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Are you ready to rethink RAM?

The revolution in use-case-aware intelligent flash could cross over into new enterprise DRAM architecture.

by Zsolt Kerekes, editor - April 2, 2014

In recent years in the flash memory market, we've seen SSD controller designers revisit and challenge the fundamental assumptions surrounding - what's the best way to interact with the raw memory?

And instead of just leaving critical timing parameters "as set" in the factory by "those who know best" (the preset magic numbers which are designed to satisfy worst-case usage within the memory population as a whole) - SSD designers have leveraged the idea that "local knowledge (in a systems context) is better" - to design new adaptive R/W controller schemes which change the raw capabilities of memory arrays - to enable faster speed, or lower power, or better reliability - or a combination of desirable features.

But what about designers in the RAM market?

Apart from 3D, packaging and interplane connection techniques - is there any similar revolutionary thinking going on in the RAM market?


In some random reading I did recently - my attention was drawn to a collection of papers delivered in August 2013 at a conference called MemCon.

I hadn't read these before - partly because I was on vacation at the time - and when that finished - I was too busy catching up with digesting the new ideas which had been channeled through the SSD industry's premier event - the Flash Memory Summit - which took place shortly after MemCon.

If you wondering why I say "FMS is the SSD industry's premier event" - then take a look at how many times it has been mentioned in the same breath as an advanced SSD concept on this site alone.

In 2014 - the relative timings of these 2 events has been adjusted to create a gap which is months rather than days - so that those poor mortals who attend both - get a chance to recuperate.

the new thinking in RAM architecture

For me today - the most interesting of all the RAM related papers on the MemCon site in my reading this week - was one which explored in a modern SSD perspective - the idea of adapting the refresh rates in DRAM to leverage the difference between "worst case" and "good enough" timings. In his presentation - Memory Scaling - a Systems Architecture Perspective (pdf) - Onur Mutlu, Assistant Professor Electrical and Computer Engineering - Carnegie Mellon University called this "Retention-Aware DRAM Refresh".

If you only like to learn one new thing a month - that in itself seems like a good enough place to stop - but it's just one of the warmup steps for the intensive workout which follows.

If you think you may up to that challenge - take a look at Onur Mutlu's accompanying paper - Memory Scaling: A Systems Architecture Perspective (pdf) - which among other things - proposes putting SSD-like thinking into the design DRAM and moving away from the idea of treating "DRAM as a passive slave device."

This paper is rich with ideas such as:-
  • tiering within the DRAM (using 2 level latency-segmented bitlines)
  • moving blocks of data within the RAM - without using the external bus - with a new design and topology of sense amps
And here's a quote from Onur Mutlu's paper which I think resonates with the enterprise SSD experience:-

"Our past work showed that application-unaware design of memory controllers, and in particular memory scheduling algorithms, leads to uncontrolled interference of applications in the memory system. Such uncontrolled interference can lead to denial of service to some applications, low system performance, and an inability to satisfy performance requirements, which makes the system uncontrollable and unpredictable."

So - will we see more flash-controller-like functions inside future DRAMs?

That depends on the system cost-benefits. (And whether such schemes can be implemented in commercially scaled semiconductor layouts - rather than the kind of conceptual lines drawn to connect virtual blocks on a whiteboard.)

Support for embedded controllers is already an integral part of the Hybrid Memory Cube (launched in October 2011) - but Carnegie Mellon's ideas about new memory designs cross the frontier line which artifically separates different chips even within an HMC architecture.

where's the money?

There can sometimes be money to be made from some of these blue sky academic research ideas.

Yesterday for example (April 1, 2014) - Marvell Semiconductor announced that a US court this week determined that the company should pay $1.54 billion to Carnegie Mellon University for alledgedly infringing patents related to hard drive patents. more - Whatever the final outcome of any appeals process - it's keeping some smart lawyers in work.
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big idea #3 - retiring and retiering enterprise DRAM

which includes a new value proposition for enterprise flash SSDs (flash as RAM) and presages a rebalancing of server memories - DRAM will shrink as a percentage of the physical RAM - which will also make it easier for emerging alternative memory types to be adopted by hardware architects and by systems software too.
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a guide to data compression techniques and where to use them for designers of SSDs and memory systems
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Editor:- May 26, 2015 - Inside the SSD controller brain the compressibility of data is one of the tools which can go into the mix of optimizing performance, endurance and competitive cost.

A recent paper - A Survey Of Architectural Approaches for Data Compression in Cache and Main Memory Systems by Sparsh Mittal and Jeffrey S. Vetter in IEEE Transactions on Parallel and Distributed Systems - reviews the published techniques available and places their relevance in the context of real and future memory types and applications.

The survey covers applications from embedded systems upto supercomputers.

In addition to being useful resource directory of related papers the article gives you a brief description of many compression techniques, where you might use them and what benefits you might expect.

See also:- list of articles and books by Sparsh Mittal which among other things covers caching techniques, reliability impacts and energy saving possibilities in a wide range of server architectures.

DRAM latencySetting the scene for DIMM wars

The DRAM market's new clothes had long been invisible.

But the SSD market was too preoccupied with lower hanging storage fruit.
latency loving reasons for fading out DRAM

Adaptive Dynamic Refresh in DRAM
Editor:- October 14, 2015 - I expected most of the practical iinovations in rethinking DRAM architecture to come from the enterprise market.

But there's an interesting exception from Green Mountain Semiconductor which is revealed in a new paper - LPDDR3/4-ECC DRAM for High-reliability IoT, Automotive and Control System Applications (pdf).

GMS designs memories for industrial. embedded and custom systems. The innovation discussed in their paper is the use of adaptive dynamic refresh as a collaborative technology with ECC which can react to ECC errors by tuning the refresh rate.

ECC adjusted adaptive DRAM refresh

GMS says the strategy is - "Increase refresh rate if too many fails and reduce rate if too few fails, always guaranteeing refesh rate mimics cell fail distribution. Self-calibrating system, no need for tightly calibrated temperature." the article (pdf)

in-situ processing in flash array obviates need for big RAM in big data - MIT research findings
Editor:- July 14, 2015 - Flash SSDs with in-situ processing in regular RAM cached servers can deliver nearly the same apps performance as fat RAM servers (but at much lower cost and lower electrical power).

That's one inference from a recent story - Cutting cost and power consumption for big data - in MIT news - which summarized a research paper at ISCA 2015 - BlueDBM: An Appliance for Big Data Analytics .

Part of the system architecture in the research included a network of FPGAs which routed data to the flash arrays and offloaded some of the application specific processing.

This is not a replacement for DRAM said Professor Arvind whose group at MIT performed the new work. But there may be many applications that can take advantage of this new style of architecture. Which companies recognize: Everybodys experimenting with different aspects of flash. Were just trying to establish another point in the design space. the article