Why upgrade to DDR4?

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Not so long ago, a standard was publishedon DDR4 SDRAM - a new generation of RAM. The standard describes the memory device in all details, but about what, in fact, the differences from the previous generation memory, or what advantages promise the transition to DDR4 is said either very little or not at all (probably in the hope of readers' imagination :-)) .
In this post I will try to explain the main differences between DDR3 and DDR4, and what are the advantages for the end users of the new memory standard.

Increase Volume and Productivity

One subtle, but at the same time important difference, is how the memory chips are organized.
An 8Gb x4 DDR4 chip usually consists of 4 groups of banks, 4 banks in each group. Each bank of such a chip contains 131.072 (2 ¹⁷ ) rows, 512 bytes each. For comparison 8Gb x4 DDR3 chip contains 8 independent banks, 65,536 (2 ¹⁶) lines per bank, 2048 bytes in each line. With an equal volume, the DDR4 chip has twice as many banks and is much shorter than the memory line. This means that the new memory can switch between memory banks much faster than DDR3 did. In particular, for 8Gb x4 DDR4 chips declared as 1600 MT / s compatible, the tFAW (Four-bank Activation Window) indicator is 20ns, which is half that of DDR3 (40ns). This means that DDR4 memory chips can open arbitrary strings in different banks twice as fast as DDR3.

A comparison of DDR3 and DDR4 shows that the largest DDR3 module that can theoretically be designed will have a size of 128GB (using QDP (quad die package - packing four chips in one package) and 8Gb crystals) (Fig. 1). For DDR4, using 16Gb crystals, and eight-layer packing of crystals in a chip, theoretically, you can create a memory module up to 512GB. The number of pins on the DDR4 modules increased to 284 to address this amount of memory. Each DDR4 memory chip can be a stack of 2, 4, or 8 DRAM crystals. A stack of 8 layers is described in additions to the specification and most likely will require the use of TSV (through silicon via) for its practical implementation.
In general, all these changes are aimed at creating larger capacity memory modules and increasing productivity.


Figure 1. Comparison of DDR3 and DDR4 modules ^(source)

Improved energy efficiency

Another important part of the DDR4 specification is the increase in energy efficiency compared to DDR3. In addition to reducing the voltage on I / O from 1.35V to 1.2V, the new standard also specifies the use of a higher voltage level inside the chips (DRAM word line 2.5V), which provides quick access in active mode and low leakage current in passive.

The electrical implementation of the data input-output interface has also changed. The new interface is called pseudo-open drain (POD, "pseudo-open drain") and its main difference is that no current flows in the circuit when a high voltage level is set on the line. The electrical interfaces of DDR3 and DDR4 are shown in Figure 2.


Figure 2. Electrical interfaces of data I / O for DDR3 and DDR4. ^{(a source)}

Reducing the voltage on the I / O, changing the electrical interface and reducing the length of the lines in the memory banks lead to a significant reduction in power consumption compared to DDR3. Preliminary estimates indicate a 30% gain. Although, of course, this depends on the nature of memory accesses, the process technology, and many other factors. Such a gain can be used in order to increase the clock frequency and, accordingly, the speed of work, or in order to save a little energy with the same performance.

Reliability

Also, many important changes relate to the reliability ( RAS ) of DDR4. For example, the specification speaks of detection and correction by memory chips of errors associated with parity control of commands and addresses.

Another example is that DDR4 chips have a connection test mode. This mode allows the memory controller to check the electrical connections (and find the “open” lines), much faster than before, and without the use of initializing sequences.
Also, the DDR4 module can be configured to discard instructions containing parity errors. In DDR3, such commands were skipped and reached the memory chips, making it much more difficult to recover from failures.
And as an example of one of the optional "features" that the specification contains, you can check the checksums for data written to memory.

All these and other features are aimed at ensuring the growth of operating frequencies and memory volumes (associated with an increase in the number of errors in operation), while guaranteeing stable operation.

With higher performance and energy efficiency, DDR4 memory in 2014 should easily take its place in multi-core server and desktop systems. And then, due to the lower price per unit volume, in addition to other advantages, DDR4 should also get to other devices.