3.1 Tracking Active Nodes

Since each node's power level can be separately controlled, total memory power can be reduced by selectively setting nodes to operate at lower power levels. However, selecting which nodes to put into lower power modes is critical to both system performance and power dissipation, since accessing a node in a low-power mode will incur resynchronization costs, stall execution, and, as a result, may increase energy consumption, offsetting any prior savings.

To avoid such costs, we need to ensure that all the nodes a process may access, i.e., its active nodes, are kept in high-power state. More specifically, we define a node to be an active node of process $i$ if and only if at least one page from the node is mapped into process $i$'s address space, and we denote the set of active nodes for $i$ as $\alpha_i$. By promoting the nodes in $\alpha_i$ to Standby mode (high-power) and demoting all other nodes (i.e., those in $\overline{\alpha}_i$) to Nap mode (low-power) when process $i$ is executing, we can reduce power while ensuring that process $i$ suffers no performance degradation due to the increased latency of nodes in low-power states.

Of course, this assumes that $\alpha_i$ can be managed to accurately reflect the active nodes for process $i$. Previous related research [4] used repeated page faults and page table scans to track the active set, but this involves very expensive, high overhead operations. To track the active set with minimal overheads, for each process we keep an array of counters, each of which is associated with a node in the system. The kernel is modified such that on all possible execution paths in which a page is allocated for, or mapped into, process $i$'s address space, the counter associated with the node containing this page is incremented. Similarly, when a page is unmapped, the counter is decremented. From these counters, $\alpha_i$ is trivially derived: a node is in $\alpha_i$ if and only if process $i$'s counter for the node is greater than zero. The overhead of maintaining $\alpha$ is only one extra instruction per mapping/unmapping operation, and is therefore negligible.