Committed to Memory

 

The first DDR4 DIMMs are for sale!

I think I have found the first DDR4 DIMMs available for consumer purchase anywhere. Crucial and a few others were showing DDR4 DIMMs at this year’s Consumer Electronics Show (CES) in January, so it’s nice to see that they translated into real products.

Engineering samples of DDR4 have been available for some time (Synopsys has several DDR4 devices that we used to characterize our DDR4 PHY testchips), but as far as I know this Crucial DIMM is the first publicly available DDR4 module. If you know of an earlier example of where a consumer could buy a DDR4 DIMM, please post it in the comments.

As I write this (March 19, 2014), through the link at crucial.com, $445.99 plus tax and shipping will get you two single-rank 8GB DIMMs, each DIMM composed of 18 of 4Gb 4-bit wide, 1.2v DDR4-2133 parts with CAS Latency 15, plus a Memory Buffer chip (that makes it a registered DIMM) and a Serial Presence Detect (SPD) chip plus some ancilliary components.

One thing to note is that the DDR4 module mentioned above is very clearly intended for servers. Support for X4 DRAM parts, Error Correcting Codes (ECC) and Registered DIMM are all features from the server/enterprise space that don’t commonly get used in consumer/desktop/notebook computers and are generally not supported by consumer CPUs nor consumer motherboards. That ties up very well with Synopsys’s predictions that DDR4 will enter the server/enterprise space first and then gradually make its way into the consumer market.

This module, albeit perhaps the first of its kind publicly available for purchase, is substantially more expensive than a similar consumer-grade DDR3 DIMM. The big question is: Why would you buy this DDR4 DIMM? There are a few possible reasons:

– You are validating some kind of system that will use DDR4 in the future
– You have an application where the reduced power consumption of DDR4 is important to you (Crucial says these DDR4 modules use 40% less power than their DDR3 equivalents)
– You are building a high-reliability, high-performance server
– You want to be the first to tell your online gaming buddies that you have a DDR4 rig

Let’s look at each of these in turn.

The first thing I want to point out is, you need a motherboard that supports DDR4 to use a DDR4 DIMM. In fact, as I searched the web today, I couldn’t find a single motherboard for sale that could accept a DDR4 DIMM… Nor a CPU from the major desktop CPU manufacturers whose datasheet says it’s DDR4-compatible… We know they are coming in the near future, so let’s assume that you’ve solved that problem, or that you’re reading this blog post at some point in the future when some DDR4 Motherboards are available.

If you are validating a DDR4 system, then you need DDR4 to do it of course – so that’s an easy decision.

The power consumption of DDR4 is attractive for many types of systems. The memory in most computing devices uses a large percentage of the total system power and being able to reduce the power has many benefits. If we’re comparing with DDR3-2133 modules that are available today, the DDR3 modules available today fall into two categories:
– DDR3 DIMMs where the DDR3 parts on the DIMM were marked by the memory manufacturer for DDR3-2133 data rates. Typically the manufacturer would have speed-sorted these devices at the time of manufacturing and these would generally be from the fast process corner which is also typically the high leakage corner and also typically the high power process corner.
– Overclocked DDR3 DIMMs where the DDR3 parts on the DIMM were marked by the memory manufacturer for a speed lower than DDR3-2133 and which have been speed-sorted by the DIMM manufacturer and/or respecified to run at a higher-than-normal voltage of 1.6v or even 1.65v, which burns more power than 1.5v.
– I was not able to find any low-voltage (1.35v) DDR3L DIMMs specified higher than DDR3-1600 speeds.
Compared to these devices, we can certainly expect less power per bit from the 1.2v IOs on the DDR4 DIMM, as well as the Data Bit Inversion function of DDR4, if you have a motherboard to support DDR4.

If you are building a high-performance, high-reliability server, this DDR4 DIMM could be attractive. Crucial’s DDR4-2133 DIMM is a registered DIMM with X4 DRAMs and ECC; whereas the fastest DDR3 DIMM I could find with those features was DDR3-1866. In the high-reliability server space, end-users are reluctant to overclock their DRAM parts, so this DDR4-2133 DIMM could be attractive by potentially offering more bandwidth than DDR3-1866 if you have a motherboard that can support it. In addition, many of the “RAS” features of DDR4 – Reliability, Availablity, Serviceability – are intended for use in high-reliability systems. Check out my recent Electronic Design article for more.

If your objective is to be able to brag about your DDR4 rig to all of your online gaming buddies (assuming that you can find a motherboard that supports it), if you have a technologically savvy group they’re likely to point out the following:
– The latency of this DIMM, at 15 clock cycles, is higher than most of the overclocked DDR3 DIMMs
– Overclocked DDR3 DIMMs are available that are rated by the DIMM manufacturers to run at higher speeds than 2133MT/s (I’ve seen some at 3000MT/s and one claiming 4000MT/s operation)
– The memory buffer on a registered DIMM adds latency compared to unbuffered DIMMs.
– The bank group arrangement in DDR4 is a new twist on memory scheduling, and unless the memory controller in the CPU is optimized for DDR4 bank groups, you’re likely to have lower bandwidth from DDR4 than DDR3 at the same clock frequency. On the other hand, when taking proper advantage of the DDR4 bank groups, it may be possible to get more bandwidth than DDR3. Shameless plug and/or a topic for another day: feel free to ask how you can take proper advantage of DDR4 bank groups in Synopsys’s uMCTL2 memory controller!

So there we have it: after 10 years of development, the first DDR4 DIMM for sale. Now you just need to find a motherboard for it…

Marc

I have to remind everyone that Synopsys does not specifically endorse any particular DIMM manufacturer or online retailer.

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