439 lines
19 KiB
Plaintext
439 lines
19 KiB
Plaintext
= Transparent Hugepage Support =
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== Objective ==
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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 is an alternative means 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 it only works for anonymous memory mappings but in the
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future it can expand over the pagecache layer starting with tmpfs.
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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: 1) the TLB miss will run faster (especially with
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virtualization using nested pagetables but almost always also on bare
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metal without virtualization) and 2) a single TLB entry will be
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mapping a much larger amount of virtual memory in turn reducing the
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number of TLB misses. With virtualization and nested pagetables the
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TLB can be mapped of larger size only if both KVM and the Linux guest
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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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- "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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if necessary, split a transparent hugepage. Therefore these components
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can continue working on the regular pages or regular pte mappings.
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- if a hugepage allocation fails because of memory fragmentation,
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regular pages should be gracefully allocated instead and mixed in
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the same vma without any failure or significant delay and without
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userland noticing
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- if some task quits and more hugepages become available (either
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immediately in the buddy or through the VM), guest physical memory
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backed by regular pages should be relocated on hugepages
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automatically (with khugepaged)
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- it doesn't require memory reservation and in turn it uses hugepages
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whenever possible (the only possible reservation here is kernelcore=
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to avoid unmovable pages to fragment all the memory but such a tweak
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is not specific to transparent hugepage support and it's a generic
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feature that applies to all dynamic high order allocations in the
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kernel)
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- this initial support only offers the feature in the anonymous memory
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regions but it'd be ideal to move it to tmpfs and the pagecache
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later
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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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== sysfs ==
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Transparent Hugepage Support can be entirely disabled (mostly for
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debugging purposes) or only enabled inside MADV_HUGEPAGE regions (to
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avoid the risk of consuming more memory resources) or enabled system
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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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hugepages in case they're not immediately free to madvise regions or
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to never try to defrag memory and simply fallback to regular pages
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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
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we 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 madvise >/sys/kernel/mm/transparent_hugepage/defrag
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echo never >/sys/kernel/mm/transparent_hugepage/defrag
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By default kernel tries to use huge zero page on read page fault.
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It's possible to disable huge zero page by writing 0 or enable it
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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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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 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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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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== Need of application restart ==
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The transparent_hugepage/enabled values only affect future
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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
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the regions registered in khugepaged.
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== Monitoring usage ==
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The number of transparent huge pages currently used by the system is
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available by reading the AnonHugePages field in /proc/meminfo. To
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identify what applications are using transparent huge pages, it is
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necessary to read /proc/PID/smaps and count the AnonHugePages fields
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for each mapping. Note that reading the smaps file is expensive and
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reading it 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 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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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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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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of pages that should be collapsed into one huge page but failed
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the allocation.
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thp_split_page 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 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_split_pmd 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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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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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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monitor this overhead.
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compact_stall 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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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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but failed.
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compact_pages_moved 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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for moving a page failed.
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compact_blocks_moved 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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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 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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hugetlbfs). Most gup users will only care about the actual physical
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address of the page and its temporary pinning to release after the I/O
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is complete, so they won't ever notice the fact the page is huge. But
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if any driver is going to mangle over the page structure of the tail
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page (like for checking page->mapping or other bits that are relevant
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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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In case you can't handle compound pages if they're returned by
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follow_page, the FOLL_SPLIT bit can be specified as parameter to
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follow_page, so that it will split the hugepages before returning
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them. Migration for example passes FOLL_SPLIT as parameter to
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follow_page because it's not hugepage aware and in fact it can't work
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at all on hugetlbfs (but it instead works fine on transparent
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hugepages thanks to FOLL_SPLIT). migration simply can't deal with
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hugepages being returned (as it's not only checking the pfn of the
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page and pinning it during the copy but it pretends to migrate the
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memory in regular page sizes and with regular pte/pmd mappings).
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== Optimizing the applications ==
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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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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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== Graceful fallback ==
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Code walking pagetables but unware about huge pmds can simply call
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split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
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pmd_offset. It's trivial to make the code transparent hugepage aware
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by just grepping for "pmd_offset" and adding split_huge_pmd where
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missing after pmd_offset returns the pmd. Thanks to the graceful
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fallback design, with a one liner change, you can avoid to write
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hundred if not thousand of lines of complex code to make your code
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hugepage aware.
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If you're not walking pagetables but you run into a physical hugepage
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but you can't handle it natively in your code, you can split it by
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calling split_huge_page(page). This is what the Linux VM does before
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it tries to swapout the hugepage for example. split_huge_page() can fail
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if the page is pinned and you must handle this correctly.
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Example to make mremap.c transparent hugepage aware with a one liner
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change:
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diff --git a/mm/mremap.c b/mm/mremap.c
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--- a/mm/mremap.c
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+++ b/mm/mremap.c
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@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
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return NULL;
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pmd = pmd_offset(pud, addr);
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+ split_huge_pmd(vma, pmd, addr);
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if (pmd_none_or_clear_bad(pmd))
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return NULL;
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== Locking in hugepage aware code ==
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We want as much code as possible hugepage aware, as calling
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split_huge_page() or split_huge_pmd() has a cost.
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To make pagetable walks huge pmd aware, all you need to do is to call
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pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
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mmap_sem in read (or write) mode to be sure an huge pmd cannot be
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created from under you by khugepaged (khugepaged collapse_huge_page
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takes the mmap_sem in write mode in addition to the anon_vma lock). If
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pmd_trans_huge returns false, you just fallback in the old code
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paths. If instead pmd_trans_huge returns true, you have to take the
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page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
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page table lock will prevent the huge pmd to be converted into a
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regular pmd from under you (split_huge_pmd can run in parallel to the
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pagetable walk). If the second pmd_trans_huge returns false, you
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should just drop the page table lock and fallback to the old code as
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before. Otherwise you can proceed to process the huge pmd and the
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hugepage natively. Once finished you can drop the page table lock.
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== Refcounts and transparent huge pages ==
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Refcounting on THP is mostly consistent with refcounting on other compound
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pages:
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- get_page()/put_page() and GUP operate in head page's ->_count.
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- ->_count in tail pages is always zero: get_page_unless_zero() never
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succeed on tail pages.
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- map/unmap of the pages with PTE entry increment/decrement ->_mapcount
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on relevant sub-page of the compound page.
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- map/unmap of the whole compound page accounted in compound_mapcount
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(stored in first tail page).
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PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one.
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This additional reference is required to get race-free detection of unmap of
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subpages when we have them mapped with both PMDs and PTEs.
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This is optimization required to lower overhead of per-subpage mapcount
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tracking. The alternative is alter ->_mapcount in all subpages on each
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map/unmap of the whole compound page.
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We set PG_double_map when a PMD of the page got split for the first time,
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but still have PMD mapping. The addtional references go away with last
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compound_mapcount.
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split_huge_page internally has to distribute the refcounts in the head
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page to the tail pages before clearing all PG_head/tail bits from the page
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structures. It can be done easily for refcounts taken by page table
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entries. But we don't have enough information on how to distribute any
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additional pins (i.e. from get_user_pages). split_huge_page() fails any
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requests to split pinned huge page: it expects page count to be equal to
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sum of mapcount of all sub-pages plus one (split_huge_page caller must
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have reference for head page).
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split_huge_page uses migration entries to stabilize page->_count and
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page->_mapcount.
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We safe against physical memory scanners too: the only legitimate way
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scanner can get reference to a page is get_page_unless_zero().
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All tail pages has zero ->_count until atomic_add(). It prevent scanner
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from geting reference to tail page up to the point. After the atomic_add()
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we don't care about ->_count value. We already known how many references
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with should uncharge from head page.
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For head page get_page_unless_zero() will succeed and we don't mind. It's
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clear where reference should go after split: it will stay on head page.
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Note that split_huge_pmd() doesn't have any limitation on refcounting:
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pmd can be split at any point and never fails.
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== Partial unmap and deferred_split_huge_page() ==
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Unmapping part of THP (with munmap() or other way) is not going to free
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memory immediately. Instead, we detect that a subpage of THP is not in use
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in page_remove_rmap() and queue the THP for splitting if memory pressure
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comes. Splitting will free up unused subpages.
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Splitting the page right away is not an option due to locking context in
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the place where we can detect partial unmap. It's also might be
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counterproductive since in many cases partial unmap unmap happens during
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exit(2) if an THP crosses VMA boundary.
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Function deferred_split_huge_page() is used to queue page for splitting.
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The splitting itself will happen when we get memory pressure via shrinker
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interface.
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