Commit 10fc05d0 authored by Mel Gorman's avatar Mel Gorman Committed by Ingo Molnar

mm: numa: Document automatic NUMA balancing sysctls

Signed-off-by: default avatarMel Gorman <>
Reviewed-by: default avatarRik van Riel <>
Cc: Andrea Arcangeli <>
Cc: Johannes Weiner <>
Cc: Srikar Dronamraju <>
Signed-off-by: default avatarPeter Zijlstra <>
Link: default avatarIngo Molnar <>
parent 37bf0637
......@@ -355,6 +355,72 @@ utilize.
Enables/disables automatic page fault based NUMA memory
balancing. Memory is moved automatically to nodes
that access it often.
Enables/disables automatic NUMA memory balancing. On NUMA machines, there
is a performance penalty if remote memory is accessed by a CPU. When this
feature is enabled the kernel samples what task thread is accessing memory
by periodically unmapping pages and later trapping a page fault. At the
time of the page fault, it is determined if the data being accessed should
be migrated to a local memory node.
The unmapping of pages and trapping faults incur additional overhead that
ideally is offset by improved memory locality but there is no universal
guarantee. If the target workload is already bound to NUMA nodes then this
feature should be disabled. Otherwise, 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 numa_balancing_scan_period_min_ms,
numa_balancing_scan_delay_ms, numa_balancing_scan_period_reset,
numa_balancing_scan_period_max_ms and numa_balancing_scan_size_mb sysctls.
numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms,
numa_balancing_scan_period_max_ms, numa_balancing_scan_period_reset,
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 sysctls control the thresholds for scan delays and
the number of pages scanned.
numa_balancing_scan_period_min_ms is the minimum delay in milliseconds
between scans. It effectively controls the maximum scanning rate for
each task.
numa_balancing_scan_delay_ms is the starting "scan delay" used for a task
when it initially forks.
numa_balancing_scan_period_max_ms is the maximum delay between scans. It
effectively controls the minimum scanning rate for each task.
numa_balancing_scan_size_mb is how many megabytes worth of pages are
scanned for a given scan.
numa_balancing_scan_period_reset is a blunt instrument that controls how
often a tasks scan delay is reset to detect sudden changes in task behaviour.
osrelease, ostype & version:
# cat osrelease
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