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Memory is slow, Disk is fast – Part 1

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Re: Memory is slow, Disk is fast – Part 1

#41

Post by marcosdumay »

numpad0 wrote: post #35
marcosdumay wrote: post #26

Earlier quoted context omitted.

Except that the article doubles (or rather, triples) down on claiming that reading from the disk is faster than from the memory despite all of CS (CS, really? That has nothing to do with CS on reality) "dogma" claiming otherwise. It's a weird article. It looks at an insightful set of numbers, fairly accesses them, and then goes on to conclude the stupidest thing.

Maybe they discovered DMA transfer from SSD on PCIe being less bottlenecked than from DRAM, and fell into a rabbit hole from there. Switching hubs always has more bandwidths than a port that it is part of, and modern CPU resides on the PCIe equivalent of an uplink port. It makes sense that there would be situations where peripheral-to-peripheral DMAs would be faster than RAM-to-peripheral transfers to my low-paygrade…

Yes, there exist situations where periferal-to-periferal is faster than RAM-to-periferal.

But that doesn't make the disk access optimizations on your OS harmful, nor even useless. The disk-to-CPU access is best done through those optimizations for several reasons, one of them being that the author's conclusion isn't entirely supported by the data.

Re: Memory is slow, Disk is fast – Part 1

#42

Post by psidebot »

Disk hardware may be faster relative to RAM, but if you're using typical serverless PaaS offerings to run a hosted application at modest operational scale, it's a heck of a lot cheaper to get a node with "normal" RAM and CPU, than it is to get decent IOPS. If you're a big iron company with big iron problems, you may need to think in different scaling terms, but for the SaaS hoi polloi the economics of hosting have somewhat preserved traditional notions of performance.

Re: Memory is slow, Disk is fast – Part 1

#43

Post by whizzter »

PaulKeeble wrote: post #38
whizzter wrote: post #36

Earlier quoted context omitted.

This from exprience or any sources on what's sane to use today? Building a niched DB and "larger-blocks" has been design direction, but how "far" to go has been a nagging question (Also are log-structured things still a benefit?).

You are also going to cause a lot of write amplification with bigger blocks and at some point its also going to limit your performance as well. What really makes this hard is it depends on how filled the drive is, how heavily the drive is utilised and for how much of the day. Time to garbage collect results in different performance to not. When you start trying to design tools to use SSDs optimally you find its heavi…

This project is not "business" bound, it's a DB abstraction so business concerns are layered outside of it (but it's a worthwhile pursuit since it rethinks some aspects I haven't seen elsewhere in all the years of DB announcements here and elsewhere).

And yes, write amplification is one major concern but the question is that considering how hardware has changed, how does one design to avoid it. Our classic 512byte, 4k,etc block sizes seems long gone and does the systems "magically" hide it or do we end up with unseen write amplification instead?

Re: Memory is slow, Disk is fast – Part 1

#46

Post by jared_hulbert »

christkv wrote: post #14

Is there a reason why are not seeing more use sram (other than cost)?

We are seeing more SRAM in the form of CPU caches, this is one of the things that is actually scaling still.

Are you asking why not use SRAM in something like a DIMM? You could do this. Here's why I wouldn't advocate for this. Assume you had zero latency SRAM in your DIMM. It still takes ~40ns to get out of the processor by the time you go through all the memory controller and phy. So you'd have an incredibly expensive but small DIMM taking up limited pins on the processor package/die. Even then you'd only cut the memory latency in half, and we'd still be stuck at a new lower flatline.

Incorporating the SRAM in die is different story, you get to scale the latency and the bandwidth closer to the other capabilities of the cores.

Re: Memory is slow, Disk is fast – Part 1

#47

Post by jared_hulbert »

howdyhowdy wrote: post #16

Why not CXL?

In this context CXL is kinda storage and kinda higher latency RAM. It's latency is worse than standard DDR5 DIMMs but the bandwidth is on the same trajectory as storage/networking.

I'm inclined to think of it like storage in this context. It's scaling, but it will require new thinking to take full advantage of.

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