Performance Benefits From Underutilized CPU
CPU is one of the hardest parts to understand in Computer Systems. I want to talk slightly about Underutilized CPU today, and why I am saying there are potential performance benefits from Underutilized CPU.
Why Underutilized CPU?
Despite processors being capable of high IPC(Instructions Per Clock/Cycle), many programs do not fully utilize the CPU's execution-unit resources. This means that during execution, there are times when the CPU's execution units are not fully busy or are idle, waiting for instructions to process.
Many programs do not fully utilize the CPU's execution-unit resources due to several factors, including the nature of the workload, the design of the program, and limitations imposed by memory access and dependencies. Here are some reasons why this might happen:
Dependency Chains: Some instructions depend on the results of previous instructions. If there are dependencies between instructions, the CPU might have to wait for the results of one instruction before it can execute the next. This can lead to stalls in the pipeline and underutilization of execution resources.
Branching: Conditional branches (if statements, loops, etc.) can disrupt the predicted flow of instructions in the pipeline. If the CPU mispredicts a branch, it might have to flush the pipeline and start fetching and executing instructions from a different path. This can result in performance penalties and underutilization of execution units.
Memory Access Latency: Memory access is often slower compared to the speed of the CPU's execution units. When a program needs to fetch data from memory, the CPU might have to wait for the data to arrive, leading to idle execution units.
Data Dependencies: Some instructions require data that is not yet available, creating data dependencies. Until the data becomes available, the CPU might stall or underutilize its execution units.
Single-Threaded Nature: Many programs are not designed to take full advantage of multiple cores or threads in modern CPUs. They might be written as single-threaded applications, not benefiting from parallelism.
Workload Nature: Some programs have inherently sequential or serial workloads that don't naturally lend themselves to parallel execution. In these cases, it's difficult to fully utilize all execution units.
I/O Operations: Programs that frequently perform I/O operations (disk read/writes, network communication) can spend significant time waiting for data to be read from or written to external sources, leading to the underutilization of execution units.
Algorithm Complexity: Some algorithms inherently have limitations that prevent them from fully utilizing execution units. For example, algorithms with inherently serial steps.
Compiler Optimizations: The compiler's ability to optimize code for parallel execution might be limited, leading to inefficiencies in utilizing execution units.
Resource Sharing: In multi-process or multi-threaded environments, contention for shared resources like memory or locks can lead to underutilization as threads or processes wait for access.
Optimizing program performance to make full use of the CPU's execution units often requires careful consideration of these factors, as well as utilizing techniques like instruction-level parallelism, loop unrolling, pipelining, and more. However, not all programs can be efficiently parallelized due to their inherent characteristics or constraints.
Memory-Safety Techniques
Memory-safety techniques benefit from underutilized CPU and partially mask their performance overhead[1].
Memory-Safety Techniques are mechanisms used to ensure that programs don't access invalid memory regions, such as buffer overflows or use-after-free errors. These techniques often introduce additional instructions and checks, which can lead to increased execution time (performance overhead).
The statement is saying that many programs do not take full advantage of the CPU's ability to execute multiple instructions per cycle (high IPC), which means there are available execution resources that could be utilized. However, because these resources are underutilized, memory-safety techniques can take advantage of these unused execution resources to perform their checks and validations without fully impacting the performance of the program.
In simpler terms, since many programs don't fully occupy the CPU's processing capabilities, there is "spare" processing power that memory-safety techniques can use to perform their checks without causing a significant slowdown. This allows the techniques to add their checks for ensuring memory safety while not fully consuming the available execution resources, partially masking their performance overhead.
In essence, by taking advantage of the CPU's underutilized resources, memory-safety techniques can provide added security without imposing as much of a performance penalty as they would if the CPU was already running at its maximum capability.
Reference
- Oleksenko, O., Kuvaiskii, D., Bhatotia, P., Felber, P., & Fetzer, C. (2018). Intel MPX Explained: A Cross-layer Analysis of the Intel MPX System Stack. Proceedings of the ACM on Measurement and Analysis of Computing Systems, 2(2), 28:1-28:30. [28]. https://doi.org/10.1145/3224423