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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

#31

Post by fizzynut »

PaulKeeble wrote: post #22
fizzynut wrote: post #19

Ai generated slop. Constantly summarising various parts of the memory hierarchy, graphs with no x axis, bad units, no real world examples, the final conclusion doesn't match the previous 10 summaries. The big problem is that it misses a lot of nuisance. If actually try to treat an SSD like ram and you randomly read and or write 4 bytes of data that isn't in a ram cache you will get performance measured in the kilobyt…

SSDs have so many cases of odd behaviour. If you limit to writing drive sector chunks, so 4k, then at some point you will run into erase issues because the flash erase size is considerably larger than the 4k sectors. But you also run into hitting the limits of the memory buffer and the amount of fast SLC as well which limits the long term sustained write speed. There are lots of these barriers you can break through a…

Yes, it can be quite brand/technology specific, but chunk sizes of 4/8/16/etc MB usually work much better for SSDs, but the only data I've found to read/write that easily lines up with those chunk sizes are things like video/textures/etc or cache buffers you fill in ram then write out in chunks.

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

#32

Post by DSMan195276 »

christkv wrote: post #14

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

It costs more because it's less dense than DRAM - the same transistor count that produces 2GB of DRAM can only fit a fraction of that in SRAM because it's 6 transistors per SRAM cell vs. 1 + capacitor for a DRAM cell.

Power usage is also generally worse since the SRAM cells use continuous power to hold their state, while DRAM cells only use power during read/write/refresh (relying on the caps to hold their charge for a short time while not actively powered).

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

#34

Post by ashvardanian »

bob1029 wrote: post #5

> clocks, IPC, and latency flatlined IPC has definitely not flatlined. Zen 4 can do something like 50-70% more IPC than a CPU from 10-15 years ago. Zen 5 is capable of ~16% more IPC than Zen 4.

In relation to other core specs, like transistor and core counts and memory bandwidth, 50% growth in 10 years is pretty much flat-lining, I’d say.

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

#35

Post by numpad0 »

marcosdumay wrote: post #26
crawshaw wrote: post #13

A title of "Memory is getting slower, Disk is getting faster" might remove a lot of the initial confusion this post causes.

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 brain.

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

#36

Post by whizzter »

fizzynut wrote: post #31
PaulKeeble wrote: post #22

Earlier quoted context omitted.

SSDs have so many cases of odd behaviour. If you limit to writing drive sector chunks, so 4k, then at some point you will run into erase issues because the flash erase size is considerably larger than the 4k sectors. But you also run into hitting the limits of the memory buffer and the amount of fast SLC as well which limits the long term sustained write speed. There are lots of these barriers you can break through a…

Yes, it can be quite brand/technology specific, but chunk sizes of 4/8/16/etc MB usually work much better for SSDs, but the only data I've found to read/write that easily lines up with those chunk sizes are things like video/textures/etc or cache buffers you fill in ram then write out in chunks.

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?).

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

#37

Post by pjdesno »

Relevant to this: memory cost per GB dropped exponentially every year from the days of the earliest computers until 2010 or so, reaching $4/GB in 2011. A decade and a half later it's still in the $2-$4/GB range.

Note also that SSDs started out only slightly cheaper per GB than DRAM - the 80GB Intel X25-M had a list price of about $500 when it was released in 2008, and references I find on the net show a street price of about $240 for the next-gen 80GB device in 2009. Nowadays you can get a 1TB NVMe drive for about the cost of 16GB of RAM, although you might want to spend a few more bucks to get a non-sketchy device.

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

#38

Post by PaulKeeble »

whizzter wrote: post #36
fizzynut wrote: post #31

Earlier quoted context omitted.

Yes, it can be quite brand/technology specific, but chunk sizes of 4/8/16/etc MB usually work much better for SSDs, but the only data I've found to read/write that easily lines up with those chunk sizes are things like video/textures/etc or cache buffers you fill in ram then write out in chunks.

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 heavily dependent on use patterns making it very hard to do this in a portable way or one that accounts for changes in the business.

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

#39

Post by masfuerte »

bolster wrote: post #20

> Data used for the charts was researched and compiled by ChatGPT, I spot checked it and found it was accurate enough to for the narrative. Last I checked, that wasn't how citations worked. I only sampled one data point (2017 AMD EPYC Rome Clock rate) which was significantly off, because in 2017 it was the Naples chipset that was released, and, unless 2017 was desperate enough to clock from 2.2Ghz to 3Ghz on the regu…

And we don't know what the prompt was. Maybe:

Hey ChatGPT, here's a narrative. Please provide some plausible stats to support it.

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

#40

Post by vlovich123 »

jeffbee wrote: post #28
vlovich123 wrote: post #18

Earlier quoted context omitted.

The article feels like LLM output. Some correct facts strung together to support an incorrect thesis the author asked it to validate, but completely ignoring inconvenient facts and reaching a wild conclusion that isn’t actually true.

That's what it says it is at the very bottom.

Well it says the charts were generated this way. But I feel like the narrative was also done this way.
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