Thursday, July 29, 2010

Retroactive Introduction to my GSOC project

I've been busy this summer, between my GSOC project, my academic progress, and life in general. I thought I should catalog some of the work that I've done for the GSOC 2010. I have previously talked about my work to port RTEMS to SPARC V9. Well, I had a GSOC project accepted for the RTEMS project which I have been working on this summer. The remainder of this blog post is excerpted from my project proposal and describes what I have been doing lately. The full project proposal can be read online at:

My project is to refactor the RTEMS task scheduler to make it more modular. The current RTEMS scheduler is tightly coupled with the thread management code. This tight coupling prevents easy implementation and testing of alternate scheduling mechanisms. Refactoring the scheduler into a new SuperCore Scheduler Manager enables RTEMS developers (namely, me) to explore new scheduling mechanisms.

After finishing my project, the RTEMS scheduler internal data structures will be encapsulated in a module that provides a protected interface such that none of the thread management code directly touches scheduling data.

Perhaps the most important outcome of my project will be enabling exploration of scheduling for symmetric multiprocessing (SMP) and multicore platforms. An SMP system is a closely-coupled system with multiple CPU chips interconnected with main memory; a multicore system is even more tightly coupled, with multiple CPUs interconnected on a single chip. Although RTEMS currently supports multiprocessing systems, the support is designed for coarse-grained systems. Each processing element (core) runs a separate RTEMS application and executive, and communication is routed through a portion of shared memory. Scheduling decisions are made locally at each core, and tasks are not able to migrate between cores.

Although my project will not provide SMP support, the goal of refactoring the scheduler is to facilitate implementing an SMP scheduler, so care will be taken to make sure the interface does not enforce any policies that might deter an SMP scheduler. By supporting global scheduling decisions and task migration, RTEMS will be able to balance a processing load across multiple CPUs. Further, unifying the executive across all the cores in the system allows RTEMS to make smarter decisions and to reduce the effort of end users in deploying SMP and multicore configurations.

Friday, July 9, 2010

Understanding Energy and Power

Lately I've been looking at power as an evaluation metric for my research. Power consumption has always been an important design concern in embedded and resource constrained devices. Recently, power has also become important in desktop and server computing environments at the chip-level and at the rack-level respectively.

Energy and power are related, and the two are so often used synonymously that confusing them is quite easy.

Power, typically measured in (kilo)watts, is a rate of production or consumption of energy. The watt unit is an expression of coulombs per second, where a coulomb is a unit of electric charge. So power is the rate of change of electric charge over a period of time. Energy is typically measured in (kilo)watt-hours, which is what shows up on your "power" bill. So you pay for the total amount of energy consumed during the period of your bill, which is actually power integrated over that interval.

A common analogy to help understand the relationship between power and energy is that power is to a flow of water as energy is to a pool of water. The flow can be very slow, dripping even, but can fill a pool given enough time; or the flow can be very fast and fill a pool even quicker. The amount of water in the pool is analogous to energy, and the rate of the flow is analogous to power.

I find it is also helpful to consider the physical equations.

To understand power and energy, think back to middle or high school and recall Ohm's law, I = V / R (equivalently V = I * R), where I is current, V is voltage, and R is resistance. Current is the movement of electric particles, measured in amperes. Voltage is the force required to drive a current between two points, measured in volts. Resistance is opposition to current, measured in ohms. Note that current and voltage are only non-zero if there is a mismatch in the electrical potential between two connected points, and that bad things happen as resistance approaches zero.

Ohm's law is about as far as most people recall (if even that far!), and we haven't yet reached energy or power. We can get some interesting equations by substituting Ohm's law into Joule's first law after some massaging, P = I^2 * R = V * I = V^2 / R, where P is the power dissipated by a resistor. Power dissipation is a more accurate term for the notion of power consumption, although the two can be used interchangeably to mean the conversion of power into some other form of transfer of energy, for example heat or sound. The power dissipation of the resistive elements of a circuit is equivalent to the instantaneous power applied to the circuit in order to generate current through it. By considering a current applied from time 0 to t to a circuit with resistance R, the electric energy created by the current passing through the resistive elements of the circuit during that time interval is E = P * t.

The moral of the story is that power is the rate of transfer of electrical charge, and that energy is an accumulation of electrical charge.