Impact of Increased Cache Misses on Runtime Performance of MPX-enabled Programs
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Low level languages like C and C++ provide high performance and direct control over memory management. But these languages are prone to memory safety violations. Intel introduced a new ISA extension-Memory Protection Extension(MPX), a hardware-assisted full-stack solution, to protect against the memory safety violations. While MPX efficiently prevents memory errors like buffer overflows and out of bound memory accesses, it comes at the cost of high performance overheads. Also, the cache locality worsens in MPX protected applications. In our research, we analyze if there is a correlation between increase in cache misses and runtime degradation in programs compiled with MPX support. We analyze 15 SPEC CPU benchmark programs for different input sizes on Windows platform, compiled with Intel's ICC compiler. We find that for input sizes train(medium) and ref(large), the average performance overheads are 140% and 144% respectively. We find that 5 out of 15 benchmarks do not have any runtime overheads and also, do not have any change in cache misses at any level. However for rest of the 10 benchmarks, we find a strong correlation between runtime overheads and cache misses overheads, with the correlation coefficients ranging from 0.8 to 0.36 for different input sizes. Based on our findings, we conclude that there is a direct correlation between runtime overheads and increase in cache misses. We also find that instructions overheads and runtime overheads have a positive correlation, with the coefficient values ranging from 0.7 to 0.33 for different input sizes.
General Audience Abstract
Low level programming languages like C and C++ are primary choices to write low-level systems software such as operating systems, virtual machines, embedded software, and performance-critical applications. But these languages are considered as unsafe and prone to memory safety errors. Intel introduced a new technique- Memory Protection Extensions (MPX) to protect against these memory errors. But prior research found that applications supported with MPX have increased runtimes (slowdowns). In our research, we analyze these slowdowns for different input sizes(medium and large) in 15 benchmark applications. Based on the input sizes, the average slowdowns range from 140% to 144%. We then examine if there is a correlation between increase in cache misses under MPX and the slowdowns. A hardware cache is a component that stores data so that future requests for that data can be served faster. Hence, cache miss is a state where the data requested for processing by a component or application is not found in the cache. Whenever a cache miss happen, the processor waits for the data to be fetched from the next cache level or from main memory before it can continue to execute. This wait influences the runtime performance of the application. Our evaluations find that 10 out of 15 applications which have increased runtimes, also have increase in cache misses. This shows a positive correlation between these two parameters. Along with that, we also found that increase in instruction size in MPX protected applications also has a direct correlation with the runtime degradation. We also quantify these relationships with a statistical measure called correlation coefficient.
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