A Case for Protecting Huge Pages from the Kernel

Controlling memory fragmentation is critical for leveraging the benefits of huge page support offered by modern architectures. The division of free memory into non-contiguous regions over time restricts huge page allocations in long run. Compaction is a popular mechanism to recover contiguous blocks of free memory on demand. However, its success rate of accumulating free regions at huge page granularity (e.g., 2MB in x86_64) is low in most situations due to the presence of unmovable kernel memory. Hence, a prudent page placement algorithm is required to control fragmentation and protect huge pages against kernel memory allocations, in order to sustain system performance over long periods of time.

In this work, we explore the interaction of kernel pages with fragmentation avoidance and recovery mechanism in Linux. Our analysis shows that stock kernel memory layout thwarts the progress of memory compaction. Furthermore, compaction can potentially induce more fragmentation depending on where kernel memory was placed by the underlying page allocator. We discuss the scope of optimization in current Linux framework and show how an effective fragmentation management can yield up to 20% performance improvement and up to 27% energy savings with the help of additional huge pages.