In-Line Interrupt Handling for Software-Managed TLBs

The general-purpose precise interrupt mechanism, which has long been used to handle exceptional conditions that occur infrequently, is now being used increasingly often to handle conditions that are neither exceptional nor infrequent. One example is the use of interrupts to perform memory management-e.g., to handle translation lookaside buffer (TLB) misses in today’s microprocessors. Because the frequency of TLB misses tends to increase with memory footprint, there is pressure on the precise interrupt mechanism to become more lightweight. When modem out-of order processors handle interrupts precisely, they typically begin by flushing the pipeline. Doing so makes the CPU available to execute handler instructions, but it wastes potentially hundreds of cycles of execution time. However; if the handler code is small, it could potentially j t in the reorder buffer along with the user-level code already there. This essentially in-lines the interrupt-handler code. One good example of where this would be both possible and useful is in the TLB-miss handler in a software-managed TLB implementation. The benejts of doing so are two-fold: ( 1 ) the instructions that would otherwise be flushed from the pipe need not be re-fetched and re-executed; and ( 2 ) any instructions that are independent of the exceptional instruction can continue to execute in parallel with the handler code. In effect, doing so provides us with lockup-free TLBs. We simulate a lockup-free data-TLB facility on a processor model with a 4-way out-of-order core reminiscent of the Alpha 21264. We find that, by using lockup-jree TLBs, one can get the performance of a fully associative TLB with a lockup-free TLB of one-fourth the size.