On Linux the Pressure Stall Information (PSI) Linux kernel interface provides a way to monitor resource pressure — situations where tasks are stalled waiting for a resource to become available. PSI covers three types of resources:
Memory pressure: tasks are stalled because the system is low on memory and the kernel is busy reclaiming it (e.g. writing out pages to swap or flushing file-backed pages).
CPU pressure: tasks are stalled waiting for CPU time because the CPU is oversubscribed.
IO pressure: tasks are stalled waiting for IO operations to complete because the IO subsystem is saturated.
PSI makes available to userspace a poll()-able file descriptor that gets
notifications whenever pressure latencies for the system or a control group
grow beyond some configured level.
When the system is under memory pressure, userspace can proactively release memory it allocated but doesn’t immediately require back to the kernel. This includes allocation caches, and other forms of caches that are not required for normal operation to continue. Similarly, when CPU or IO pressure is detected, services can take appropriate action such as reducing parallelism, deferring background work, or shedding load.
The effects of resource pressure during runtime generally are growing latencies during operation: applications and services slow down until pressure is relieved. Hence, to ensure stable service latencies it is essential to detect pressure early and respond appropriately.
systemd itself makes use of PSI, and helps applications to do so too.
Specifically:
Most of systemd’s long running components watch for PSI memory pressure events, and release allocation caches and other resources once seen.
systemd’s service manager provides a protocol for asking services to monitor PSI events and configure the appropriate pressure thresholds, for memory, CPU, and IO pressure independently.
systemd’s sd-event event loop API provides high-level calls
sd_event_add_memory_pressure(), sd_event_add_cpu_pressure(), and
sd_event_add_io_pressure() enabling programs using it to efficiently hook
into the PSI pressure protocol provided by the service manager, with very few
lines of code.
For each resource type, if pressure handling for a specific service is enabled
via the corresponding *PressureWatch= setting (i.e. MemoryPressureWatch=,
CPUPressureWatch=, or IOPressureWatch=), two environment variables are set
by the service manager:
$MEMORY_PRESSURE_WATCH / $CPU_PRESSURE_WATCH / $IO_PRESSURE_WATCH —
contains an absolute path in the file system to the file to watch for
pressure events. This will usually point to a PSI file such as the
memory.pressure, cpu.pressure, or io.pressure file of the service’s
cgroup. In order to make debugging easier, and allow later extension it is
recommended for applications to also allow this path to refer to an AF_UNIX
stream socket in the file system or a FIFO inode in the file system.
Regardless of which of the three types of inodes this absolute path refers
to, all three are poll()-able for pressure events. The variable can also be
set to the literal string /dev/null. If so the service code should take this
as indication that pressure monitoring for this resource is not desired and
should be turned off.
$MEMORY_PRESSURE_WRITE / $CPU_PRESSURE_WRITE / $IO_PRESSURE_WRITE —
optional. If set by the service manager it contains Base64 encoded data (that
may contain arbitrary binary values, including NUL bytes) that should be
written into the path provided via the corresponding *_PRESSURE_WATCH
variable right after opening it. Typically, if talking directly to a PSI
kernel file this will contain information about the threshold settings
configurable in the service manager.
The protocol works the same for all three resource types. The remainder of this section uses memory pressure as the example, but the same logic applies to CPU and IO pressure with the corresponding environment variable names.
When a service initializes it hence should look for
$MEMORY_PRESSURE_WATCH. If set, it should try to open the specified path. If
it detects the path to refer to a regular file it should assume it refers to a
PSI kernel file. If so, it should write the data from $MEMORY_PRESSURE_WRITE
into the file descriptor (after Base64-decoding it, and only if the variable is
set) and then watch for POLLPRI events on it. If it detects the path refers
to a FIFO inode, it should open it, write the $MEMORY_PRESSURE_WRITE data
into it (as above) and then watch for POLLIN events on it. Whenever POLLIN
is seen it should read and discard any data queued in the FIFO. If the path
refers to an AF_UNIX socket in the file system, the application should
connect() a stream socket to it, write $MEMORY_PRESSURE_WRITE into it (as
above) and watch for POLLIN, discarding any data it might receive.
To summarize:
If $MEMORY_PRESSURE_WATCH points to a regular file: open and watch for
POLLPRI, never read from the file descriptor.
If $MEMORY_PRESSURE_WATCH points to a FIFO: open and watch for POLLIN,
read/discard any incoming data.
If $MEMORY_PRESSURE_WATCH points to an AF_UNIX socket: connect and watch
for POLLIN, read/discard any incoming data.
If $MEMORY_PRESSURE_WATCH contains the literal string /dev/null, turn off
memory pressure handling.
(And in each case, immediately after opening/connecting to the path, write the
decoded $MEMORY_PRESSURE_WRITE data into it.)
Whenever a POLLPRI/POLLIN event is seen the service is under pressure. It
should use this as hint to release suitable redundant resources, for example:
glibc’s memory allocation cache, via
malloc_trim(). Similarly,
allocation caches implemented in the service itself.
Any other local caches, such as DNS caches, or web caches (in particular if service is a web browser).
Terminate any idle worker threads or processes.
Run a garbage collection (GC) cycle, if the runtime environment supports it.
Terminate the process if idle, and can be automatically started when needed next.
Which actions precisely to take depends on the service in question and the type of pressure. Note that the notifications are delivered when resource latency already degraded beyond some point. Hence when discussing which resources to keep and which to discard, keep in mind it’s typically acceptable that latencies incurred recovering discarded resources at a later point are acceptable, given that latencies already are affected negatively.
In case the path supplied via $MEMORY_PRESSURE_WATCH points to a PSI kernel
API file, or to an AF_UNIX socket, opening it multiple times is safe and reliable,
and should deliver notifications to each of the opened file descriptors. This
is specifically useful for services that consist of multiple processes, and
where each of them shall be able to release resources on memory pressure.
The POLLPRI/POLLIN conditions will be triggered every time pressure is
detected, but not continuously. It is thus safe to keep poll()-ing on the
same file descriptor continuously, and executing resource release operations
whenever the file descriptor triggers without having to expect overloading the
process.
(Currently, the protocol defined here only allows configuration of a single
“degree” of pressure per resource type, there’s no distinction made on how
strong the pressure is. In future, if it becomes apparent that there’s clear
need to extend this we might eventually add different degrees, most likely by
adding additional environment variables such as $MEMORY_PRESSURE_WRITE_LOW
and $MEMORY_PRESSURE_WRITE_HIGH or similar, which may contain different
settings for lower or higher pressure thresholds.)
The service manager provides two per-service settings for each resource type that control pressure handling:
MemoryPressureWatch= / CPUPressureWatch= / IOPressureWatch= controls
whether to enable the pressure protocol for the respective resource type for
the service in question. See
systemd.resource-control(5)
for details.
MemoryPressureThresholdSec= / CPUPressureThresholdSec= /
IOPressureThresholdSec= allows configuring the threshold when to signal
pressure to the services. It takes a time value (usually in the millisecond
range) that defines a threshold per 2s time window: if resource latencies grow
beyond this threshold notifications are generated towards the service,
requesting it to release resources.
The /etc/systemd/system.conf file provides two settings for each resource
type that may be used to select the default values for the above settings. If
the threshold isn’t configured via the per-service nor system-wide option, it
defaults to 200ms.
When pressure monitoring is enabled for a service this primarily does three things:
It enables the corresponding cgroup accounting for the service (this is a requirement for per-cgroup PSI).
It sets the aforementioned two environment variables for processes invoked for the service, based on the control group of the service and provided settings.
The corresponding PSI control group file (memory.pressure, cpu.pressure,
or io.pressure) associated with the service’s cgroup is delegated to the
service (i.e. permissions are relaxed so that unprivileged service payload
code can open the file for writing).
sd-eventThe
sd-event
event loop library provides API calls that encapsulate the functionality
described above:
sd_event_add_memory_pressure(),
sd_event_add_cpu_pressure(), and sd_event_add_io_pressure() implement the
service-side of the pressure protocol for each resource type and integrate it
with an sd-event event loop. Each reads the corresponding two environment
variables, connects/opens the specified file, writes the specified data to it,
then watches it for events.
The sd_event_trim_memory() call may be called to trim the calling
processes’ memory. It’s a wrapper around glibc’s malloc_trim(), but first
releases allocation caches maintained by libsystemd internally. This function
serves as the default when a NULL callback is supplied to
sd_event_add_memory_pressure(). Note that the default handler for
sd_event_add_cpu_pressure() and sd_event_add_io_pressure() is a no-op;
a custom callback should be provided for CPU and IO pressure to take
meaningful action.
When implementing a service using sd-event, for automatic memory pressure
handling, it’s typically sufficient to add a line such as:
(void) sd_event_add_memory_pressure(event, NULL, NULL, NULL);
– right after allocating the event loop object event. For CPU and IO pressure,
a custom handler should be provided to take appropriate action:
(void) sd_event_add_cpu_pressure(event, NULL, my_cpu_pressure_handler, userdata);
(void) sd_event_add_io_pressure(event, NULL, my_io_pressure_handler, userdata);
Other programming environments might have native APIs to watch memory pressure/low memory events. Most notable is probably GLib’s GMemoryMonitor. As of GLib 2.86.0, it uses the per-cgroup PSI kernel file to monitor for memory pressure, but does not yet read the environment variables recommended above.
In older versions, it used the per-system Linux PSI interface as the backend, but operated differently than the above: memory pressure events were picked up by a system service, which then propagated this through D-Bus to the applications. This was typically less than ideal, since this means each notification event had to traverse three processes before being handled. This traversal created additional latencies at a time where the system is already experiencing adverse latencies. Moreover, it focused on system-wide PSI events, even though service-local ones are generally the better approach.