Transparent Hugepage Support

Objective

Performance critical computing applications dealing with large memory working sets are already running on top of libhugetlbfs and in turn hugetlbfs. Transparent HugePage Support (THP) is an alternative mean of using huge pages for the backing of virtual memory with huge pages that supports the automatic promotion and demotion of page sizes and without the shortcomings of hugetlbfs.

Currently THP only works for anonymous memory mappings and tmpfs/shmem. But in the future it can expand to other filesystems.

Note

in the examples below we presume that the basic page size is 4K and the huge page size is 2M, although the actual numbers may vary depending on the CPU architecture.

The reason applications are running faster is because of two factors. The first factor is almost completely irrelevant and it’s not of significant interest because it’ll also have the downside of requiring larger clear-page copy-page in page faults which is a potentially negative effect. The first factor consists in taking a single page fault for each 2M virtual region touched by userland (so reducing the enter/exit kernel frequency by a 512 times factor). This only matters the first time the memory is accessed for the lifetime of a memory mapping. The second long lasting and much more important factor will affect all subsequent accesses to the memory for the whole runtime of the application. The second factor consist of two components:

1. the TLB miss will run faster (especially with virtualization using nested pagetables but almost always also on bare metal without virtualization)

2. a single TLB entry will be mapping a much larger amount of virtual memory in turn reducing the number of TLB misses. With virtualization and nested pagetables the TLB can be mapped of larger size only if both KVM and the Linux guest are using hugepages but a significant speedup already happens if only one of the two is using hugepages just because of the fact the TLB miss is going to run faster.

Modern kernels support “multi-size THP” (mTHP), which introduces the ability to allocate memory in blocks that are bigger than a base page but smaller than traditional PMD-size (as described above), in increments of a power-of-2 number of pages. mTHP can back anonymous memory (for example 16K, 32K, 64K, etc). These THPs continue to be PTE-mapped, but in many cases can still provide similar benefits to those outlined above: Page faults are significantly reduced (by a factor of e.g. 4, 8, 16, etc), but latency spikes are much less prominent because the size of each page isn’t as huge as the PMD-sized variant and there is less memory to clear in each page fault. Some architectures also employ TLB compression mechanisms to squeeze more entries in when a set of PTEs are virtually and physically contiguous and approporiately aligned. In this case, TLB misses will occur less often.

THP can be enabled system wide or restricted to certain tasks or even memory ranges inside task’s address space. Unless THP is completely disabled, there is khugepaged daemon that scans memory and collapses sequences of basic pages into PMD-sized huge pages.

The THP behaviour is controlled via sysfs interface and using madvise(2) and prctl(2) system calls.

Transparent Hugepage Support maximizes the usefulness of free memory if compared to the reservation approach of hugetlbfs by allowing all unused memory to be used as cache or other movable (or even unmovable entities). It doesn’t require reservation to prevent hugepage allocation failures to be noticeable from userland. It allows paging and all other advanced VM features to be available on the hugepages. It requires no modifications for applications to take advantage of it.

Applications however can be further optimized to take advantage of this feature, like for example they’ve been optimized before to avoid a flood of mmap system calls for every malloc(4k). Optimizing userland is by far not mandatory and khugepaged already can take care of long lived page allocations even for hugepage unaware applications that deals with large amounts of memory.

In certain cases when hugepages are enabled system wide, application may end up allocating more memory resources. An application may mmap a large region but only touch 1 byte of it, in that case a 2M page might be allocated instead of a 4k page for no good. This is why it’s possible to disable hugepages system-wide and to only have them inside MADV_HUGEPAGE madvise regions.

Embedded systems should enable hugepages only inside madvise regions to eliminate any risk of wasting any precious byte of memory and to only run faster.

Applications that gets a lot of benefit from hugepages and that don’t risk to lose memory by using hugepages, should use madvise(MADV_HUGEPAGE) on their critical mmapped regions.

sysfs

Global THP controls

Transparent Hugepage Support for anonymous memory can be entirely disabled (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE regions (to avoid the risk of consuming more memory resources) or enabled system wide. This can be achieved per-supported-THP-size with one of:

echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled

where <size> is the hugepage size being addressed, the available sizes for which vary by system.

For example:

echo always >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled

Alternatively it is possible to specify that a given hugepage size will inherit the top-level “enabled” value:

echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled

For example:

echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled

The top-level setting (for use with “inherit”) can be set by issuing one of the following commands:

echo always >/sys/kernel/mm/transparent_hugepage/enabled
echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
echo never >/sys/kernel/mm/transparent_hugepage/enabled

By default, PMD-sized hugepages have enabled=”inherit” and all other hugepage sizes have enabled=”never”. If enabling multiple hugepage sizes, the kernel will select the most appropriate enabled size for a given allocation.

It’s also possible to limit defrag efforts in the VM to generate anonymous hugepages in case they’re not immediately free to madvise regions or to never try to defrag memory and simply fallback to regular pages unless hugepages are immediately available. Clearly if we spend CPU time to defrag memory, we would expect to gain even more by the fact we use hugepages later instead of regular pages. This isn’t always guaranteed, but it may be more likely in case the allocation is for a MADV_HUGEPAGE region.

echo always >/sys/kernel/mm/transparent_hugepage/defrag
echo defer >/sys/kernel/mm/transparent_hugepage/defrag
echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag
echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
echo never >/sys/kernel/mm/transparent_hugepage/defrag

always

means that an application requesting THP will stall on allocation failure and directly reclaim pages and compact memory in an effort to allocate a THP immediately. This may be desirable for virtual machines that benefit heavily from THP use and are willing to delay the VM start to utilise them.

defer

means that an application will wake kswapd in the background to reclaim pages and wake kcompactd to compact memory so that THP is available in the near future. It’s the responsibility of khugepaged to then install the THP pages later.

defer+madvise

will enter direct reclaim and compaction like always, but only for regions that have used madvise(MADV_HUGEPAGE); all other regions will wake kswapd in the background to reclaim pages and wake kcompactd to compact memory so that THP is available in the near future.

madvise

will enter direct reclaim like always but only for regions that are have used madvise(MADV_HUGEPAGE). This is the default behaviour.

never

should be self-explanatory.

By default kernel tries to use huge, PMD-mappable zero page on read page fault to anonymous mapping. It’s possible to disable huge zero page by writing 0 or enable it back by writing 1:

echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page

Some userspace (such as a test program, or an optimized memory allocation library) may want to know the size (in bytes) of a PMD-mappable transparent hugepage:

cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size

khugepaged will be automatically started when one or more hugepage sizes are enabled (either by directly setting “always” or “madvise”, or by setting “inherit” while the top-level enabled is set to “always” or “madvise”), and it’ll be automatically shutdown when the last hugepage size is disabled (either by directly setting “never”, or by setting “inherit” while the top-level enabled is set to “never”).

Khugepaged controls

Note

khugepaged currently only searches for opportunities to collapse to PMD-sized THP and no attempt is made to collapse to other THP sizes.

khugepaged runs usually at low frequency so while one may not want to invoke defrag algorithms synchronously during the page faults, it should be worth invoking defrag at least in khugepaged. However it’s also possible to disable defrag in khugepaged by writing 0 or enable defrag in khugepaged by writing 1:

echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag

You can also control how many pages khugepaged should scan at each pass:

/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan

and how many milliseconds to wait in khugepaged between each pass (you can set this to 0 to run khugepaged at 100% utilization of one core):

/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs

and how many milliseconds to wait in khugepaged if there’s an hugepage allocation failure to throttle the next allocation attempt:

/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs

The khugepaged progress can be seen in the number of pages collapsed (note that this counter may not be an exact count of the number of pages collapsed, since “collapsed” could mean multiple things: (1) A PTE mapping being replaced by a PMD mapping, or (2) All 4K physical pages replaced by one 2M hugepage. Each may happen independently, or together, depending on the type of memory and the failures that occur. As such, this value should be interpreted roughly as a sign of progress, and counters in /proc/vmstat consulted for more accurate accounting):

/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed

for each pass:

/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans

max_ptes_none specifies how many extra small pages (that are not already mapped) can be allocated when collapsing a group of small pages into one large page:

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none

A higher value leads to use additional memory for programs. A lower value leads to gain less thp performance. Value of max_ptes_none can waste cpu time very little, you can ignore it.

max_ptes_swap specifies how many pages can be brought in from swap when collapsing a group of pages into a transparent huge page:

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap

A higher value can cause excessive swap IO and waste memory. A lower value can prevent THPs from being collapsed, resulting fewer pages being collapsed into THPs, and lower memory access performance.

max_ptes_shared specifies how many pages can be shared across multiple processes. khugepaged might treat pages of THPs as shared if any page of that THP is shared. Exceeding the number would block the collapse:

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared

A higher value may increase memory footprint for some workloads.

Boot parameter

You can change the sysfs boot time defaults of Transparent Hugepage Support by passing the parameter transparent_hugepage=always or transparent_hugepage=madvise or transparent_hugepage=never to the kernel command line.

Hugepages in tmpfs/shmem

You can control hugepage allocation policy in tmpfs with mount option huge=. It can have following values:

always

Attempt to allocate huge pages every time we need a new page;

never

Do not allocate huge pages;

within_size

Only allocate huge page if it will be fully within i_size. Also respect fadvise()/madvise() hints;

advise

Only allocate huge pages if requested with fadvise()/madvise();

The default policy is never.

mount -o remount,huge= /mountpoint works fine after mount: remounting huge=never will not attempt to break up huge pages at all, just stop more from being allocated.

There’s also sysfs knob to control hugepage allocation policy for internal shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or MAP_ANONYMOUS), GPU drivers’ DRM objects, Ashmem.

In addition to policies listed above, shmem_enabled allows two further values:

deny

For use in emergencies, to force the huge option off from all mounts;

force

Force the huge option on for all - very useful for testing;

Need of application restart

The transparent_hugepage/enabled and transparent_hugepage/hugepages-<size>kB/enabled values and tmpfs mount option only affect future behavior. So to make them effective you need to restart any application that could have been using hugepages. This also applies to the regions registered in khugepaged.

Monitoring usage

Note

Currently the below counters only record events relating to PMD-sized THP. Events relating to other THP sizes are not included.

The number of PMD-sized anonymous transparent huge pages currently used by the system is available by reading the AnonHugePages field in /proc/meminfo. To identify what applications are using PMD-sized anonymous transparent huge pages, it is necessary to read /proc/PID/smaps and count the AnonHugePages fields for each mapping. (Note that AnonHugePages only applies to traditional PMD-sized THP for historical reasons and should have been called AnonHugePmdMapped).

The number of file transparent huge pages mapped to userspace is available by reading ShmemPmdMapped and ShmemHugePages fields in /proc/meminfo. To identify what applications are mapping file transparent huge pages, it is necessary to read /proc/PID/smaps and count the FileHugeMapped fields for each mapping.

Note that reading the smaps file is expensive and reading it frequently will incur overhead.

There are a number of counters in /proc/vmstat that may be used to monitor how successfully the system is providing huge pages for use.

thp_fault_alloc

is incremented every time a huge page is successfully allocated and charged to handle a page fault.

thp_collapse_alloc

is incremented by khugepaged when it has found a range of pages to collapse into one huge page and has successfully allocated a new huge page to store the data.

thp_fault_fallback

is incremented if a page fault fails to allocate or charge a huge page and instead falls back to using small pages.

thp_fault_fallback_charge

is incremented if a page fault fails to charge a huge page and instead falls back to using small pages even though the allocation was successful.

thp_collapse_alloc_failed

is incremented if khugepaged found a range of pages that should be collapsed into one huge page but failed the allocation.

thp_file_alloc

is incremented every time a file huge page is successfully allocated.

thp_file_fallback

is incremented if a file huge page is attempted to be allocated but fails and instead falls back to using small pages.

thp_file_fallback_charge

is incremented if a file huge page cannot be charged and instead falls back to using small pages even though the allocation was successful.

thp_file_mapped

is incremented every time a file huge page is mapped into user address space.

thp_split_page

is incremented every time a huge page is split into base pages. This can happen for a variety of reasons but a common reason is that a huge page is old and is being reclaimed. This action implies splitting all PMD the page mapped with.

thp_split_page_failed

is incremented if kernel fails to split huge page. This can happen if the page was pinned by somebody.

thp_deferred_split_page

is incremented when a huge page is put onto split queue. This happens when a huge page is partially unmapped and splitting it would free up some memory. Pages on split queue are going to be split under memory pressure.

thp_split_pmd

is incremented every time a PMD split into table of PTEs. This can happen, for instance, when application calls mprotect() or munmap() on part of huge page. It doesn’t split huge page, only page table entry.

thp_zero_page_alloc

is incremented every time a huge zero page used for thp is successfully allocated. Note, it doesn’t count every map of the huge zero page, only its allocation.

thp_zero_page_alloc_failed

is incremented if kernel fails to allocate huge zero page and falls back to using small pages.

thp_swpout

is incremented every time a huge page is swapout in one piece without splitting.

thp_swpout_fallback

is incremented if a huge page has to be split before swapout. Usually because failed to allocate some continuous swap space for the huge page.

In /sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats, There are also individual counters for each huge page size, which can be utilized to monitor the system’s effectiveness in providing huge pages for usage. Each counter has its own corresponding file.

anon_fault_alloc

is incremented every time a huge page is successfully allocated and charged to handle a page fault.

anon_fault_fallback

is incremented if a page fault fails to allocate or charge a huge page and instead falls back to using huge pages with lower orders or small pages.

anon_fault_fallback_charge

is incremented if a page fault fails to charge a huge page and instead falls back to using huge pages with lower orders or small pages even though the allocation was successful.

anon_swpout

is incremented every time a huge page is swapped out in one piece without splitting.

anon_swpout_fallback

is incremented if a huge page has to be split before swapout. Usually because failed to allocate some continuous swap space for the huge page.

As the system ages, allocating huge pages may be expensive as the system uses memory compaction to copy data around memory to free a huge page for use. There are some counters in /proc/vmstat to help monitor this overhead.

compact_stall

is incremented every time a process stalls to run memory compaction so that a huge page is free for use.

compact_success

is incremented if the system compacted memory and freed a huge page for use.

compact_fail

is incremented if the system tries to compact memory but failed.

It is possible to establish how long the stalls were using the function tracer to record how long was spent in __alloc_pages() and using the mm_page_alloc tracepoint to identify which allocations were for huge pages.

Optimizing the applications

To be guaranteed that the kernel will map a THP immediately in any memory region, the mmap region has to be hugepage naturally aligned. posix_memalign() can provide that guarantee.

Hugetlbfs

You can use hugetlbfs on a kernel that has transparent hugepage support enabled just fine as always. No difference can be noted in hugetlbfs other than there will be less overall fragmentation. All usual features belonging to hugetlbfs are preserved and unaffected. libhugetlbfs will also work fine as usual.