Documentation/accounting/psi.txt - linux - Git at Google

 ================================
 PSI - Pressure Stall Information
 ================================

 :Date: April, 2018
 :Author: Johannes Weiner <hannes@cmpxchg.org>

 When CPU, memory or IO devices are contended, workloads experience
 latency spikes, throughput losses, and run the risk of OOM kills.

 Without an accurate measure of such contention, users are forced to
 either play it safe and under-utilize their hardware resources, or
 roll the dice and frequently suffer the disruptions resulting from
 excessive overcommit.

 The psi feature identifies and quantifies the disruptions caused by
 such resource crunches and the time impact it has on complex workloads
 or even entire systems.

 Having an accurate measure of productivity losses caused by resource
 scarcity aids users in sizing workloads to hardware--or provisioning
 hardware according to workload demand.

 As psi aggregates this information in realtime, systems can be managed
 dynamically using techniques such as load shedding, migrating jobs to
 other systems or data centers, or strategically pausing or killing low
 priority or restartable batch jobs.

 This allows maximizing hardware utilization without sacrificing
 workload health or risking major disruptions such as OOM kills.

 Pressure interface
 ==================

 Pressure information for each resource is exported through the
 respective file in /proc/pressure/ -- cpu, memory, and io.

 The format for CPU is as such:

 some avg10=0.00 avg60=0.00 avg300=0.00 total=0

 and for memory and IO:

 some avg10=0.00 avg60=0.00 avg300=0.00 total=0
 full avg10=0.00 avg60=0.00 avg300=0.00 total=0

 The "some" line indicates the share of time in which at least some
 tasks are stalled on a given resource.

 The "full" line indicates the share of time in which all non-idle
 tasks are stalled on a given resource simultaneously. In this state
 actual CPU cycles are going to waste, and a workload that spends
 extended time in this state is considered to be thrashing. This has
 severe impact on performance, and it's useful to distinguish this
 situation from a state where some tasks are stalled but the CPU is
 still doing productive work. As such, time spent in this subset of the
 stall state is tracked separately and exported in the "full" averages.

 The ratios (in %) are tracked as recent trends over ten, sixty, and
 three hundred second windows, which gives insight into short term events
 as well as medium and long term trends. The total absolute stall time
 (in us) is tracked and exported as well, to allow detection of latency
 spikes which wouldn't necessarily make a dent in the time averages,
 or to average trends over custom time frames.

 Cgroup2 interface
 =================

 In a system with a CONFIG_CGROUP=y kernel and the cgroup2 filesystem
 mounted, pressure stall information is also tracked for tasks grouped
 into cgroups. Each subdirectory in the cgroupfs mountpoint contains
 cpu.pressure, memory.pressure, and io.pressure files; the format is
 the same as the /proc/pressure/ files.
	================================
	PSI - Pressure Stall Information
	================================

	:Date: April, 2018
	:Author: Johannes Weiner <hannes@cmpxchg.org>

	When CPU, memory or IO devices are contended, workloads experience
	latency spikes, throughput losses, and run the risk of OOM kills.

	Without an accurate measure of such contention, users are forced to
	either play it safe and under-utilize their hardware resources, or
	roll the dice and frequently suffer the disruptions resulting from
	excessive overcommit.

	The psi feature identifies and quantifies the disruptions caused by
	such resource crunches and the time impact it has on complex workloads
	or even entire systems.

	Having an accurate measure of productivity losses caused by resource
	scarcity aids users in sizing workloads to hardware--or provisioning
	hardware according to workload demand.

	As psi aggregates this information in realtime, systems can be managed
	dynamically using techniques such as load shedding, migrating jobs to
	other systems or data centers, or strategically pausing or killing low
	priority or restartable batch jobs.

	This allows maximizing hardware utilization without sacrificing
	workload health or risking major disruptions such as OOM kills.

	Pressure interface
	==================

	Pressure information for each resource is exported through the
	respective file in /proc/pressure/ -- cpu, memory, and io.

	The format for CPU is as such:

	some avg10=0.00 avg60=0.00 avg300=0.00 total=0

	and for memory and IO:

	some avg10=0.00 avg60=0.00 avg300=0.00 total=0
	full avg10=0.00 avg60=0.00 avg300=0.00 total=0

	The "some" line indicates the share of time in which at least some
	tasks are stalled on a given resource.

	The "full" line indicates the share of time in which all non-idle
	tasks are stalled on a given resource simultaneously. In this state
	actual CPU cycles are going to waste, and a workload that spends
	extended time in this state is considered to be thrashing. This has
	severe impact on performance, and it's useful to distinguish this
	situation from a state where some tasks are stalled but the CPU is
	still doing productive work. As such, time spent in this subset of the
	stall state is tracked separately and exported in the "full" averages.

	The ratios (in %) are tracked as recent trends over ten, sixty, and
	three hundred second windows, which gives insight into short term events
	as well as medium and long term trends. The total absolute stall time
	(in us) is tracked and exported as well, to allow detection of latency
	spikes which wouldn't necessarily make a dent in the time averages,
	or to average trends over custom time frames.

	Cgroup2 interface
	=================

	In a system with a CONFIG_CGROUP=y kernel and the cgroup2 filesystem
	mounted, pressure stall information is also tracked for tasks grouped
	into cgroups. Each subdirectory in the cgroupfs mountpoint contains
	cpu.pressure, memory.pressure, and io.pressure files; the format is
	the same as the /proc/pressure/ files.