1.. _psi: 2 3================================ 4PSI - Pressure Stall Information 5================================ 6 7:Date: April, 2018 8:Author: Johannes Weiner <hannes@cmpxchg.org> 9 10When CPU, memory or IO devices are contended, workloads experience 11latency spikes, throughput losses, and run the risk of OOM kills. 12 13Without an accurate measure of such contention, users are forced to 14either play it safe and under-utilize their hardware resources, or 15roll the dice and frequently suffer the disruptions resulting from 16excessive overcommit. 17 18The psi feature identifies and quantifies the disruptions caused by 19such resource crunches and the time impact it has on complex workloads 20or even entire systems. 21 22Having an accurate measure of productivity losses caused by resource 23scarcity aids users in sizing workloads to hardware--or provisioning 24hardware according to workload demand. 25 26As psi aggregates this information in realtime, systems can be managed 27dynamically using techniques such as load shedding, migrating jobs to 28other systems or data centers, or strategically pausing or killing low 29priority or restartable batch jobs. 30 31This allows maximizing hardware utilization without sacrificing 32workload health or risking major disruptions such as OOM kills. 33 34Pressure interface 35================== 36 37Pressure information for each resource is exported through the 38respective file in /proc/pressure/ -- cpu, memory, and io. 39 40The format for CPU is as such:: 41 42 some avg10=0.00 avg60=0.00 avg300=0.00 total=0 43 44and for memory and IO:: 45 46 some avg10=0.00 avg60=0.00 avg300=0.00 total=0 47 full avg10=0.00 avg60=0.00 avg300=0.00 total=0 48 49The "some" line indicates the share of time in which at least some 50tasks are stalled on a given resource. 51 52The "full" line indicates the share of time in which all non-idle 53tasks are stalled on a given resource simultaneously. In this state 54actual CPU cycles are going to waste, and a workload that spends 55extended time in this state is considered to be thrashing. This has 56severe impact on performance, and it's useful to distinguish this 57situation from a state where some tasks are stalled but the CPU is 58still doing productive work. As such, time spent in this subset of the 59stall state is tracked separately and exported in the "full" averages. 60 61The ratios (in %) are tracked as recent trends over ten, sixty, and 62three hundred second windows, which gives insight into short term events 63as well as medium and long term trends. The total absolute stall time 64(in us) is tracked and exported as well, to allow detection of latency 65spikes which wouldn't necessarily make a dent in the time averages, 66or to average trends over custom time frames. 67 68Monitoring for pressure thresholds 69================================== 70 71Users can register triggers and use poll() to be woken up when resource 72pressure exceeds certain thresholds. 73 74A trigger describes the maximum cumulative stall time over a specific 75time window, e.g. 100ms of total stall time within any 500ms window to 76generate a wakeup event. 77 78To register a trigger user has to open psi interface file under 79/proc/pressure/ representing the resource to be monitored and write the 80desired threshold and time window. The open file descriptor should be 81used to wait for trigger events using select(), poll() or epoll(). 82The following format is used:: 83 84 <some|full> <stall amount in us> <time window in us> 85 86For example writing "some 150000 1000000" into /proc/pressure/memory 87would add 150ms threshold for partial memory stall measured within 881sec time window. Writing "full 50000 1000000" into /proc/pressure/io 89would add 50ms threshold for full io stall measured within 1sec time window. 90 91Triggers can be set on more than one psi metric and more than one trigger 92for the same psi metric can be specified. However for each trigger a separate 93file descriptor is required to be able to poll it separately from others, 94therefore for each trigger a separate open() syscall should be made even 95when opening the same psi interface file. 96 97Monitors activate only when system enters stall state for the monitored 98psi metric and deactivates upon exit from the stall state. While system is 99in the stall state psi signal growth is monitored at a rate of 10 times per 100tracking window. 101 102The kernel accepts window sizes ranging from 500ms to 10s, therefore min 103monitoring update interval is 50ms and max is 1s. Min limit is set to 104prevent overly frequent polling. Max limit is chosen as a high enough number 105after which monitors are most likely not needed and psi averages can be used 106instead. 107 108When activated, psi monitor stays active for at least the duration of one 109tracking window to avoid repeated activations/deactivations when system is 110bouncing in and out of the stall state. 111 112Notifications to the userspace are rate-limited to one per tracking window. 113 114The trigger will de-register when the file descriptor used to define the 115trigger is closed. 116 117Userspace monitor usage example 118=============================== 119 120:: 121 122 #include <errno.h> 123 #include <fcntl.h> 124 #include <stdio.h> 125 #include <poll.h> 126 #include <string.h> 127 #include <unistd.h> 128 129 /* 130 * Monitor memory partial stall with 1s tracking window size 131 * and 150ms threshold. 132 */ 133 int main() { 134 const char trig[] = "some 150000 1000000"; 135 struct pollfd fds; 136 int n; 137 138 fds.fd = open("/proc/pressure/memory", O_RDWR | O_NONBLOCK); 139 if (fds.fd < 0) { 140 printf("/proc/pressure/memory open error: %s\n", 141 strerror(errno)); 142 return 1; 143 } 144 fds.events = POLLPRI; 145 146 if (write(fds.fd, trig, strlen(trig) + 1) < 0) { 147 printf("/proc/pressure/memory write error: %s\n", 148 strerror(errno)); 149 return 1; 150 } 151 152 printf("waiting for events...\n"); 153 while (1) { 154 n = poll(&fds, 1, -1); 155 if (n < 0) { 156 printf("poll error: %s\n", strerror(errno)); 157 return 1; 158 } 159 if (fds.revents & POLLERR) { 160 printf("got POLLERR, event source is gone\n"); 161 return 0; 162 } 163 if (fds.revents & POLLPRI) { 164 printf("event triggered!\n"); 165 } else { 166 printf("unknown event received: 0x%x\n", fds.revents); 167 return 1; 168 } 169 } 170 171 return 0; 172 } 173 174Cgroup2 interface 175================= 176 177In a system with a CONFIG_CGROUP=y kernel and the cgroup2 filesystem 178mounted, pressure stall information is also tracked for tasks grouped 179into cgroups. Each subdirectory in the cgroupfs mountpoint contains 180cpu.pressure, memory.pressure, and io.pressure files; the format is 181the same as the /proc/pressure/ files. 182 183Per-cgroup psi monitors can be specified and used the same way as 184system-wide ones. 185