The PSI backend is based on Kernel PSI [1]. It monitors the memory
allocation time with in a given time window. If the allocation time
is more than the given threshold, Kernel signal the application to
deliver the memory pressure events.
The current thresholds are:
LOW: 70ms out of 2 seconds for partial stall
MEDIUM: 100ms out of 2 seconds for partial stall
CRITICAL: 100ms out of 2 seconds for full stall
[1] https://docs.kernel.org/accounting/psi.html
