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DDR5 memory is on its way, twice as fast as DDR4

pcworld.com

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Re: DDR5 memory is on its way, twice as fast as DDR4

#61

Post by Retric »

ajross wrote: post #56
Retric wrote: post #25

Earlier quoted context omitted.

Latency is limited by the speed of electricity which is a large fraction of the speed of light, so unless you move DDR closer to the CPU it physically can't get 2x as fast... ever. light speed * 0.951 (electricity) / 2 (round trip) / 2 feet (CPU to furthest RAM chip along a wire) ~= 250 million cycles per second. However, cache is now huge. So, it's latency is usually less of an issue than sequential reads/writes and…

Out of curiosity, why did you ballpark the length of the wire to "2 feet" (close to an order of magnitude high, way under 1 digit of precision) but specify the wave propagation speed along that wire to one part in a thousand (way more precision than appropriate: line capacitance changes significantly with trace width, pcb thickness to ground plane, pcb composition, etc...)? You're point isn't invalid but it's sort of…

I very much rounded the output to represent minimal accuracy and while 9.51 e-1 is not accurate to 3 digits 1 digits of accuracy * 1 digits of accuracy = less than 1 digits of accuracy.

Anyway, if you actually trace the furthest bit of a ram chip to the CPU it's a longer path than you might think > 1 foot but As to my point tCL is a round trip latency and actually not that far from optimal. DDR is not designed for pure random access so much as cheap access to lot's of ram so yes there are many trade offs, but they are more reasonable when your close to hard limits.

Re: DDR5 memory is on its way, twice as fast as DDR4

#62

Post by theandrewbailey »

jerf wrote: post #58
theandrewbailey wrote: post #55

Earlier quoted context omitted.

Agreed. Load times were bad in the past. No one's asked me yet why these smaller SSD thingys are so much more expensive than larger hard drives, but I prepared the answer a long time ago: They make your computer feel 10x faster. Only cheapskates and the stubborn would pass on an SSD.

"Only cheapskates and the stubborn would pass on an SSD." Even a year ago I was still going "eeehhhhhh, weeelllll, maaayybee..." but sometime in the last year I'd say we crossed over for any professional. It's no longer a matter of paying $100 for a big-enough spinning rust or $600 for a too-small SSD, now it's more like "Do I want a really fast 512GB or a slow 4TB for the same price?", which are both as of right now…

The people that I had in mind asking me would not know what an SSD is (the less technically literate). The people who go to Dell, HP, etc, and are trying to decide on a "good" laptop.

Re: DDR5 memory is on its way, twice as fast as DDR4

#64

Post by yuhong »

ksec wrote: post #45
phkahler wrote: post #18

Earlier quoted context omitted.

>> The only real advantage for a new spec is to bump the density. And DDR4 can apparently support 128GB per module. Those apparently exist, but you can't find even a 32GB at Newegg.

I am not aware of anyone other Samsung has this http://thememoryguy.com/samsungs-colossal-128gb-dimm/ But that was the end of 2015, we are into 2017 and i have yet to see this again on the market. And they are likely very expensive. I really hope DDR5 bumps the spec to 512GB rather then 256GB. Unfortunately the DDR memory / GB prices is actually on the rise. The only thing i could see changes in the market is when al…

It would probably make sense to talk about the density of each chip here. 16Gbit DDR4 is already in the spec though it is not being made yet.`

Re: DDR5 memory is on its way, twice as fast as DDR4

#65

Post by dom0 »

petra wrote: post #59
Retric wrote: post #28

Earlier quoted context omitted.

Heat, RAM puts out a lot more than you might think. That said, for low power devices like cellphones you can use different packaging and cut latency. Also, AMD for example uses stacked ram (HBM), but that needs very special packaging and active cooling. http://www.amd.com/en-us/innovations/software-technologies/h... https://en.wikipedia.org/wiki/High_Bandwidth_Memory

If heat is the problem, and since the memory wall is a 20-year problem, was there some new cooling tech ? or is it just standard industry inertia(and just being satisfied with half solutions) ?

No, it's a fundamental issue with the technology. Roughly, the ratio of power density to capacity-that-can-hold-information-for-a-refresh-interval stays more or less constant, independent of fab node. In fact, newer generations (DDR3, DDR4) have been pushing it here (cf. rowhammer).

I doubt we will see a drastic improvement in DRAM latency unless the underlying process changes a lot (i.e. different physical storage mechanism or very different manufacturing).

Re: DDR5 memory is on its way, twice as fast as DDR4

#66

Post by gatsby99 »

Retric wrote: post #61
ajross wrote: post #56

Earlier quoted context omitted.

Out of curiosity, why did you ballpark the length of the wire to "2 feet" (close to an order of magnitude high, way under 1 digit of precision) but specify the wave propagation speed along that wire to one part in a thousand (way more precision than appropriate: line capacitance changes significantly with trace width, pcb thickness to ground plane, pcb composition, etc...)? You're point isn't invalid but it's sort of…

I very much rounded the output to represent minimal accuracy and while 9.51 e-1 is not accurate to 3 digits 1 digits of accuracy * 1 digits of accuracy = less than 1 digits of accuracy. Anyway, if you actually trace the furthest bit of a ram chip to the CPU it's a longer path than you might think > 1 foot but As to my point tCL is a round trip latency and actually not that far from optimal. DDR is not designed for pu…

Best practice is to have traces less than 3"

Re: DDR5 memory is on its way, twice as fast as DDR4

#67

Post by Retric »

gatsby99 wrote: post #66
Retric wrote: post #61

Earlier quoted context omitted.

I very much rounded the output to represent minimal accuracy and while 9.51 e-1 is not accurate to 3 digits 1 digits of accuracy * 1 digits of accuracy = less than 1 digits of accuracy. Anyway, if you actually trace the furthest bit of a ram chip to the CPU it's a longer path than you might think > 1 foot but As to my point tCL is a round trip latency and actually not that far from optimal. DDR is not designed for pu…

Best practice is to have traces less than 3"

Traces only go from CPU pin to Module, latency includes from memory controller CPU pin, CPU pin to PCB, PCB to module, module to RAM chip edge, ram chip edge to actual memory location.

Here is a picture, from the center of the CPU, to the bottom edge of the furthest part of the DIM then up. http://harddiskdirect.com/media/catalog/product/cache/1/imag...

PS: A DIMM is just over 5 inches wide and 0.7 inches tall so if was centered physically on top of a CPU that's 3 inches right there.

Re: DDR5 memory is on its way, twice as fast as DDR4

#69

Post by gatsby99 »

Retric wrote: post #67
gatsby99 wrote: post #66

Earlier quoted context omitted.

Best practice is to have traces less than 3"

Traces only go from CPU pin to Module, latency includes from memory controller CPU pin, CPU pin to PCB, PCB to module, module to RAM chip edge, ram chip edge to actual memory location. Here is a picture, from the center of the CPU, to the bottom edge of the furthest part of the DIM then up. http://harddiskdirect.com/media/catalog/product/cache/1/imag... PS: A DIMM is just over 5 inches wide and 0.7 inches tall so if…

You don't know what you are talking about. Memory controllers are typically onchip.

Re: DDR5 memory is on its way, twice as fast as DDR4

#70

Post by Zekio »

theandrewbailey wrote: post #53
TazeTSchnitzel wrote: post #19

Are there applications which will see significant benefit from a new generation of DRAM, or higher speeds? Games, for instance, seem to see only very small performance differences from changing RAM speeds.

Yet you can still see gamers worrying about RAM timings.

Well they should worry but only if they use Ryzen CPU's which depend quite a bit on RAM
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