is not a good predictor of latency. (Figure from [120].)
Similarly, the number of PEs in the hardware and their peak throughput are not sufficient for evaluating the throughput and latency. It is critical to report actual runtime of the DNN models on hardware to account for other effects such as utilization of PEs, as highlighted in Equation (3.2). Ideally, this evaluation should be performed on clearly specified DNN models, for instance those that are part of the MLPerf benchmarking suite. In addition, batch size should be reported in conjunction with the throughput in order to evaluate latency.
3.3 ENERGY EFFICIENCY AND POWER CONSUMPTION
Energy efficiency is used to indicate the amount of data that can be processed or the number of executions of a task that can be completed for a given unit of energy. High energy efficiency is important when processing DNNs at the edge in embedded devices with limited battery capacity (e.g., smartphones, smart sensors, robots, and wearables). Edge processing may be preferred over the cloud for certain applications due to latency, privacy, or communication bandwidth limitations. Energy efficiency is often generically reported as the number of operations per joule. In the case of inference, energy efficiency is reported as inferences per joule or the inverse as energy consumption in terms of joules per inference.
Power consumption is used to indicate the amount of energy consumed per unit time. Increased power consumption results in increased heat dissipation; accordingly, the maximum power consumption is dictated by a design criterion typically called the thermal design power (TDP), which is the power that the cooling system is designed to dissipate. Power consumption is important when processing DNNs in the cloud as data centers have stringent power ceilings due to cooling costs; similarly, handheld and wearable devices also have tight power constraints since the user is often quite sensitive to heat and the form factor of the device limits the cooling mechanisms (e.g., no fans). Power consumption is typically reported in watts or joules per second.
Power consumption in conjunction with energy efficiency limits the throughput as follows: inferences joules inferences
Therefore, if we can improve energy efficiency by increasing the number of inferences per joule, we can increase the number of inferences per second and thus throughput of the system.
There are several factors that affect the energy efficiency. The number of inferences per joule can be decomposed into
where the number of operations per joule is dictated by both the hardware and DNN model, while the number of operations per inference is dictated by the DNN model.
There are various design considerations for the hardware that will affect the energy per operation (i.e., joules per operation). The energy per operation can be broken down into the energy required to move the input and output data, and the energy required to perform the MAC computation
For each component the joules per operation6 is computed as
where C is the total switching capacitance, VDD is the supply voltage, and α is the switching activity, which indicates how often the capacitance is charged.
The energy consumption is dominated by the data movement as the capacitance of data movement tends to be much higher that the capacitance for arithmetic operations such as a MAC (Figure 3.3). Furthermore, the switching capacitance increases the further the data needs to travel to reach the PE, which consists of the distance to get out of the memory where the data is stored and the distance to cross the network between the memory and the PE. Accordingly, larger memories and longer interconnects (e.g., off-chip) tend to consume more energy than smaller and closer memories due to the capacitance of the long wires employed. In order to reduce the energy consumption of data movement, we can exploit data reuse where the data is moved once from distant large memory (e.g., off-chip DRAM) and reused for multiple operations from a local smaller memory (e.g., on-chip buffer or scratchpad within the PE). Optimizing data movement is a major consideration in the design of DNN accelerators; the design of the dataflow, which defines the processing order, to increase data reuse within the memory hierarchy is discussed in Chapter 5. In addition, advanced device and memory technologies can be used to reduce the switching capacitance between compute and memory, as described in Chapter 10.
Figure 3.3: The energy consumption for various arithmetic operations and memory accesses in a 45 nm process. The relative energy cost (computed relative to the 8b add) is shown on a log scale. The energy consumption of data movement (red) is significantly higher than arithmetic operations (blue). (Figure adapted from [121].)
This raises the issue of the appropriate scope over which energy efficiency and power consumption should be reported. Including the entire system (out to the fans and power supplies) is beyond the scope of this book. Conversely, ignoring off-chip memory accesses, which can vary greatly between chip designs, can easily result in a misleading perception of the efficiency of the system. Therefore, it is critical to not only report the energy efficiency and power consumption of the chip, but also the energy efficiency and power consumption of the off-chip memory (e.g., DRAM) or the amount of off-chip accesses (e.g., DRAM accesses) if no specific memory technology is specified; for the latter, it can be reported in terms of the total amount of data that is read and written off-chip per inference.
Reducing the joules per MAC operation itself can be achieved by reducing the switching activity and/or capacitance at a circuit level or micro-architecture level. This can also be achieved by reducing precision (e.g., reducing the bit width of the MAC operation), as shown in Figure 3.3 and discussed in Chapter 7. Note that the impact of reducing precision on accuracy must also be considered.
For instruction-based systems such as CPUs and GPUs, this can also be achieved by reducing instruction bookkeeping overhead. For example, using large aggregate instructions (e.g., single-instruction, multiple-data (SIMD)/Vector Instructions; single-instruction, multiple-threads (SIMT)/Tensor Instructions), a single instruction can be used to initiate multiple operations.
Similar to the throughput metric discussed in Section 3.2, the number of operations per inference depends on the DNN model, however the operations per joules may be a function of the ability of the hardware to exploit sparsity to avoid performing ineffectual MAC operations. Equation (3.9) shows how operations per joule can be decomposed into:
1. the number of effectual operations plus unexploited ineffectual operations per joule, which remains somewhat constant for a given hardware architecture design;
2. the ratio of effectual operations over effectual operations plus unexploited ineffectual operations, which refers to the ability of the hardware to exploit ineffectual operations (ideally unexploited ineffectual operations should be zero, and this ratio should be one); and
3. the number of effectual operations out of (total) operations, which is related to the amount of sparsity and depends on the DNN model.