IBM using PCM to implement better AI – round 6

Saw a recent article that discussed IBM’s research into new computing architectures that are inspired by brain computational techniques (see A new brain inspired architecture … ). The article reports on research done by IBM R&D into using Phase Change Memory (PCM) technology to implement various versions of computer architectures for AI (see Tutorial: Brain inspired computation using PCM, in the AIP Journal of Applied Physics).

As you may recall, we have been reporting on IBM Research into different computing architectures to support AI processing for quite awhile now, (see: Parts 1, 2, 3, 4, & 5). In our last post, More power efficient deep learning through IBM and PCM, we reported on a unique hybrid PCM-silicon solution to deep learning computation.

Readers should also be familiar with PCM as well as it’s been discussed at length in a number of our posts (see The end of NAND is near, maybe; The future of data storage is MRAM; and New chip architectures with CPU, storage & sensors …). MRAM, ReRAM and current 3D XPoint seem to be all different forms of PCM (I think).

In the current research, IBM discusses three different approaches to support AI  utilizing PCM devices. All three approaches stem from the physical characteristics of PCM.

(Some) PCM physics

FIG. 2. (a) Phase-change memory is based on the rapid and reversible phase transition of certain types of materials between crystalline and amorphous phases by the application of suitable electrical pulses. (b) Transmission electron micrograph of a mushroom-type PCM device in a RESET state. It can be seen that the bottom electrode is blocked by the amorphous phase.

It turns out that PCM devices have many  characteristics that lend themselves to be useful for specialized computation. PCM devices crystalize and melt in order to change state. The properties associated with melting and crystallization of the PCM media cell can be used to support unique forms of computation. Some of these PCM characteristics include::

  • Analog, not digital memory – PCM devices are, at the core, an analog memory device. We mean that they don’t record just a 0 or 1 (actually resistant or conductive) state, but rather a continuum of values between those two.
  • PCM devices have an accumulation capability –   each PCM cell actually  accumulates a level of activation. This means that one cell can be more or less likely to change state depending on prior activity.
  • PCM devices are noisy – PCM cells arenot perfect recorders of state chang signals  but rather have a well known, random noise which impacts the state level attained, that can be used to introduce randomness into processing.

The other major advantage of PCM devices is that they take a lot less power than a GPU-CPU to work.

Three ways to use PCM for AI learning

FIG. 4. “In-memory computing,” computation is performed in place by exploiting the physical attributes of memory devices organized as a “computational memory” unit. For example, if data A is stored in a computational memory unit and if we would like to perform f(A), then it is not required to bring A to the processing unit. This saves energy and time that would have to be spent in the case of conventional computing system and memory unit. Adapted from Ref. 19.

The Applied Physics article describes three ways to use PCM devices in AI learning. These three include:

  1. Computational storage – which uses the analog capabilities of PCM to perform  arithmetic and learning computations. In a sort of combined compute and storage device.
  2. AI co-processor – which uses PCM devices, in an “all PCM nodes connected to all other PCM nodes” operation that could be used to perform neural network learning. In an AI co-processor there would be multiple all connected PCM modules, each emulating a neural network layer.
  3. Spiking neural networks –  which uses PCM activation accumulation characteristics & inherent randomness to mimic, biological spiking neuron activation.
FIG. 11.
A proposed chip architecture for a co-processor for deep learning based on PCM arrays.28

It’s the last approach that intrigues me.

Spiking neural nets (SNN)

FIG. 12. (a) Schematic illustration of a synaptic connection and the corresponding pre- and post-synaptic neurons. The synaptic connection strengthens or weakens based on the spike activity of these neurons; a process referred to as synaptic plasticity. (b) A well-known plasticity mechanism is spike-time-dependent plasticity (STDP), leading to weight changes that depend on the relative timing between the pre- and post-synaptic neuronal spike activities. Adapted from Ref. 31.

Biological neurons accumulate charge from all input (connected) neurons and when they reach some input threshold, generate an output signal or spike. This spike is then used to start the process with another neuron up stream from it

Biological neurons also exhibit randomness in their threshold-spiking process.

Emulating spiking neurons, n today’s neural nets, takes computation.  Also randomness takes more.

But with PCM SNN, both the spiking process and its randomness, comes from device physics. Using PCM to create SNN seems a logical progression.

PCM as storage, as memory, as compute or all the above

In the storage business, we look at Optane (see our 3D Xpoint post) SSDs as blazingly fast storage. Intel has also announced that they will use 3D Xpoint in a memory form factor which should provide sadly slower, but larger memory devices.

But using PCM for compute, is a radical departure from the von Neumann computer architectures we know and love today. HPE has been discussing another new computing architecture with their memristor technology, but only in prototype form.

It seems IBM, is also prototyping hardware done this path.

Welcome to the next computing revolution.

Photo & Caption Credit(s): Photo and caption from Figure 2 in AIP Journal of Applied Physics article

Photo and caption from Figure 4 in AIP Journal of Applied Physics article

Photo and caption from Figure 11 in AIP Journal of Applied Physics article

Photo and caption from Figure 12 in AIP Journal of Applied Physics article