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Date: Wed, 9 Oct 2013 10:24:12 -0700 From: tip-bot for Mel Gorman <tipbot@...or.com> To: linux-tip-commits@...r.kernel.org Cc: linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org, peterz@...radead.org, hannes@...xchg.org, riel@...hat.com, aarcange@...hat.com, srikar@...ux.vnet.ibm.com, mgorman@...e.de, tglx@...utronix.de Subject: [tip:sched/core] mm: numa: Document automatic NUMA balancing sysctls Commit-ID: 10fc05d0e551146ad6feb0ab8902d28a2d3c5624 Gitweb: http://git.kernel.org/tip/10fc05d0e551146ad6feb0ab8902d28a2d3c5624 Author: Mel Gorman <mgorman@...e.de> AuthorDate: Mon, 7 Oct 2013 11:28:40 +0100 Committer: Ingo Molnar <mingo@...nel.org> CommitDate: Wed, 9 Oct 2013 12:39:20 +0200 mm: numa: Document automatic NUMA balancing sysctls Signed-off-by: Mel Gorman <mgorman@...e.de> Reviewed-by: Rik van Riel <riel@...hat.com> Cc: Andrea Arcangeli <aarcange@...hat.com> Cc: Johannes Weiner <hannes@...xchg.org> Cc: Srikar Dronamraju <srikar@...ux.vnet.ibm.com> Signed-off-by: Peter Zijlstra <peterz@...radead.org> Link: http://lkml.kernel.org/r/1381141781-10992-3-git-send-email-mgorman@suse.de Signed-off-by: Ingo Molnar <mingo@...nel.org> --- Documentation/sysctl/kernel.txt | 66 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 66 insertions(+) diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index 9d4c1d1..1428c66 100644 --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -355,6 +355,72 @@ utilize. ============================================================== +numa_balancing + +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, +numa_balancing_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 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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