Chapter 35. Configuring an operating system to optimize memory access

Pages added in this category represent parts of files in permanent storage. These pages, from the page cache, can be mapped or unmapped in an application’s address spaces. You can use applications to map files into their address space using the mmap system calls, or to operate on files via the buffered I/O read or write system calls.

Buffered I/O system calls, as well as applications that map pages directly, can re-utilize unmapped pages. As a result, these pages are stored in the cache by the kernel, especially when the system is not running any memory intensive tasks, to avoid re-issuing costly I/O operations over the same set of pages.

Defines the conditions that determine whether a large memory request is accepted or denied.The default value is 0.

By default, the kernel performs checks if a virtual memory allocation request fits into the present amount of memory (total + swap) and rejects only large requests. Otherwise virtual memory allocations are granted, and this means they allow memory overcommitment.

Setting the overcommit_memory parameter’s value:

Sets the size of the reserved free pages pool. It is also responsible for setting the min_page, low_page, and high_page thresholds that govern the behavior of the Linux kernel’s page reclaim algorithms. It also specifies the minimum number of kilobytes to keep free across the system. This calculates a specific value for each low memory zone, each of which is assigned a number of reserved free pages in proportion to their size.

Setting the vm.min_free_kbytes parameter’s value:

In the event that a system runs out of memory, and the panic_on_oom parameter is set to 0, the oom_killer function kills processes, starting with the process that has the highest oom_score, until the system recovers.

The oom_adj parameter determines the oom_score of a process. This parameter is set per process identifier. A value of -17 disables the oom_killer for that process. Other valid values range from -16 to 15.

The swappiness value, ranging from 0 to 200, controls the degree to which the system favors reclaiming memory from the anonymous memory pool, or the page cache memory pool.

Setting the swappiness parameter’s value:

Determines the maximum number of file handles for the entire system. The default value on Red Hat Enterprise Linux 8 is either 8192 or one tenth of the free memory pages available at the time the kernel starts, whichever is higher.

Raising this value can resolve errors caused by a lack of available file handles.