docs/vm: transhuge.txt: convert to ReST format
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
5431998233
commit
44f380fe90
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@ -1,6 +1,11 @@
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= Transparent Hugepage Support =
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.. _transhuge:
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== Objective ==
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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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@ -33,7 +38,8 @@ are using hugepages but a significant speedup already happens if only
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one of the two is using hugepages just because of the fact the TLB
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miss is going to run faster.
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== Design ==
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Design
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======
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- "graceful fallback": mm components which don't have transparent hugepage
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knowledge fall back to breaking huge pmd mapping into table of ptes and,
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@ -88,16 +94,17 @@ 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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== sysfs ==
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sysfs
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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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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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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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@ -108,44 +115,53 @@ 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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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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::
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"always" means that an application requesting THP will stall on allocation
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failure and directly reclaim pages and compact memory in an effort to
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allocate a THP immediately. This may be desirable for virtual machines
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that benefit heavily from THP use and are willing to delay the VM start
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to utilise them.
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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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"defer" 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 THP is
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available in the near future. It's the responsibility of khugepaged
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to then install the THP pages later.
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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+madvise" will enter direct reclaim and compaction like "always", but
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only for regions that have used madvise(MADV_HUGEPAGE); all other regions
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will wake kswapd in the background to reclaim pages and wake kcompactd to
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compact memory so that THP is available in the near future.
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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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"madvise" 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 behaviour.
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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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"never" should be self-explanatory.
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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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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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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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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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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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@ -155,84 +171,86 @@ 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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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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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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pass::
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/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
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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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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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/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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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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/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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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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/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
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for each pass:
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for each pass::
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/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
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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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``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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of small pages into one large page::
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/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
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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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``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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/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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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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(without "") to the kernel command line.
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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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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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``huge=``. It can have following values:
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- "always":
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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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never
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Do not allocate huge pages;
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- "within_size":
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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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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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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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``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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@ -243,110 +261,130 @@ 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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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
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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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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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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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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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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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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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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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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 is incremented every time a huge page is successfully
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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 is incremented by khugepaged when it has found
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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 is incremented if a page fault fails to allocate
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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 is incremented if khugepaged found a range
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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 is incremented every time a file huge page is successfully
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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 is incremented every time a file huge page is mapped into
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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 is incremented every time a huge page is split into base
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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 is incremented if kernel fails to split huge
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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 is incremented when a huge page is put onto split
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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 is incremented every time a PMD split into table of PTEs.
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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 is incremented every time a huge zero page is
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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 is incremented if kernel fails to allocate
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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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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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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 is incremented every time a process stalls to run
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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 is incremented if the system compacted memory and
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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 is incremented if the system tries to compact memory
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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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compact_pages_moved is incremented each time a page is moved. If
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compact_pages_moved
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is incremented each time a page is moved. If
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this value is increasing rapidly, it implies that the system
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is copying a lot of data to satisfy the huge page allocation.
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It is possible that the cost of copying exceeds any savings
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from reduced TLB misses.
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compact_pagemigrate_failed is incremented when the underlying mechanism
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compact_pagemigrate_failed
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is incremented when the underlying mechanism
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for moving a page failed.
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compact_blocks_moved is incremented each time memory compaction examines
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compact_blocks_moved
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is incremented each time memory compaction examines
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a huge page aligned range of pages.
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It is possible to establish how long the stalls were using the function
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@ -354,7 +392,8 @@ tracer to record how long was spent in __alloc_pages_nodemask 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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== get_user_pages and follow_page ==
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get_user_pages and follow_page
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==============================
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get_user_pages and follow_page if run on a hugepage, will return the
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head or tail pages as usual (exactly as they would do on
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@ -367,10 +406,11 @@ for the head page and not the tail page), it should be updated to jump
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to check head page instead. Taking reference on any head/tail page would
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prevent page from being split by anyone.
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NOTE: these aren't new constraints to the GUP API, and they match the
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same constrains that applies to hugetlbfs too, so any driver capable
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of handling GUP on hugetlbfs will also work fine on transparent
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hugepage backed mappings.
|
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.. note::
|
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these aren't new constraints to the GUP API, and they match the
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||||
same constrains that applies to hugetlbfs too, so any driver capable
|
||||
of handling GUP on hugetlbfs will also work fine on transparent
|
||||
hugepage backed mappings.
|
||||
|
||||
In case you can't handle compound pages if they're returned by
|
||||
follow_page, the FOLL_SPLIT bit can be specified as parameter to
|
||||
|
|
@ -383,13 +423,15 @@ hugepages being returned (as it's not only checking the pfn of the
|
|||
page and pinning it during the copy but it pretends to migrate the
|
||||
memory in regular page sizes and with regular pte/pmd mappings).
|
||||
|
||||
== Optimizing the applications ==
|
||||
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 ==
|
||||
Hugetlbfs
|
||||
=========
|
||||
|
||||
You can use hugetlbfs on a kernel that has transparent hugepage
|
||||
support enabled just fine as always. No difference can be noted in
|
||||
|
|
@ -397,7 +439,8 @@ 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.
|
||||
|
||||
== Graceful fallback ==
|
||||
Graceful fallback
|
||||
=================
|
||||
|
||||
Code walking pagetables but unaware about huge pmds can simply call
|
||||
split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
|
||||
|
|
@ -415,20 +458,21 @@ it tries to swapout the hugepage for example. split_huge_page() can fail
|
|||
if the page is pinned and you must handle this correctly.
|
||||
|
||||
Example to make mremap.c transparent hugepage aware with a one liner
|
||||
change:
|
||||
change::
|
||||
|
||||
diff --git a/mm/mremap.c b/mm/mremap.c
|
||||
--- a/mm/mremap.c
|
||||
+++ b/mm/mremap.c
|
||||
@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
|
||||
return NULL;
|
||||
diff --git a/mm/mremap.c b/mm/mremap.c
|
||||
--- a/mm/mremap.c
|
||||
+++ b/mm/mremap.c
|
||||
@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
|
||||
return NULL;
|
||||
|
||||
pmd = pmd_offset(pud, addr);
|
||||
+ split_huge_pmd(vma, pmd, addr);
|
||||
if (pmd_none_or_clear_bad(pmd))
|
||||
return NULL;
|
||||
pmd = pmd_offset(pud, addr);
|
||||
+ split_huge_pmd(vma, pmd, addr);
|
||||
if (pmd_none_or_clear_bad(pmd))
|
||||
return NULL;
|
||||
|
||||
== Locking in hugepage aware code ==
|
||||
Locking in hugepage aware code
|
||||
==============================
|
||||
|
||||
We want as much code as possible hugepage aware, as calling
|
||||
split_huge_page() or split_huge_pmd() has a cost.
|
||||
|
|
@ -448,7 +492,8 @@ should just drop the page table lock and fallback to the old code as
|
|||
before. Otherwise you can proceed to process the huge pmd and the
|
||||
hugepage natively. Once finished you can drop the page table lock.
|
||||
|
||||
== Refcounts and transparent huge pages ==
|
||||
Refcounts and transparent huge pages
|
||||
====================================
|
||||
|
||||
Refcounting on THP is mostly consistent with refcounting on other compound
|
||||
pages:
|
||||
|
|
@ -510,7 +555,8 @@ clear where reference should go after split: it will stay on head page.
|
|||
Note that split_huge_pmd() doesn't have any limitation on refcounting:
|
||||
pmd can be split at any point and never fails.
|
||||
|
||||
== Partial unmap and deferred_split_huge_page() ==
|
||||
Partial unmap and deferred_split_huge_page()
|
||||
============================================
|
||||
|
||||
Unmapping part of THP (with munmap() or other way) is not going to free
|
||||
memory immediately. Instead, we detect that a subpage of THP is not in use
|
||||
|
|
|
|||
Loading…
Reference in New Issue