
DDR5 memory is on its way, twice as fast as DDR4 - TeraBit_Tech
http://www.pcworld.com/article/3186737/components/staying-alive-ddr5-memory-is-on-its-way.html
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trey-jones
So, it's been a while since I tinkered with hardware, but I'm making the blind
assumption that although the bus speed is twice as fast as DDR4, there is also
twice the read latency, as has always been the case with DDR memory, meaning
the net read latency improvement is basically nil.

Of course we expect every generation of Double Data Rate memory to double the
data rate. But it's always been a bit misleading to say "twice as fast".

~~~
Retric
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 total size.

~~~
jjoonathan
I keep hearing this, but it doesn't jibe with the RAM timings I see. tRAS is
usually 50ns or so, and propagation delay is 2ns or so (see below). What
gives?

My suspicion is that amplifying the differential voltages produced by
femtocoulombs of charge at 100% reliability is a harder problem than moving
the DRAM chips closer to the CPU, and that speed-of-light has gotten the blame
in "pop architecture" due to sloppy overgeneralization.

    
    
        2*2 feet is long for a memory bus, but it's partially cancelled
        out by the high velocity factor (sorry, "speed of electricity")
        of 0.951 in parent's calculations. Instead, I'm going with 0.5*2
        feet at a velocity factor of 0.5 for a 2ns roundtrip propagation delay.

~~~
Retric
[https://en.wikipedia.org/wiki/CAS_latency](https://en.wikipedia.org/wiki/CAS_latency)

First word latency: DDR3 SDRAM 6.37ns = 1/156,985,871 of a second.

~~~
jjoonathan
tCL is (very) roughly the RAM equivalent of sequential access time. I'm
talking random access time, tRAS, which includes closing the last row
(precharging the diff amps) and opening the next (waiting for the amps to
stabilize). It's called Random Access Memory, so I think it's more fair to
judge it by its random access time.

~~~
Retric
tCL is very much a round trip latency not just sequential access. If you want
sequential access 4 times you can pay for tCL once.
[https://www.altera.com/ja_JP/pdfs/literature/hb/external-
mem...](https://www.altera.com/ja_JP/pdfs/literature/hb/external-
memory/emi_optimizing_efficiency.pdf)

DDR is not really setup for random access, because it's setup for talking to
L2/L3 cache not registers. tRAS ~= tCL + tRCD + tRP which kind of but not
exactly exactly what you want for random access.

~~~
jjoonathan
Yes, tCL is a round trip time for accessing an already open row, and it puts a
sharp upper bound of ~5ns on propagation delay. Random access time,
approximated by tRAS, is still ~50ns. You contend that tCL is the bigger
problem, I contend that tRAS is the bigger problem. The key statistic is
<#CAS/#RAS>, which I haven't been able to find easy values for.

I don't understand your argument that the L2 and L3 caches tip the balance in
favor of <#CAS>. If anything, I'd expect them to do the opposite, because the
SRAM and DRAM both aim to exploit locality, meaning that better L2 and L3
caches would reduce #CAS faster than #RAS. Of course, armchair reasoning about
such complex systems as memory controllers and caches can only go so far, so I
wouldn't be shocked to be wrong, but I would certainly demand actual
statistics before changing my view.

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sniglom
"Analysts didn't expect DDR5 to be developed" Is that the same analysts
stating that Apple will have an event at the same time as last year?

If an improvement can be done cheap and easy it will be done. Implementing a
memory controller for DDR5 is probably a trivial job very similar to current
DDR4.

Integrated GPUs has been on the rise for a long time. This is good news for
reusing current memory controller designs while providing higher bandwidth.

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sp332
JEDEC officially specs DDR4 RAM up to 2133 MHz. But manufacturers seem to have
left this far behind already, with Corsair and G.Skill shipping modules at
4200 MHz. I don't see why JEDEC doesn't just add a few more rows to the table
of official speeds. The only real advantage for a new spec is to bump the
density.

~~~
phkahler
>> 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.

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pmalynin
Above 8 gigs per module you're going to need buffered ram. Check in the ECC
section.

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sscotth
Not true. I use non-ECC 16GB sticks.

[https://m.newegg.com/products/N82E16820231974](https://m.newegg.com/products/N82E16820231974)

~~~
yuhong
That is because we now have 8Gbit DDR4 chips.

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refrigerator
Would the equivalent amount of DDR5 actually feel any different to DDR4 to the
end user? It seems like hardware is getting better but basic software (web
browser etc) is getting more complicated/bloated at the same rate, so things
don't actually _feel_ 10x faster than they did a few years ago.

~~~
jerf
Do you have an SSD yet? A good one?

Computers with good SSDs and not too much bloatware (relevant on Windows) do
feel faster to me than computers used to be.

I think people can get nostalgic accidentally and overestimate how fast things
were in the past. I remember multi-minute loads for games on my Commodore 64.
I remember multi-minute Windows 3.1 bootups. I remember watching a JPEG
progressively creep in on the early web. I remember watching my school
projects in C++ take noticeable time to compile despite what we would now
consider their absurdly simple nature. I remember when I was reluctant to
click on a 1MB download.

But you do want to avoid having a modern Windows machine on a slow spinning-
rust hard drive. Yeow.

~~~
theandrewbailey
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.

~~~
jerf
"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 ~$150 give or take $30
depending on your quality needs. (I just checked.) That's still quite the
spread on size, but you can fit a lot in 512GB.

~~~
theandrewbailey
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.

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dogma1138
Is the image of an DDR5 DIMM? because from the look of it it has some serious
power management and storage components looks more like an NVRAM DIMM to me.

Edit: the image name has 8GB NVDIMM in it, so yeah this isn't DDR5.

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vondur
How much difference is there between GDDR and regular DDR? I think that GDDR
runs at higher voltages and temps than regular DDR. I'm assuming if GDDR5 is
available, then it's kind of inevitable that regular DDR 5 would be coming at
some point.

~~~
theandrewbailey
I thought the main difference with graphical RAM was that the monitor output
DAC could read from memory at all times, regardless of what the GPU is doing
with it. And GDDR _n_ is not the same generation as DDR _n_ ; the last GDDRs
(4,5) have been based off DDR3, I think.

~~~
dom0
Graphics RAM was dual-ported (=two readers at the same time) in the early
days, but hasn't been for a long time. The concurrent accesses are managed by
the GPU internally now.

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akmittal
Most laptops shipped this year still has DDR3/LPDDR3 RAM

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dogma1138
Because LPDDR4 wasn't available in sufficient quantities or not supported by
the platform SkyLake only supported LPDDR3 and the current KabyLake SKUs also
do not support LPDDR4.

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Nexxxeh
Is LPDDR3 what is also referred to as DDR3L, or is that different?

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my123
DDR3L is completely different than LPDDR3. DDR3L is a low-voltage DDR3
version, while LPDDR3 is completely separate.

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Zekio
I wonder how this will affect the Infinite Fabric in Ryzen CPU which runs at
half clock speed of the ram

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Tharkun
Is anything being done in DDR5 to mitigate rowhammer at the design level?

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TazeTSchnitzel
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.

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the8472
Games are a special breed of application since they delegate a lot of work to
the GPU and often are GPU-bound, not CPU-bound. Other workloads often end up
with the CPU waiting for memory accesses. The most obvious example, besides
stream-processing, would be garbage-collected languages. Simply scanning the
heap will benefit from all the memory bandwidth it can get.

Even if speeds don't improve perceptibly because the CPU is simply waiting for
the human 99% of the time it still may benefit laptop battery time due to
increased power efficiency and the CPU completing that 1% sooner and returning
to low power states.

~~~
TazeTSchnitzel
> Games are a special breed of application since they delegate a lot of work
> to the GPU and often are GPU-bound, not CPU-bound.

Yeah, and the GPU (at least a dedicated one) has its own VRAM, so accesses for
which RAM speed would otherwise be important end up relying on VRAM speed, I
assume.

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rocky1138
Is DDR4 memory a major bottleneck in systems today?

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KSS42
Yes, in servers, because of high core counts and demanding "big data"
applications.

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sayright
HN article above this one: "Going faster doesn’t make you happier.." :)

