189 lines
6.6 KiB
ReStructuredText
189 lines
6.6 KiB
ReStructuredText
.. _psi:
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================================
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PSI - Pressure Stall Information
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================================
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:Date: April, 2018
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:Author: Johannes Weiner <hannes@cmpxchg.org>
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When CPU, memory or IO devices are contended, workloads experience
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latency spikes, throughput losses, and run the risk of OOM kills.
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Without an accurate measure of such contention, users are forced to
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either play it safe and under-utilize their hardware resources, or
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roll the dice and frequently suffer the disruptions resulting from
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excessive overcommit.
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The psi feature identifies and quantifies the disruptions caused by
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such resource crunches and the time impact it has on complex workloads
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or even entire systems.
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Having an accurate measure of productivity losses caused by resource
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scarcity aids users in sizing workloads to hardware--or provisioning
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hardware according to workload demand.
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As psi aggregates this information in realtime, systems can be managed
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dynamically using techniques such as load shedding, migrating jobs to
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other systems or data centers, or strategically pausing or killing low
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priority or restartable batch jobs.
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This allows maximizing hardware utilization without sacrificing
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workload health or risking major disruptions such as OOM kills.
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Pressure interface
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==================
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Pressure information for each resource is exported through the
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respective file in /proc/pressure/ -- cpu, memory, and io.
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The format is as such::
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some avg10=0.00 avg60=0.00 avg300=0.00 total=0
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full avg10=0.00 avg60=0.00 avg300=0.00 total=0
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The "some" line indicates the share of time in which at least some
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tasks are stalled on a given resource.
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The "full" line indicates the share of time in which all non-idle
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tasks are stalled on a given resource simultaneously. In this state
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actual CPU cycles are going to waste, and a workload that spends
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extended time in this state is considered to be thrashing. This has
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severe impact on performance, and it's useful to distinguish this
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situation from a state where some tasks are stalled but the CPU is
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still doing productive work. As such, time spent in this subset of the
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stall state is tracked separately and exported in the "full" averages.
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CPU full is undefined at the system level, but has been reported
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since 5.13, so it is set to zero for backward compatibility.
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The ratios (in %) are tracked as recent trends over ten, sixty, and
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three hundred second windows, which gives insight into short term events
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as well as medium and long term trends. The total absolute stall time
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(in us) is tracked and exported as well, to allow detection of latency
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spikes which wouldn't necessarily make a dent in the time averages,
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or to average trends over custom time frames.
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Monitoring for pressure thresholds
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==================================
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Users can register triggers and use poll() to be woken up when resource
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pressure exceeds certain thresholds.
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A trigger describes the maximum cumulative stall time over a specific
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time window, e.g. 100ms of total stall time within any 500ms window to
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generate a wakeup event.
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To register a trigger user has to open psi interface file under
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/proc/pressure/ representing the resource to be monitored and write the
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desired threshold and time window. The open file descriptor should be
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used to wait for trigger events using select(), poll() or epoll().
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The following format is used::
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<some|full> <stall amount in us> <time window in us>
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For example writing "some 150000 1000000" into /proc/pressure/memory
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would add 150ms threshold for partial memory stall measured within
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1sec time window. Writing "full 50000 1000000" into /proc/pressure/io
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would add 50ms threshold for full io stall measured within 1sec time window.
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Triggers can be set on more than one psi metric and more than one trigger
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for the same psi metric can be specified. However for each trigger a separate
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file descriptor is required to be able to poll it separately from others,
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therefore for each trigger a separate open() syscall should be made even
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when opening the same psi interface file. Write operations to a file descriptor
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with an already existing psi trigger will fail with EBUSY.
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Monitors activate only when system enters stall state for the monitored
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psi metric and deactivates upon exit from the stall state. While system is
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in the stall state psi signal growth is monitored at a rate of 10 times per
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tracking window.
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The kernel accepts window sizes ranging from 500ms to 10s, therefore min
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monitoring update interval is 50ms and max is 1s. Min limit is set to
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prevent overly frequent polling. Max limit is chosen as a high enough number
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after which monitors are most likely not needed and psi averages can be used
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instead.
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Unprivileged users can also create monitors, with the only limitation that the
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window size must be a multiple of 2s, in order to prevent excessive resource
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usage.
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When activated, psi monitor stays active for at least the duration of one
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tracking window to avoid repeated activations/deactivations when system is
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bouncing in and out of the stall state.
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Notifications to the userspace are rate-limited to one per tracking window.
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The trigger will de-register when the file descriptor used to define the
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trigger is closed.
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Userspace monitor usage example
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===============================
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::
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#include <errno.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <poll.h>
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#include <string.h>
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#include <unistd.h>
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/*
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* Monitor memory partial stall with 1s tracking window size
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* and 150ms threshold.
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*/
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int main() {
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const char trig[] = "some 150000 1000000";
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struct pollfd fds;
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int n;
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fds.fd = open("/proc/pressure/memory", O_RDWR | O_NONBLOCK);
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if (fds.fd < 0) {
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printf("/proc/pressure/memory open error: %s\n",
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strerror(errno));
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return 1;
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}
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fds.events = POLLPRI;
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if (write(fds.fd, trig, strlen(trig) + 1) < 0) {
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printf("/proc/pressure/memory write error: %s\n",
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strerror(errno));
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return 1;
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}
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printf("waiting for events...\n");
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while (1) {
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n = poll(&fds, 1, -1);
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if (n < 0) {
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printf("poll error: %s\n", strerror(errno));
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return 1;
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}
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if (fds.revents & POLLERR) {
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printf("got POLLERR, event source is gone\n");
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return 0;
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}
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if (fds.revents & POLLPRI) {
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printf("event triggered!\n");
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} else {
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printf("unknown event received: 0x%x\n", fds.revents);
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return 1;
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}
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}
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return 0;
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}
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Cgroup2 interface
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=================
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In a system with a CONFIG_CGROUPS=y kernel and the cgroup2 filesystem
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mounted, pressure stall information is also tracked for tasks grouped
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into cgroups. Each subdirectory in the cgroupfs mountpoint contains
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cpu.pressure, memory.pressure, and io.pressure files; the format is
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the same as the /proc/pressure/ files.
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Per-cgroup psi monitors can be specified and used the same way as
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system-wide ones.
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