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PSI - Pressure Stall Information
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: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.