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Chapter 7. Memory

This chapter outlines the memory management capabilities of Red Hat Enterprise Linux 7. Section 7.1, “Considerations” discusses memory related factors that affect performance. Section 7.2, “Monitoring and Diagnosing Performance Problems” teaches you how to use Red Hat Enterprise Linux 7 tools to diagnose performance problems related to memory utilization or configuration details. Section 7.5, “Configuring System Memory Capacity” and Section 7.3, “Configuring HugeTLB Huge Pages” discuss the tools and strategies you can use to solve memory related performance problems in Red Hat Enterprise Linux 7.

7.1. Considerations

By default, Red Hat Enterprise Linux 7 is optimized for moderate workloads. If your application or workload requires a large amount of memory, changing the way your system manages virtual memory may improve the performance of your application.

7.1.1. Larger Page Size

Physical memory is managed in chunks called pages. On most architectures supported by Red Hat Enterprise Linux 7, the default size of a memory page is 4 KB. This default page size has proved to be suitable for general-purpose operating systems, such as Red Hat Enterprise Linux 7, which support many different kinds of workloads.
However, specific applications can benefit from using larger page sizes in certain cases. For example, an application that works with a large and relatively fixed data set of hundreds of megabytes or even dozens of gigabytes can have performance issues when using 4 KB pages. Such data sets can require hundreds of thousands of 4 KB pages, which can lead to overhead in the operating system and the CPU.
Red Hat Enterprise Linux 7 enables the use of larger page sizes for applications working with big data sets. Using larger page sizes can improve the performance of such applications.
Two different large page features are available in Red Hat Enterprise Linux 7: the HugeTLB feature, also called static huge pages in this guide, and the Transparent Huge Page feature.

7.1.2. Translation Lookaside Buffer Size

Reading address mappings from the page table is time-consuming and resource-expensive, so CPUs are built with a cache for recently-used addresses: the Translation Lookaside Buffer (TLB). However, the default TLB can only cache a certain number of address mappings. If a requested address mapping is not in the TLB (that is, the TLB is missed), the system still needs to read the page table to determine the physical to virtual address mapping.
Because of the relationship between application memory requirements and the size of pages used to cache address mappings, applications with large memory requirements are more likely to suffer performance degradation from TLB misses than applications with minimal memory requirements. It is therefore important to avoid TLB misses wherever possible.
Both HugeTLB and Transparent Huge Page features allow applications to use pages larger than 4 KB. This allows addresses stored in the TLB to reference more memory, which reduces TLB misses and improves application performance.