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docs/vm: transhuge: split userspace bits to admin-guide/mm/transhuge
Now that the administrative information for transparent huge pages is nicely separated, move it to its own page under the admin guide. Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mike Rapoport
Jonathan Corbet
parent
aa00eaa9af
commit
45c9a74f64
@ -4313,7 +4313,8 @@
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Format: [always|madvise|never]
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Can be used to control the default behavior of the system
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with respect to transparent hugepages.
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See Documentation/vm/transhuge.rst for more details.
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See Documentation/admin-guide/mm/transhuge.rst
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for more details.
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tsc= Disable clocksource stability checks for TSC.
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Format: <string>
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@ -27,4 +27,5 @@ the Linux memory management.
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numa_memory_policy
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pagemap
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soft-dirty
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transhuge
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userfaultfd
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418
Documentation/admin-guide/mm/transhuge.rst
Normal file
418
Documentation/admin-guide/mm/transhuge.rst
Normal file
@ -0,0 +1,418 @@
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.. _admin_guide_transhuge:
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============================
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Transparent Hugepage Support
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============================
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Objective
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=========
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Performance critical computing applications dealing with large memory
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working sets are already running on top of libhugetlbfs and in turn
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hugetlbfs. Transparent HugePage Support (THP) is an alternative mean of
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using huge pages for the backing of virtual memory with huge pages
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that supports the automatic promotion and demotion of page sizes and
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without the shortcomings of hugetlbfs.
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Currently THP only works for anonymous memory mappings and tmpfs/shmem.
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But in the future it can expand to other filesystems.
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.. note::
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in the examples below we presume that the basic page size is 4K and
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the huge page size is 2M, although the actual numbers may vary
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depending on the CPU architecture.
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The reason applications are running faster is because of two
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factors. The first factor is almost completely irrelevant and it's not
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of significant interest because it'll also have the downside of
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requiring larger clear-page copy-page in page faults which is a
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potentially negative effect. The first factor consists in taking a
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single page fault for each 2M virtual region touched by userland (so
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reducing the enter/exit kernel frequency by a 512 times factor). This
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only matters the first time the memory is accessed for the lifetime of
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a memory mapping. The second long lasting and much more important
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factor will affect all subsequent accesses to the memory for the whole
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runtime of the application. The second factor consist of two
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components:
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1) the TLB miss will run faster (especially with virtualization using
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nested pagetables but almost always also on bare metal without
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virtualization)
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2) a single TLB entry will be mapping a much larger amount of virtual
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memory in turn reducing the number of TLB misses. With
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virtualization and nested pagetables the TLB can be mapped of
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larger size only if both KVM and the Linux guest are using
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hugepages but a significant speedup already happens if only one of
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the two is using hugepages just because of the fact the TLB miss is
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going to run faster.
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THP can be enabled system wide or restricted to certain tasks or even
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memory ranges inside task's address space. Unless THP is completely
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disabled, there is ``khugepaged`` daemon that scans memory and
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collapses sequences of basic pages into huge pages.
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The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
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interface and using madivse(2) and prctl(2) system calls.
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Transparent Hugepage Support maximizes the usefulness of free memory
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if compared to the reservation approach of hugetlbfs by allowing all
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unused memory to be used as cache or other movable (or even unmovable
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entities). It doesn't require reservation to prevent hugepage
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allocation failures to be noticeable from userland. It allows paging
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and all other advanced VM features to be available on the
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hugepages. It requires no modifications for applications to take
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advantage of it.
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Applications however can be further optimized to take advantage of
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this feature, like for example they've been optimized before to avoid
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a flood of mmap system calls for every malloc(4k). Optimizing userland
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is by far not mandatory and khugepaged already can take care of long
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lived page allocations even for hugepage unaware applications that
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deals with large amounts of memory.
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In certain cases when hugepages are enabled system wide, application
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may end up allocating more memory resources. An application may mmap a
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large region but only touch 1 byte of it, in that case a 2M page might
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be allocated instead of a 4k page for no good. This is why it's
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possible to disable hugepages system-wide and to only have them inside
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MADV_HUGEPAGE madvise regions.
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Embedded systems should enable hugepages only inside madvise regions
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to eliminate any risk of wasting any precious byte of memory and to
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only run faster.
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Applications that gets a lot of benefit from hugepages and that don't
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risk to lose memory by using hugepages, should use
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madvise(MADV_HUGEPAGE) on their critical mmapped regions.
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.. _thp_sysfs:
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sysfs
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=====
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Global THP controls
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-------------------
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Transparent Hugepage Support for anonymous memory can be entirely disabled
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(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
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regions (to avoid the risk of consuming more memory resources) or enabled
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system wide. This can be achieved with one of::
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echo always >/sys/kernel/mm/transparent_hugepage/enabled
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echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
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echo never >/sys/kernel/mm/transparent_hugepage/enabled
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It's also possible to limit defrag efforts in the VM to generate
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anonymous hugepages in case they're not immediately free to madvise
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regions or to never try to defrag memory and simply fallback to regular
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pages unless hugepages are immediately available. Clearly if we spend CPU
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time to defrag memory, we would expect to gain even more by the fact we
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use hugepages later instead of regular pages. This isn't always
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guaranteed, but it may be more likely in case the allocation is for a
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MADV_HUGEPAGE region.
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::
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echo always >/sys/kernel/mm/transparent_hugepage/defrag
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echo defer >/sys/kernel/mm/transparent_hugepage/defrag
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echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag
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echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
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echo never >/sys/kernel/mm/transparent_hugepage/defrag
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always
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means that an application requesting THP will stall on
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allocation failure and directly reclaim pages and compact
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memory in an effort to allocate a THP immediately. This may be
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desirable for virtual machines that benefit heavily from THP
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use and are willing to delay the VM start to utilise them.
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defer
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means that an application will wake kswapd in the background
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to reclaim pages and wake kcompactd to compact memory so that
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THP is available in the near future. It's the responsibility
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of khugepaged to then install the THP pages later.
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defer+madvise
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will enter direct reclaim and compaction like ``always``, but
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only for regions that have used madvise(MADV_HUGEPAGE); all
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other regions will wake kswapd in the background to reclaim
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pages and wake kcompactd to compact memory so that THP is
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available in the near future.
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madvise
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will enter direct reclaim like ``always`` but only for regions
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that are have used madvise(MADV_HUGEPAGE). This is the default
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behaviour.
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never
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should be self-explanatory.
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By default kernel tries to use huge zero page on read page fault to
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anonymous mapping. It's possible to disable huge zero page by writing 0
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or enable it back by writing 1::
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echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
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echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
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Some userspace (such as a test program, or an optimized memory allocation
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library) may want to know the size (in bytes) of a transparent hugepage::
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cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size
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khugepaged will be automatically started when
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transparent_hugepage/enabled is set to "always" or "madvise, and it'll
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be automatically shutdown if it's set to "never".
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Khugepaged controls
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-------------------
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khugepaged runs usually at low frequency so while one may not want to
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invoke defrag algorithms synchronously during the page faults, it
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should be worth invoking defrag at least in khugepaged. However it's
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also possible to disable defrag in khugepaged by writing 0 or enable
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defrag in khugepaged by writing 1::
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echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
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echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
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You can also control how many pages khugepaged should scan at each
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pass::
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/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
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and how many milliseconds to wait in khugepaged between each pass (you
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can set this to 0 to run khugepaged at 100% utilization of one core)::
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/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
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and how many milliseconds to wait in khugepaged if there's an hugepage
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allocation failure to throttle the next allocation attempt::
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/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
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The khugepaged progress can be seen in the number of pages collapsed::
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/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
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for each pass::
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/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
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``max_ptes_none`` specifies how many extra small pages (that are
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not already mapped) can be allocated when collapsing a group
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of small pages into one large page::
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/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
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A higher value leads to use additional memory for programs.
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A lower value leads to gain less thp performance. Value of
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max_ptes_none can waste cpu time very little, you can
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ignore it.
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``max_ptes_swap`` specifies how many pages can be brought in from
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swap when collapsing a group of pages into a transparent huge page::
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/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
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A higher value can cause excessive swap IO and waste
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memory. A lower value can prevent THPs from being
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collapsed, resulting fewer pages being collapsed into
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THPs, and lower memory access performance.
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Boot parameter
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==============
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You can change the sysfs boot time defaults of Transparent Hugepage
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Support by passing the parameter ``transparent_hugepage=always`` or
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``transparent_hugepage=madvise`` or ``transparent_hugepage=never``
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to the kernel command line.
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Hugepages in tmpfs/shmem
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========================
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You can control hugepage allocation policy in tmpfs with mount option
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``huge=``. It can have following values:
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always
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Attempt to allocate huge pages every time we need a new page;
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never
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Do not allocate huge pages;
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within_size
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Only allocate huge page if it will be fully within i_size.
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Also respect fadvise()/madvise() hints;
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advise
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Only allocate huge pages if requested with fadvise()/madvise();
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The default policy is ``never``.
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``mount -o remount,huge= /mountpoint`` works fine after mount: remounting
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``huge=never`` will not attempt to break up huge pages at all, just stop more
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from being allocated.
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There's also sysfs knob to control hugepage allocation policy for internal
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shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount
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is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or
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MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
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In addition to policies listed above, shmem_enabled allows two further
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values:
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deny
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For use in emergencies, to force the huge option off from
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all mounts;
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force
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Force the huge option on for all - very useful for testing;
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Need of application restart
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===========================
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The transparent_hugepage/enabled values and tmpfs mount option only affect
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future behavior. So to make them effective you need to restart any
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application that could have been using hugepages. This also applies to the
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regions registered in khugepaged.
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Monitoring usage
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================
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The number of anonymous transparent huge pages currently used by the
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system is available by reading the AnonHugePages field in ``/proc/meminfo``.
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To identify what applications are using anonymous transparent huge pages,
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it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages fields
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for each mapping.
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The number of file transparent huge pages mapped to userspace is available
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by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``.
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To identify what applications are mapping file transparent huge pages, it
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is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields
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for each mapping.
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Note that reading the smaps file is expensive and reading it
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frequently will incur overhead.
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There are a number of counters in ``/proc/vmstat`` that may be used to
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monitor how successfully the system is providing huge pages for use.
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thp_fault_alloc
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is incremented every time a huge page is successfully
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allocated to handle a page fault. This applies to both the
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first time a page is faulted and for COW faults.
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thp_collapse_alloc
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is incremented by khugepaged when it has found
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a range of pages to collapse into one huge page and has
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successfully allocated a new huge page to store the data.
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thp_fault_fallback
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is incremented if a page fault fails to allocate
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a huge page and instead falls back to using small pages.
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thp_collapse_alloc_failed
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is incremented if khugepaged found a range
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of pages that should be collapsed into one huge page but failed
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the allocation.
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thp_file_alloc
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is incremented every time a file huge page is successfully
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allocated.
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thp_file_mapped
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is incremented every time a file huge page is mapped into
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user address space.
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thp_split_page
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is incremented every time a huge page is split into base
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pages. This can happen for a variety of reasons but a common
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reason is that a huge page is old and is being reclaimed.
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This action implies splitting all PMD the page mapped with.
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thp_split_page_failed
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is incremented if kernel fails to split huge
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page. This can happen if the page was pinned by somebody.
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thp_deferred_split_page
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is incremented when a huge page is put onto split
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queue. This happens when a huge page is partially unmapped and
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splitting it would free up some memory. Pages on split queue are
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going to be split under memory pressure.
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thp_split_pmd
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is incremented every time a PMD split into table of PTEs.
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This can happen, for instance, when application calls mprotect() or
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munmap() on part of huge page. It doesn't split huge page, only
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page table entry.
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thp_zero_page_alloc
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is incremented every time a huge zero page is
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successfully allocated. It includes allocations which where
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dropped due race with other allocation. Note, it doesn't count
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every map of the huge zero page, only its allocation.
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thp_zero_page_alloc_failed
|
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is incremented if kernel fails to allocate
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huge zero page and falls back to using small pages.
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thp_swpout
|
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is incremented every time a huge page is swapout in one
|
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piece without splitting.
|
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|
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thp_swpout_fallback
|
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is incremented if a huge page has to be split before swapout.
|
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Usually because failed to allocate some continuous swap space
|
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for the huge page.
|
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As the system ages, allocating huge pages may be expensive as the
|
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system uses memory compaction to copy data around memory to free a
|
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huge page for use. There are some counters in ``/proc/vmstat`` to help
|
||||
monitor this overhead.
|
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|
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compact_stall
|
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is incremented every time a process stalls to run
|
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memory compaction so that a huge page is free for use.
|
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|
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compact_success
|
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is incremented if the system compacted memory and
|
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freed a huge page for use.
|
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|
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compact_fail
|
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is incremented if the system tries to compact memory
|
||||
but failed.
|
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|
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compact_pages_moved
|
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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.
|
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|
||||
compact_pagemigrate_failed
|
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is incremented when the underlying mechanism
|
||||
for moving a page failed.
|
||||
|
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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
|
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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.
|
||||
|
||||
Optimizing the applications
|
||||
===========================
|
||||
|
||||
To be guaranteed that the kernel will map a 2M page immediately in any
|
||||
memory region, the mmap region has to be hugepage naturally
|
||||
aligned. posix_memalign() can provide that guarantee.
|
||||
|
||||
Hugetlbfs
|
||||
=========
|
||||
|
||||
You can use hugetlbfs on a kernel that has transparent hugepage
|
||||
support enabled just fine as always. No difference can be noted in
|
||||
hugetlbfs other than there will be less overall fragmentation. All
|
||||
usual features belonging to hugetlbfs are preserved and
|
||||
unaffected. libhugetlbfs will also work fine as usual.
|
||||
@ -4,418 +4,8 @@
|
||||
Transparent Hugepage Support
|
||||
============================
|
||||
|
||||
Objective
|
||||
=========
|
||||
|
||||
Performance critical computing applications dealing with large memory
|
||||
working sets are already running on top of libhugetlbfs and in turn
|
||||
hugetlbfs. Transparent HugePage Support (THP) is an alternative mean of
|
||||
using huge pages for the backing of virtual memory with huge pages
|
||||
that supports the automatic promotion and demotion of page sizes and
|
||||
without the shortcomings of hugetlbfs.
|
||||
|
||||
Currently THP only works for anonymous memory mappings and tmpfs/shmem.
|
||||
But in the future it can expand to other filesystems.
|
||||
|
||||
.. note::
|
||||
in the examples below we presume that the basic page size is 4K and
|
||||
the huge page size is 2M, although the actual numbers may vary
|
||||
depending on the CPU architecture.
|
||||
|
||||
The reason applications are running faster is because of two
|
||||
factors. The first factor is almost completely irrelevant and it's not
|
||||
of significant interest because it'll also have the downside of
|
||||
requiring larger clear-page copy-page in page faults which is a
|
||||
potentially negative effect. The first factor consists in taking a
|
||||
single page fault for each 2M virtual region touched by userland (so
|
||||
reducing the enter/exit kernel frequency by a 512 times factor). This
|
||||
only matters the first time the memory is accessed for the lifetime of
|
||||
a memory mapping. The second long lasting and much more important
|
||||
factor will affect all subsequent accesses to the memory for the whole
|
||||
runtime of the application. The second factor consist of two
|
||||
components:
|
||||
|
||||
1) the TLB miss will run faster (especially with virtualization using
|
||||
nested pagetables but almost always also on bare metal without
|
||||
virtualization)
|
||||
|
||||
2) a single TLB entry will be mapping a much larger amount of virtual
|
||||
memory in turn reducing the number of TLB misses. With
|
||||
virtualization and nested pagetables the TLB can be mapped of
|
||||
larger size only if both KVM and the Linux guest are using
|
||||
hugepages but a significant speedup already happens if only one of
|
||||
the two is using hugepages just because of the fact the TLB miss is
|
||||
going to run faster.
|
||||
|
||||
THP can be enabled system wide or restricted to certain tasks or even
|
||||
memory ranges inside task's address space. Unless THP is completely
|
||||
disabled, there is ``khugepaged`` daemon that scans memory and
|
||||
collapses sequences of basic pages into huge pages.
|
||||
|
||||
The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
|
||||
interface and using madivse(2) and prctl(2) system calls.
|
||||
|
||||
Transparent Hugepage Support maximizes the usefulness of free memory
|
||||
if compared to the reservation approach of hugetlbfs by allowing all
|
||||
unused memory to be used as cache or other movable (or even unmovable
|
||||
entities). It doesn't require reservation to prevent hugepage
|
||||
allocation failures to be noticeable from userland. It allows paging
|
||||
and all other advanced VM features to be available on the
|
||||
hugepages. It requires no modifications for applications to take
|
||||
advantage of it.
|
||||
|
||||
Applications however can be further optimized to take advantage of
|
||||
this feature, like for example they've been optimized before to avoid
|
||||
a flood of mmap system calls for every malloc(4k). Optimizing userland
|
||||
is by far not mandatory and khugepaged already can take care of long
|
||||
lived page allocations even for hugepage unaware applications that
|
||||
deals with large amounts of memory.
|
||||
|
||||
In certain cases when hugepages are enabled system wide, application
|
||||
may end up allocating more memory resources. An application may mmap a
|
||||
large region but only touch 1 byte of it, in that case a 2M page might
|
||||
be allocated instead of a 4k page for no good. This is why it's
|
||||
possible to disable hugepages system-wide and to only have them inside
|
||||
MADV_HUGEPAGE madvise regions.
|
||||
|
||||
Embedded systems should enable hugepages only inside madvise regions
|
||||
to eliminate any risk of wasting any precious byte of memory and to
|
||||
only run faster.
|
||||
|
||||
Applications that gets a lot of benefit from hugepages and that don't
|
||||
risk to lose memory by using hugepages, should use
|
||||
madvise(MADV_HUGEPAGE) on their critical mmapped regions.
|
||||
|
||||
.. _thp_sysfs:
|
||||
|
||||
sysfs
|
||||
=====
|
||||
|
||||
Global THP controls
|
||||
-------------------
|
||||
|
||||
Transparent Hugepage Support for anonymous memory can be entirely disabled
|
||||
(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
|
||||
regions (to avoid the risk of consuming more memory resources) or enabled
|
||||
system wide. This can be achieved with one of::
|
||||
|
||||
echo always >/sys/kernel/mm/transparent_hugepage/enabled
|
||||
echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
|
||||
echo never >/sys/kernel/mm/transparent_hugepage/enabled
|
||||
|
||||
It's also possible to limit defrag efforts in the VM to generate
|
||||
anonymous hugepages in case they're not immediately free to madvise
|
||||
regions or to never try to defrag memory and simply fallback to regular
|
||||
pages unless hugepages are immediately available. Clearly if we spend CPU
|
||||
time to defrag memory, we would expect to gain even more by the fact we
|
||||
use hugepages later instead of regular pages. This isn't always
|
||||
guaranteed, but it may be more likely in case the allocation is for a
|
||||
MADV_HUGEPAGE region.
|
||||
|
||||
::
|
||||
|
||||
echo always >/sys/kernel/mm/transparent_hugepage/defrag
|
||||
echo defer >/sys/kernel/mm/transparent_hugepage/defrag
|
||||
echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag
|
||||
echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
|
||||
echo never >/sys/kernel/mm/transparent_hugepage/defrag
|
||||
|
||||
always
|
||||
means that an application requesting THP will stall on
|
||||
allocation failure and directly reclaim pages and compact
|
||||
memory in an effort to allocate a THP immediately. This may be
|
||||
desirable for virtual machines that benefit heavily from THP
|
||||
use and are willing to delay the VM start to utilise them.
|
||||
|
||||
defer
|
||||
means that an application will wake kswapd in the background
|
||||
to reclaim pages and wake kcompactd to compact memory so that
|
||||
THP is available in the near future. It's the responsibility
|
||||
of khugepaged to then install the THP pages later.
|
||||
|
||||
defer+madvise
|
||||
will enter direct reclaim and compaction like ``always``, but
|
||||
only for regions that have used madvise(MADV_HUGEPAGE); all
|
||||
other regions will wake kswapd in the background to reclaim
|
||||
pages and wake kcompactd to compact memory so that THP is
|
||||
available in the near future.
|
||||
|
||||
madvise
|
||||
will enter direct reclaim like ``always`` but only for regions
|
||||
that are have used madvise(MADV_HUGEPAGE). This is the default
|
||||
behaviour.
|
||||
|
||||
never
|
||||
should be self-explanatory.
|
||||
|
||||
By default kernel tries to use huge zero page on read page fault to
|
||||
anonymous mapping. It's possible to disable huge zero page by writing 0
|
||||
or enable it back by writing 1::
|
||||
|
||||
echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
|
||||
echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
|
||||
|
||||
Some userspace (such as a test program, or an optimized memory allocation
|
||||
library) may want to know the size (in bytes) of a transparent hugepage::
|
||||
|
||||
cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size
|
||||
|
||||
khugepaged will be automatically started when
|
||||
transparent_hugepage/enabled is set to "always" or "madvise, and it'll
|
||||
be automatically shutdown if it's set to "never".
|
||||
|
||||
Khugepaged controls
|
||||
-------------------
|
||||
|
||||
khugepaged runs usually at low frequency so while one may not want to
|
||||
invoke defrag algorithms synchronously during the page faults, it
|
||||
should be worth invoking defrag at least in khugepaged. However it's
|
||||
also possible to disable defrag in khugepaged by writing 0 or enable
|
||||
defrag in khugepaged by writing 1::
|
||||
|
||||
echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
|
||||
echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
|
||||
|
||||
You can also control how many pages khugepaged should scan at each
|
||||
pass::
|
||||
|
||||
/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
|
||||
|
||||
and how many milliseconds to wait in khugepaged between each pass (you
|
||||
can set this to 0 to run khugepaged at 100% utilization of one core)::
|
||||
|
||||
/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
|
||||
|
||||
and how many milliseconds to wait in khugepaged if there's an hugepage
|
||||
allocation failure to throttle the next allocation attempt::
|
||||
|
||||
/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
|
||||
|
||||
The khugepaged progress can be seen in the number of pages collapsed::
|
||||
|
||||
/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
|
||||
|
||||
for each pass::
|
||||
|
||||
/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
|
||||
|
||||
``max_ptes_none`` specifies how many extra small pages (that are
|
||||
not already mapped) can be allocated when collapsing a group
|
||||
of small pages into one large page::
|
||||
|
||||
/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
|
||||
|
||||
A higher value leads to use additional memory for programs.
|
||||
A lower value leads to gain less thp performance. Value of
|
||||
max_ptes_none can waste cpu time very little, you can
|
||||
ignore it.
|
||||
|
||||
``max_ptes_swap`` specifies how many pages can be brought in from
|
||||
swap when collapsing a group of pages into a transparent huge page::
|
||||
|
||||
/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
|
||||
|
||||
A higher value can cause excessive swap IO and waste
|
||||
memory. A lower value can prevent THPs from being
|
||||
collapsed, resulting fewer pages being collapsed into
|
||||
THPs, and lower memory access performance.
|
||||
|
||||
Boot parameter
|
||||
==============
|
||||
|
||||
You can change the sysfs boot time defaults of Transparent Hugepage
|
||||
Support by passing the parameter ``transparent_hugepage=always`` or
|
||||
``transparent_hugepage=madvise`` or ``transparent_hugepage=never``
|
||||
to the kernel command line.
|
||||
|
||||
Hugepages in tmpfs/shmem
|
||||
========================
|
||||
|
||||
You can control hugepage allocation policy in tmpfs with mount option
|
||||
``huge=``. It can have following values:
|
||||
|
||||
always
|
||||
Attempt to allocate huge pages every time we need a new page;
|
||||
|
||||
never
|
||||
Do not allocate huge pages;
|
||||
|
||||
within_size
|
||||
Only allocate huge page if it will be fully within i_size.
|
||||
Also respect fadvise()/madvise() hints;
|
||||
|
||||
advise
|
||||
Only allocate huge pages if requested with fadvise()/madvise();
|
||||
|
||||
The default policy is ``never``.
|
||||
|
||||
``mount -o remount,huge= /mountpoint`` works fine after mount: remounting
|
||||
``huge=never`` will not attempt to break up huge pages at all, just stop more
|
||||
from being allocated.
|
||||
|
||||
There's also sysfs knob to control hugepage allocation policy for internal
|
||||
shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount
|
||||
is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or
|
||||
MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
|
||||
|
||||
In addition to policies listed above, shmem_enabled allows two further
|
||||
values:
|
||||
|
||||
deny
|
||||
For use in emergencies, to force the huge option off from
|
||||
all mounts;
|
||||
force
|
||||
Force the huge option on for all - very useful for testing;
|
||||
|
||||
Need of application restart
|
||||
===========================
|
||||
|
||||
The transparent_hugepage/enabled values and tmpfs mount option only affect
|
||||
future 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 anonymous transparent huge pages currently used by the
|
||||
system is available by reading the AnonHugePages field in ``/proc/meminfo``.
|
||||
To identify what applications are using anonymous transparent huge pages,
|
||||
it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages fields
|
||||
for each mapping.
|
||||
|
||||
The number of file transparent huge pages mapped to userspace is available
|
||||
by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``.
|
||||
To identify what applications are mapping file transparent huge pages, it
|
||||
is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped 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_file_alloc
|
||||
is incremented every time a file huge page is successfully
|
||||
allocated.
|
||||
|
||||
thp_file_mapped
|
||||
is incremented every time a file huge page is mapped into
|
||||
user address space.
|
||||
|
||||
thp_split_page
|
||||
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.
|
||||
This action implies splitting all PMD the page mapped with.
|
||||
|
||||
thp_split_page_failed
|
||||
is incremented if kernel fails to split huge
|
||||
page. This can happen if the page was pinned by somebody.
|
||||
|
||||
thp_deferred_split_page
|
||||
is incremented when a huge page is put onto split
|
||||
queue. This happens when a huge page is partially unmapped and
|
||||
splitting it would free up some memory. Pages on split queue are
|
||||
going to be split under memory pressure.
|
||||
|
||||
thp_split_pmd
|
||||
is incremented every time a PMD split into table of PTEs.
|
||||
This can happen, for instance, when application calls mprotect() or
|
||||
munmap() on part of huge page. It doesn't split huge page, only
|
||||
page table entry.
|
||||
|
||||
thp_zero_page_alloc
|
||||
is incremented every time a huge zero page is
|
||||
successfully allocated. It includes allocations which where
|
||||
dropped due race with other allocation. Note, it doesn't count
|
||||
every map of the huge zero page, only its allocation.
|
||||
|
||||
thp_zero_page_alloc_failed
|
||||
is incremented if kernel fails to allocate
|
||||
huge zero page and falls back to using small pages.
|
||||
|
||||
thp_swpout
|
||||
is incremented every time a huge page is swapout in one
|
||||
piece without splitting.
|
||||
|
||||
thp_swpout_fallback
|
||||
is incremented if a huge page has to be split before swapout.
|
||||
Usually because failed to allocate some continuous swap space
|
||||
for the huge page.
|
||||
|
||||
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.
|
||||
|
||||
Optimizing the applications
|
||||
===========================
|
||||
|
||||
To be guaranteed that the kernel will map a 2M page immediately in any
|
||||
memory region, the mmap region has to be hugepage naturally
|
||||
aligned. posix_memalign() can provide that guarantee.
|
||||
|
||||
Hugetlbfs
|
||||
=========
|
||||
|
||||
You can use hugetlbfs on a kernel that has transparent hugepage
|
||||
support enabled just fine as always. No difference can be noted in
|
||||
hugetlbfs other than there will be less overall fragmentation. All
|
||||
usual features belonging to hugetlbfs are preserved and
|
||||
unaffected. libhugetlbfs will also work fine as usual.
|
||||
This document describes design principles Transparent Hugepage (THP)
|
||||
Support and its interaction with other parts of the memory management.
|
||||
|
||||
Design principles
|
||||
=================
|
||||
|
||||
Loading…
Reference in New Issue
Block a user