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Diffstat (limited to 'Documentation/vm/transhuge.txt')
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diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt index 29bdf62aac09..f734bb2a78dc 100644 --- a/Documentation/vm/transhuge.txt +++ b/Documentation/vm/transhuge.txt @@ -166,6 +166,68 @@ behavior. So to make them effective you need to restart any application that could have been using hugepages. This also applies to the regions registered in khugepaged. +== Monitoring usage == + +The number of transparent huge pages currently used by the system is +available by reading the AnonHugePages field in /proc/meminfo. To +identify what applications are using transparent huge pages, it is +necessary to read /proc/PID/smaps and count the AnonHugePages fields +for each mapping. Note that reading the smaps file is expensive and +reading it frequently will incur overhead. + +There are a number of counters in /proc/vmstat that may be used to +monitor how successfully the system is providing huge pages for use. + +thp_fault_alloc is incremented every time a huge page is successfully + allocated to handle a page fault. This applies to both the + first time a page is faulted and for COW faults. + +thp_collapse_alloc is incremented by khugepaged when it has found + a range of pages to collapse into one huge page and has + successfully allocated a new huge page to store the data. + +thp_fault_fallback is incremented if a page fault fails to allocate + a huge page and instead falls back to using small pages. + +thp_collapse_alloc_failed is incremented if khugepaged found a range + of pages that should be collapsed into one huge page but failed + the allocation. + +thp_split is incremented every time a huge page is split into base + pages. This can happen for a variety of reasons but a common + reason is that a huge page is old and is being reclaimed. + +As the system ages, allocating huge pages may be expensive as the +system uses memory compaction to copy data around memory to free a +huge page for use. There are some counters in /proc/vmstat to help +monitor this overhead. + +compact_stall is incremented every time a process stalls to run + memory compaction so that a huge page is free for use. + +compact_success is incremented if the system compacted memory and + freed a huge page for use. + +compact_fail is incremented if the system tries to compact memory + but failed. + +compact_pages_moved is incremented each time a page is moved. If + this value is increasing rapidly, it implies that the system + is copying a lot of data to satisfy the huge page allocation. + It is possible that the cost of copying exceeds any savings + from reduced TLB misses. + +compact_pagemigrate_failed is incremented when the underlying mechanism + for moving a page failed. + +compact_blocks_moved is incremented each time memory compaction examines + a huge page aligned range of pages. + +It is possible to establish how long the stalls were using the function +tracer to record how long was spent in __alloc_pages_nodemask and +using the mm_page_alloc tracepoint to identify which allocations were +for huge pages. + == get_user_pages and follow_page == get_user_pages and follow_page if run on a hugepage, will return the |