linux/Documentation/scheduler/sched-debug.rst

=================
Scheduler debugfs
=================

Booting a kernel with debugfs enabled will give access to
scheduler specific debug files under /sys/kernel/debug/sched. Some of
those files are described below.

numa_balancing
==============

`numa_balancing` directory is used to hold files to control NUMA
balancing feature.  If the system overhead from the feature is too
high then the rate the kernel samples for NUMA hinting faults may be
controlled by the `scan_period_min_ms, scan_delay_ms,
scan_period_max_ms, scan_size_mb` files.


scan_period_min_ms, scan_delay_ms, scan_period_max_ms, scan_size_mb
-------------------------------------------------------------------

Automatic NUMA balancing scans tasks address space and unmaps pages to
detect if pages are properly placed or if the data should be migrated to a
memory node local to where the task is running.  Every "scan delay" the task
scans the next "scan size" number of pages in its address space. When the
end of the address space is reached the scanner restarts from the beginning.

In combination, the "scan delay" and "scan size" determine the scan rate.
When "scan delay" decreases, the scan rate increases.  The scan delay and
hence the scan rate of every task is adaptive and depends on historical
behaviour. If pages are properly placed then the scan delay increases,
otherwise the scan delay decreases.  The "scan size" is not adaptive but
the higher the "scan size", the higher the scan rate.

Higher scan rates incur higher system overhead as page faults must be
trapped and potentially data must be migrated. However, the higher the scan
rate, the more quickly a tasks memory is migrated to a local node if the
workload pattern changes and minimises performance impact due to remote
memory accesses. These files control the thresholds for scan delays and
the number of pages scanned.

``scan_period_min_ms`` is the minimum time in milliseconds to scan a
tasks virtual memory. It effectively controls the maximum scanning
rate for each task.

``scan_delay_ms`` is the starting "scan delay" used for a task when it
initially forks.

``scan_period_max_ms`` is the maximum time in milliseconds to scan a
tasks virtual memory. It effectively controls the minimum scanning
rate for each task.

``scan_size_mb`` is how many megabytes worth of pages are scanned for
a given scan.