430 lines
16 KiB
ReStructuredText
430 lines
16 KiB
ReStructuredText
.. _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 madvise(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 (note
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that this counter may not be an exact count of the number of pages
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collapsed, since "collapsed" could mean multiple things: (1) A PTE mapping
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being replaced by a PMD mapping, or (2) All 4K physical pages replaced by
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one 2M hugepage. Each may happen independently, or together, depending on
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the type of memory and the failures that occur. As such, this value should
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be interpreted roughly as a sign of progress, and counters in /proc/vmstat
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consulted for more accurate accounting)::
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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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``max_ptes_shared`` specifies how many pages can be shared across multiple
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processes. Exceeding the number would block the collapse::
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/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared
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A higher value may increase memory footprint for some workloads.
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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.
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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_fault_fallback_charge
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is incremented if a page fault fails to charge a huge page and
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instead falls back to using small pages even though the
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allocation was successful.
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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_fallback
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is incremented if a file huge page is attempted to be allocated
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but fails and instead falls back to using small pages.
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thp_file_fallback_charge
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is incremented if a file huge page cannot be charged and instead
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falls back to using small pages even though the allocation was
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successful.
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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 used for thp is
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successfully allocated. Note, it doesn't count every map of
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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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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
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monitor this overhead.
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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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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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compact_fail
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is incremented if the system tries to compact memory
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but failed.
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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() and
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using the mm_page_alloc tracepoint to identify which allocations were
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for huge pages.
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Optimizing the applications
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===========================
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To be guaranteed that the kernel will map a 2M page immediately in any
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memory region, the mmap region has to be hugepage naturally
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aligned. posix_memalign() can provide that guarantee.
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Hugetlbfs
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=========
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You can use hugetlbfs on a kernel that has transparent hugepage
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support enabled just fine as always. No difference can be noted in
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hugetlbfs other than there will be less overall fragmentation. All
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usual features belonging to hugetlbfs are preserved and
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unaffected. libhugetlbfs will also work fine as usual.
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