming.

Anyway, it has PC3 SIMMs in and given current prices I thought maybe I could get back the disposal costs, but I see 10x 8GB sticks going for around £3!

How hard would it be to fake even DDR4 RAM with a bunch of obsolete SIMMs? I would imagine (from a position of no actual knowledge) something that sits in front of a bunch of interleaved slow SIMMs to fake DDR5, perhaps with a small amount of actual very expensive RAM as a buffer (like SSDs do). Can RAM run arbitrarily slow so long as clock timing is maintained? That is, does it have the ability to insert wait states or something?

I appreciate the answer is probably "your ciggies are definitely illegal", but thinking about this has to be better than being out there melting today.

[ejeffrey]

In principle, more chips + interleaving logic could have as much bandwidth as you want.   But the logic needed to implement the interleaving would have to be fast and custom and therefore expensive.  Also, the latency would be pretty bad.  DDR5 isn't much lower latency than original SDRAM, but the intermediate logic would add significant latency that would be undesirable.  Finally, the burst size would be a problem.  To get the highest throughput from DRAM you need to burst.  But if you take 8x older generation DRAM chips and mux them, your burst size per RAM would be 1/8th the size.

So overall I don't think you could make this work feasibly, even with custom silicon.  There is a reason that DRAM still uses wide, parallel, single-ended data busses and is always clustered as tightly around the processor as packaging will allow.

[PlainName]

Thanks. Not quite so silly as first thought, then

I figured the logic would be expensive, but compared to the same amount of real RAM it might still be cheaper. And sometimes you just need oodles of RAM but not necessarily the fasted speed - my ML330G6, for instance, was doing a great job of being an ESXi server with a handful of VMs in it. Replacing it would today cost a fortune just in RAM regardless of what's talking to it, and it doesn't actually need DDR5 speed.

[thm_w]

I think the problem is the low level access the system has to the RAM. You'd have to set very slow jedec timings, maybe if they don't have limits you can get away with a very high timing numbers?

There are these which I believe need specific HW support:
https://blog.workinghardinit.work/2019/07/22/a-quick-intro-to-nvdimm-f/
https://www.penguinsolutions.com/en-us/resources/blog/what-is-cxl-memory-expansion

I figured the logic would be expensive, but compared to the same amount of real RAM it might still be cheaper. And sometimes you just need oodles of RAM but not necessarily the fasted speed - my ML330G6, for instance, was doing a great job of being an ESXi server with a handful of VMs in it. Replacing it would today cost a fortune just in RAM regardless of what's talking to it, and it doesn't actually need DDR5 speed.

You can always buy a DDR4 system.

[PlainName]

There are these which I believe need specific HW support:

Interesting, thanks.

You can always buy a DDR4 system.

Just had to when my main PC died (this was all before the hot weather too), but only so I could use my existing 32GB DDR4. Prices of that are merely 'how fing much?!?' nowadays.

As luck would have it, just before Xmas a friend needed a new laptop drive so I persuaded them to get a 1tb nvme at around £60. I got one as well and, because I'm running out of slots, also a 2tb one for about £110. A week later and they were three times the price.

[SiliconWizard]

If you can stick to DDR4, you can certainly build a machine with a lot more RAM than you could afford with DDR5 currently. There's a lot of DDR4 on the second-hand market. Now brand new DDR4 tends to be nearly as expensive as DDR5 now as stocks are plummeting, but the second-hand market still has a lot.

I got 128 GB DDR4 ECC from a decommissioned server for about 120 €. If you have a CPU/motherboard that can handle ECC DDR4, that's an excellent deal as DDR4 ECC from decommissioned servers tends to be abundant (due to a lot of servers having been upgraded). I for one had a Xeon CPU for this but I believe many "recent" consumer AMD CPUs supporting DDR4 can handle ECC DDR4 without a problem (which is not the case for Intel consumer CPUs).

[PlainName]

Yeah, I am wondering if buying now to beat scarcity is better than holding off hoping that AI will get stomach ache and not eat so much.

[Haenk]

I got 128 GB DDR4 ECC from a decommissioned server for about 120 €. If you have a CPU/motherboard that can handle ECC DDR4, that's an excellent deal as DDR4 ECC from decommissioned servers tends to be abundant (due to a lot of servers having been upgraded). I for one had a Xeon CPU for this but I believe many "recent" consumer AMD CPUs supporting DDR4 can handle ECC DDR4 without a problem (which is not the case for Intel consumer CPUs).

Unfortunately, that's usually Registered/Buffered - and usually non-server mainboards will not run with these installed. ECC non registered modules are quite rare.
Some chinese hacked together server chipsets and normal "user" sockets on cheap mainboards that might support registered modules. Consumer mainboards from the usual suspects like ASUS will *not* support these. Rarely they can use ECC modules (however will not support ECC functionality).

[thm_w]

Meta now adding custom ICs to be able to use DDR4 to their servers:
https://www.pcmag.com/news/even-meta-is-using-ddr4-to-get-around-ddr5-memory-shortages

The price of large size DDR4 modules may go up more (32-64GB).

[ejeffrey]

CXL is a cache coherent low latency extension to PCIe.  So this memory should be usable as "native" memory with random access but is separate from the main memory controller.  The speed and latency will be worse than native dram but much better than any sort of flash or block accessible memory.