1=================
2Scheduler debugfs
3=================
4
5Booting a kernel with CONFIG_SCHED_DEBUG=y will give access to
6scheduler specific debug files under /sys/kernel/debug/sched. Some of
7those files are described below.
8
9numa_balancing
10==============
11
12`numa_balancing` directory is used to hold files to control NUMA
13balancing feature.  If the system overhead from the feature is too
14high then the rate the kernel samples for NUMA hinting faults may be
15controlled by the `scan_period_min_ms, scan_delay_ms,
16scan_period_max_ms, scan_size_mb` files.
17
18
19scan_period_min_ms, scan_delay_ms, scan_period_max_ms, scan_size_mb
20-------------------------------------------------------------------
21
22Automatic NUMA balancing scans tasks address space and unmaps pages to
23detect if pages are properly placed or if the data should be migrated to a
24memory node local to where the task is running.  Every "scan delay" the task
25scans the next "scan size" number of pages in its address space. When the
26end of the address space is reached the scanner restarts from the beginning.
27
28In combination, the "scan delay" and "scan size" determine the scan rate.
29When "scan delay" decreases, the scan rate increases.  The scan delay and
30hence the scan rate of every task is adaptive and depends on historical
31behaviour. If pages are properly placed then the scan delay increases,
32otherwise the scan delay decreases.  The "scan size" is not adaptive but
33the higher the "scan size", the higher the scan rate.
34
35Higher scan rates incur higher system overhead as page faults must be
36trapped and potentially data must be migrated. However, the higher the scan
37rate, the more quickly a tasks memory is migrated to a local node if the
38workload pattern changes and minimises performance impact due to remote
39memory accesses. These files control the thresholds for scan delays and
40the number of pages scanned.
41
42``scan_period_min_ms`` is the minimum time in milliseconds to scan a
43tasks virtual memory. It effectively controls the maximum scanning
44rate for each task.
45
46``scan_delay_ms`` is the starting "scan delay" used for a task when it
47initially forks.
48
49``scan_period_max_ms`` is the maximum time in milliseconds to scan a
50tasks virtual memory. It effectively controls the minimum scanning
51rate for each task.
52
53``scan_size_mb`` is how many megabytes worth of pages are scanned for
54a given scan.
55