ulimit:

Used to set user limits of system-wide resources. Provides control over resources available to the shell and processes started by it. Some common ulimit commands may include:

• ulimit -s [n | unlimited]: Set the stack size to n kbytes, or unlimited to allow the stack size to grow without limit.
• ulimit -l (number): Set the maximum size that can be locked into memory.

Performance Governors (Linux):

In-kernel CPU frequency governors are pre-configured power schemes for the CPU. The CPUfreq governors use P-states to change frequencies and lower power consumption. The dynamic governors can switch between CPU frequencies, based on CPU utilization to allow for power savings while not sacrificing performance.

To set the governor, use the following commmand: "cpupower frequency-set -r -g {desired_governor}"

Disabling Linux services:

Certain Linux services may be disabled to minimize tasks that may consume CPU cycles.

irqbalance:

Disabled through "service irqbalance stop". Depending on the workload involved, the irqbalance service reassigns various IRQ's to system CPUs. Though this service might help in some situations, disabling it can also help environments which need to minimize or eliminate latency to more quickly respond to events.

Tuning Kernel parameters:

The following Linux Kernel parameters were tuned to better optimize performance of some areas of the system:

• dirty_background_ratio: Set through "echo 40 > /proc/sys/vm/dirty_background_ratio". This setting can help Linux disk caching and performance by setting the percentage of system memory that can be filled with dirty pages.
• dirty_ratio: Set through "echo 8 > /proc/sys/vm/dirty_ratio". This setting is the absolute maximum amount of system memory that can be filled with dirty pages before everything must get committed to disk.
• ksm/sleep_millisecs: Set through "echo 200 > /sys/kernel/mm/ksm/sleep_millisecs". This setting controls how many milliseconds the ksmd (KSM daeomn) should sleep before the next scan.
• khugepaged/scan_sleep_millisecs: Set through "echo 50000 > /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs". This setting controls how many milliseconds to wait in khugepaged is there is a hugepage allocation failure to throttle the next allocation attempt.
• swappiness: The swappiness value can range from 1 to 100. A value of 100 will cause the kernel to swap out inactive processes frequently in favor of file system performance, resulting in large disk cache sizes. A value of 1 tells the kernel to only swap processes to disk if absolutely necessary. This can be set through a command like "echo 1 > /proc/sys/vm/swappiness"
• numa_balancing: Disabled through "echo 0 > /proc/sys/kernel/numa_balancing". This feature will automatically migrate data on demand so memory nodes are aligned to the local CPU that is accessing data. Depending on the workload involved, enabling this can boost the performance if the workload performs well on NUMA hardware. If the workload is statically set to balance between nodes, then this service may not provide a benefit.
• Zone Reclaim Mode: Zone reclaim allows the reclaiming of pages from a zone if the number of free pages falls below a watermark even if other zones still have enough pages available. Reclaiming a page can be more beneficial than taking the performance penalties that are associated with allocating a page on a remote zone, especially for NUMA machines. To tell the kernel to free local node memory rather than grabbing free memory from remote nodes, use a command like "echo 1 > /proc/sys/vm/zone_reclaim_mode"

tuned-adm:

The tuned-adm tool is a commandline interface for switching between different tuning profiles available to the tuned tuning daeomn available in supported Linux distros. The default configuration file is located in /etc/tuned.conf and the supported profiles can be found in /etc/tune-profiles.

Some profiles that may be available by default include: default, desktop-powersave, server-powersave, laptop-ac-powersave, laptop-battery-powersave, spindown-disk, throughput-performance, latency-performance, enterprise-storage

To set a profile, one can issue the command "tuned-adm profile (profile_name)". Here are details about relevant profiles.

• throughput-performance: Server profile for typical throughput tuning. This profile disables tuned and ktune power saving features, enables sysctl settings that may improve disk and network IO throughphut performance, switches to the deadline scheduler, and sets the CPU governor to performance.
• latency-performance: Server profile for typical latency tuning. This profile disables tuned and ktune power saving features, enables the deadline IO scheduler, and sets the CPU governor to performance.
• enterprise-storage: Server profile to high disk throughput tuning. This profile disables tuned and ktune power saving features, enables the deadline IO scheduler, enables hugepages and disables disk barriers, increases disk readahead values, and sets the CPU governor to performance

Transparent Huge Pages (THP):

THP is an abstraction layer that automates most aspects of creating, managing, and using huge pages. THP is designed to hide much of the complexity in using huge pages from system administrators and developers, as normal huge pages must be assigned at boot time, can be difficult to manage manually, and often require significant changes to code in order to be used effectively. Transparent Hugepages increase the memory page size from 4 kilobytes to 2 megabytes. Transparent Hugepages provide significant performance advantages on systems with highly contended resources and large memory workloads. If memory utilization is too high or memory is badly fragmented which prevents hugepages being allocated, the kernel will assign smaller 4k pages instead. Most recent Linux OS releases have THP enabled by default.

Linux Huge Page settings:

If you need finer control and manually set the Huge Pages you can follow the below steps:

• Create a mount point for the huge pages: "mkdir /mnt/hugepages"
• The huge page file system needs to be mounted when the systems reboots. Add the following to a system boot configuration file before any services are started: "mount -t hugetlbfs nodev /mnt/hugepages"
• Set vm/nr_hugepages=N in your /etc/sysctl.conf file where N is the maximum number of pages the system may allocate.
• Reboot to have the changes take effect.

Note that further information about huge pages may be found in your Linux documentation file: /usr/src/linux/Documentation/vm/hugetlbpage.txt

Environment Variables:

GOMP_CPU_AFFINITY:

Used to bind threads to specific CPUs. The variable should contain a space-separated or comma-separated list of CPUs. This list may contain different kinds of entries: either single CPU numbers in any order, a range of CPUs (M-N) or a range with some stride (M-N:S). CPU numbers are zero based. For example, GOMP_CPU_AFFINITY="0 3 1-2 4-15:2" will bind the initial thread to CPU 0, the second to CPU 3, the third to CPU 1, the fourth to CPU 2, the fifth to CPU 4, the sixth through tenth to CPUs 6, 8, 10, 12, and 14 respectively and then start assigning back from the beginning of the list. GOMP_CPU_AFFINITY=0 binds all threads to CPU 0. There is no libgomp library routine to determine whether a CPU affinity specification is in effect. As a workaround, language-specific library functions, e.g., getenv in C or GET_ENVIRONMENT_VARIABLE in Fortran, may be used to query the setting of the GOMP_CPU_AFFINITY environment variable. A defined CPU affinity on startup cannot be changed or disabled during the runtime of the application. If both GOMP_CPU_AFFINITY and OMP_PROC_BIND are set, OMP_PROC_BIND has a higher precedence. If neither has been set and OMP_PROC_BIND is unset, or when OMP_PROC_BIND is set to FALSE, the host system will handle the assignment of threads to CPUs.

OMP_DYNAMIC:

Dynamic adjustment of threads. Enable or disable the dynamic adjustment of the number of threads within a team. The value of this environment variable shall be TRUE or FALSE. If undefined, dynamic adjustment is disabled by default.

OMP_SCHEDULE:

How threads are scheduled. Allows to specify schedule type and chunk size. The value of the variable shall have the form: type[,chunk] where type is one of static, dynamic or guided. The optional chunk size shall be a positive integer. If undefined, dynamic scheduling and a chunk size of 1 is used.

OMP_THREAD_LIMIT:

Set the maximum number of threads. Specifies the number of threads to use for the whole program. The value of this variable shall be a positive integer. If undefined, the number of threads is not limited.

MALLOC_CONF:

This environment variable affects the execution of the allocation functions. If the environment variable MALLOC_CONF is set, the characters it contains will be interpreted as options.