I guess I am a little slow here

Posted May 24, 2012 13:24 UTC (Thu) by felixfix (subscriber, #242)
Parent article: Preparing for nonvolatile RAM

I too remember core memory.

I do not understand the problem as posed here, nor the responses. Why not just treat it exactly like a regular suspend and wakeup? The sole difference is that some peripherals may have lost power, including the usually-non-removable disk drives.

What is the point of trying to make it look like a disk? It reminds of those fun projects to run Linux on a 6502 emulator running under Windows running under Wine on Linux. Fun, interesting ... but ultimately silly.

I guess I am a little slow here

Posted May 24, 2012 18:44 UTC (Thu) by iabervon (subscriber, #722) [Link] (8 responses)

It's a bit different from usual suspend/resume if you have both DRAM and NVM, and you could have some memory areas lose power and others persist. You're also likely to have some interesting effects with the BIOS going through its boot instead of its resume, and leaving devices configured in an unexpected way.

I think the point of making it look like a disk is the same as the point of having tmpfs: it's nice to have filesystems (with their actual benefit of namespaces, naming, and authorization properties) with full memory bandwidth.

It's also possible that it would be beneficial to be able to keep data in NVM usefully while switching to a new kernel, which requires some sort of in-storage data structures which are stable across kernel versions.

I guess I am a little slow here

Posted May 24, 2012 19:16 UTC (Thu) by dlang (guest, #313) [Link] (5 responses)

if NVM is as fast as DRAM and higher desnity, why would people still have DRAM in their system?

Uses for NVM

Posted May 24, 2012 23:42 UTC (Thu) by giraffedata (guest, #1954) [Link] (3 responses)

if NVM is as fast as DRAM and higher density, why would people still have DRAM in their system?

Don't forget cheaper.

I think the real question may be: after you replace all your DRAM with NVM and do the obvious suspend/resume exploitation, what more can you do with all the additional NVM you have in excess of what used to be DRAM. The article's reference to "available in larger sizes" alludes to this.

A bigger file cache in the kernel was mentioned. This would address the problem some systems have today that their file cache size is limited by how long it takes to prime it after each boot.

The filesystem idea seems to allude to using it for stuff we used to keep on SSDs, but were limited by the cost of dragging it over a SATA wire into a CPU register (stopping off at DRAM along the way).

Uses for NVM

Posted May 24, 2012 23:57 UTC (Thu) by dlang (guest, #313) [Link] (2 responses)

when they say "available in larger sizes", what sort of sizes are they talking about?

if they are talking about sized 2x -4x larger than current DRAM devices, the answer is simple, you will use all that you have as normal RAM and not have 'extra' to find a use for.

If they are talking 20x+ larger, then it may start replacing flash in systems. but unless they are talking significantly larger than that, they won't start replacing spinning rust

Uses for NVM

Posted Jun 2, 2012 9:45 UTC (Sat) by Duncan (guest, #6647) [Link] (1 responses)

This realistic, cautionary note seems IMO to be the best comment so far (in time), both above and below yours. Let's look at what we have:

Using pricewatch.com as a guide on going street-price, current near-best prices (note that for most gig quantities, prices are higher, and of course spinning rust comes in far larger gig quantities... I simply quickly scanned and picked what appeared to be the best gigs per $ in each category) :

DRAM ~ $4/gig ($32ish, 8 gig)

SSDs ~ $1/gig ($230, 240 gig)

Flash ~ 50 cents a gig ($16, 32 gig USB stick)

Spinning rust 3.5 inch ~ 5 cents a gig ($145, 3000 gig)
Spinning rust 2.5 inch ~ 9 cents a gig ($$60, 640 gig)

Now it's worth considering just what this NVRAM is being compared against. It's being compared against $4+/gig DRAM, similar latency, higher density, lower cost, except that it happens to be non-volatile.

It's *NOT* being compared against the next step down in both price and latency, SSDs, except lower latency, higher bandwidth, tho its non-volatile nature might make that a more logical direct comparison.

That certainly says a great deal about at least the intended price point. They'd rather be compared against $4/gig DRAM than against $1/gig SSDs.

OK, that sets some pretty close bounds on the practical price-point, well under an order of magnitude. We're looking at, probably, $2-3/gig at today's capacity/price points, tho of course all three technologies (SSD, NVRAM, DRAM) can be predicted to be somewhat below that by the time it comes out in reasonable quantities and target sub-dram prices (the time frame wasn't mentioned, but maybe a couple years?)

You've made a very practical point that at the target sub-dram price-point (tho you said size/capacity, but price per capacity is I believe the controlling factor, or we'd all be using battery-backed DRAM storage for our tibibytes of videos!), perhaps half that of dram, in practice we're simply talking a cheaper dram replacement that has one very different property than current dram -- non-volatility.

That means it's *NOT* going to be the end of the world as we know it. We're simply looking at, for the most part, a cheaper dram with one rather interesting quality compared to current dram. We're NOT, at least near term, going to be replacing spinning rust, for sure.

While it's likely to replace current tech SSDs at the high end, that's likely to simply push them down-market a bit, much as SSDs forced spinning rust down-market quite a bit. Current tech SSDs will in turn drop back toward on par with flash, where it seems they settled for quite awhile, altho they're double flash's price ATM. That will hopefully in turn push flash prices down... so were this NVRAM available at the target price-points today, we might see something like this instead of the above.

DRAM, $3/gig

NVRAM, $2/gig,

SSD, 80 cents/gig

Flash, 40 cents/gig

Spinning rust, 4 cents/gig 3.5, 8 cents/gig 2.5

Or for a time NVRAM might floor the dram market, to say $2/gig (relative), with NVRAM at $1.50/gig, then actually increase the price of dram as it gets eclipsed and drops from competitive so that prices for dram don't fall in line with the rest of the market, thus going up relative to it, such that people might end up with half-gig dram machines again, the rest nvram, and dram costing (relative, the whole market would of course be cheaper by then) say $6-8/gig low end as a result. (The dynamic would thus be much like that for eclipsed memory technologies like DDR-1 and PC100/133, today. You can compare their prices per gig on pricewatch, or your favorite alternative, if you like. I just took a WAG above, but just checked and $6-8/gig for DDR is pretty close, actually, PC100/133 about double that! Prices for eclipsed tech do drop, but not by nearly as much, relative to current tech.)

The big point of course being that all this talk about killing filesystems or even SSDs and spinning rust... is VERY premature, to say the least! Otherwise, the comparison would as I said, be to SSDs, but lower latency and higher bandwidth, not to dram, but non-volatile.

Of course predicting ten years out is a tough business. By then, this could well either look like a flash in the pan or could have taken over the whole market. But more likely, it'll be simply incremental, spinning rust will still be around for our then approaching petabyte needs and nvram may or may not have replaced dram and/or ssds but may, if it survives, be used like one or the other, or both, and the world will go on much like it does today, but different, just as today is much like 2002, but different.

Uses for NVM

Posted Jun 2, 2012 20:43 UTC (Sat) by dlang (guest, #313) [Link]

if NVRAM is bigger, cheaper, and faster than DRAM (or worst case, equivalent in any or all of these categories), why would anyone still use DRAM?

This is similar to the way that flash has essentially eliminated both ROM and EEPROM from the market.

I guess I am a little slow here

Posted May 25, 2012 0:41 UTC (Fri) by iabervon (subscriber, #722) [Link]

He actually said "similar latency and throughput"; the difference could be enough that we'll see systems with some of each.

I guess I am a little slow here

Posted May 24, 2012 23:49 UTC (Thu) by felixfix (subscriber, #242) [Link] (1 responses)

Why would the BIOS not know it has NVRAM?

Some things simply are not upgradeable. If you have an old computer with DRAM, you almost certainly won't be able to replace the DRAM with NVRAM.

And if the NVRAM computer comes with a BIOS that thinks it needs to set up refresh cycles and such, it is much too broken to buy, and anyone who does gets what they deserve.

I guess I am a little slow here

Posted May 25, 2012 1:06 UTC (Fri) by iabervon (subscriber, #722) [Link]

When the power comes up and the machine has NVM, the BIOS doesn't necessarily know whether the NVM actually contains anything useful (or salvageable-- the processors may have lost power without suspending cleanly, which the OS may or may not be clever enough to sort out). And you might be trying to boot an old OS or be switching OSes entirely. So I expect that what will happen is that the BIOS will pass control to the bootloader on powerup (having clobbered the first 640k of RAM, most likely), and the bootloader has to recognize that we have a kernel in NVM already and return to its recovery point instead of loading a kernel off of the boot medium.

Of course, you could also just say, "unexpected power failures are still fatal; the only benefit of NVM is that you can remove all power while cleanup suspended." I'd expect that to be the behavior if the OS doesn't know about NVM, but I think being able to recover from unexpected power failures is more interesting.