OS Tuning
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:
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.
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.
Other options beside a generic performance governor can be set, such as the Performance governor and Powersave governor:
--governor , -g
The governor defines the power characteristics of the system CPU, which in turn affects CPU performance. Each governor has its own unique behavior, purpose, and suitability in terms of workload.
On many Linux systems one can set the governor for all CPUs through the cpupower utility with following commands:
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.
Transparent Hugepages (THP)
THP is an abstraction layer that automates most aspects of creating, managing,and using huge pages. It is designed to hide much of the complexity in using huge pages from system administrators and developers. Huge pages increase the memory page size from 4 kilobytes to 2 megabytes. This provides 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.
THP usage is controlled by the sysfs setting /sys/kernel/mm/transparent_hugepage/enabled.
Possible values:
THP creation is controlled by the sysfs setting /sys/kernel/mm/transparent_hugepage/defrag.
Possible values:
An application that "always" requests THP often can benefit from waiting for an allocation until those huge pages can be assembled.
For more information see the Linux transparent hugepage documentation.
Hyper-Threading [ALL](Default = Enable):
Enables Hyper Threading (Software Method to Enable/Disable Logical Processor threads.)
SNC (Sub NUMA) (Default = Disabled):
SNC breaks up the LLC into disjointed clusters based on address range, with each cluster bound to a subset of the memory controllers in the system. It improves average latency to the LLCSNC disable will support 1-cluster (XPT/KTI Prefetch enable) 4-IMC way interleave. SNC2 Enable supports 2-clusters SNC and 2-way IMC interleave. Enable SNC2 will gray out iMC_Interleave knob and UmaBasedClustering knob.
LLC dead line alloc (Default = Enabled):
In the Skylake cache scheme, mid-level cache (MLC) evictions are filled into the last level cache (LLC). If a line is evicted from the MLC to the LLC, the Skylake core can flag the evicted MLC lines as "dead". This means that the lines are not likely to be read again. This option allows dead lines to be dropped and never fill the LLC if the option is disabled. Values for this BIOS option can be:
Patrol Scrub (Default = Enabled):
This option allows for correction of soft memory errors. Over the length of system runtime, the risk of producing multi-bit and uncorrected errors is reduced with this option. Values for this BIOS setting can be:
Xtended Prediciton Table (XPT) Prefetch (Default = Enabled):
This option configures the processor Xtended Prediciton Table (XPT) prefetch feature. The XPT prefetcher exists on top of other prefetchers that that can prefetch data in the core DCU, MLC, and LLC. The XPT prefetcher will issue a speculative DRAM read request in parallel to an LLC lookup. This prefetch bypasses the LLC, saving latency. In some cases, setting this option to disabled can improve performance. In some cases, setting this option to disabled can improve performance. Typically, setting this option to enable provides better performance. This option must be enabled when Sub-NUMA Clustering is enabled. Values for this BIOS option can be:
Energy/Performance Bias (Default = Balanced Performance):
Use input from ENERGY_PERF_BIAS_CONFIG mode selection. PERF/Balanced Perf/Balanced Power/Power
Intel VT for Directed I/O (Default = Enable):
Enable/Disable Intel Virtualization Technology for Directed I/O (VT-d) by reporting the I/O device assignment to VMM through DMAR ACPI Tables.
Last updated March 18, 2021.