Pressure Stall Information (PSI) - Alerts

PURPOSE

This project aims to deliver Pressure Stall Information (PSI) alerts via standard Linux graphical desktop notifications (through libnotify compatible daemons and CLI programs), and email (email-to-SMS is also supported). This can alert the system administrator of CPU, I/O, or Memory (RAM) pressure in near real time.

PREREQUISITES

• A Linux system with kernel 5.2.0 or greater, with the /proc filesystem enabled
• systemd
• zsh
• sysstat (for pidstat)
• ssh (OpenSSH, for desktop notifications)
• psi-by-example (a modified version of this is included in this project as a submodule)
• a libnotify-compatible desktop notification system

History

When I first learned about Pressure Stall Information (PSI), I was intrigued. This provides a real-time view into the performance and typical resource contention Linux system administrators need to worry about: CPU, I/O, and Memory (RAM). During this research, I found this post complete with a C code example; albeit, it was light on I/O details and the example C code the author provided didn't even include Memory pressure at all (so modified it to include Memory pressure).

A quick and dirty description of PSI: whenever one or more processes are waiting for some measurable resource (CPU, I/O, or RAM), the percentage of processes waiting on the resource will begin to increase. Initially, the percentage will be low, but as resource contention increases, more and more processes will be waiting to be processed by the CPU for that resource. If not all processes are waiting on this resource, PSI calls this the "some" contention for resources. If all processes are waiting on the resource, this is known as the "full" resource contention.

The pressure information is exposed in the /proc filesystem in these three virtual files: /proc/pressure/cpu, /proc/pressure/io, /proc/pressure/memory. Each file reports both some and full, and has the following output:

some avg10=0.02 avg60=0.43 avg300=0.55 total=711489361
full avg10=0.02 avg60=0.43 avg300=0.54 total=681874430

This example is taken from /proc/pressure/io, for I/O pressure. The full CPU pressure information really depends on the cgroups, which this project doesn't pay close attention to at this time. The percentages are a measure of the average resource pressure over the last 10s, 60s, and 300s (5 minutes). The total is the number of microseconds that any processes were waiting for the resource; this is a counter that is reset on boot, and will continously update as processes wait for the resource. They always have to wait for the resource, even if it's on the order of hundreds of microseconds or less. Even if the percentages were all zeroes, the total counter will be nonzero (at least for the some metrics), and even the full metrics will have a nonzero total except for CPU, because the full CPU total only really applies to cgroups (and are out of scope for this project at present).

The monitor code (from psi-by-example listed above) only considers the "some" pressure for all three resources, which will usually alert before the system becomes critical (and in the case of full Memory usage/thrashing, completely unusable for any workload). Thus the alerts should come in well before the full resource pressure gets maxed out.

Now, I don't know C very well, but this monitor.c code was easy enough to extend to include memory pressure. However, the create_load.c only creates CPU and I/O load (memory load is too detrimental to system performance).

This was developed on an SSDNodes VPS (Virtual Private Server), which is a KVM virtual machine, backed by SSD hardware. It is very well provisioned with virtual hardware: 8 vCPUs, 32GiB RAM, and 640GiB SSD disk space. Currently, there is very little load on this system, even with four different websites on it, with corresponding database engines, and an nginx reverse proxy. I plan on putting mailcow-dockerized on this VPS soon, which has the potential to increase the load significantly.

Now, once the regular workload of this VPS increases, my current configuration may become too noisy. However, I've tried to configure psi-alerts.sh in such a way that it only alerts once when the pressure on a resource increases, and won't alert again until that pressure subsides (and the some percentages drop below the configurable threshold for at least five minutes).

TODO

• finish INSTALL section
  • figure out how we're actually going to build all submodules
  • and install them
• finish CONFIGURE section
  • about defining an instance and editing it
    • sudo systemctl edit psi-alerts@<user>.service
    • mainly for Environment= variables
  • consider reworking this for a user service, not a system service
    • this could make desktop notifications simpler, and not having to use SSH keys without passphrases
    • need to become much more familiar with user services
• consider reworking all code in a compiled language (other than C)
  • time to learn Go
  • or continue learning Rust
  • need to know how to use kernel syscalls in these languages (if possible)
  • also, convert psi-alerts.sh script to either of these languages