377 lines
17 KiB
Plaintext
Executable File
377 lines
17 KiB
Plaintext
Executable File
= 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 is an alternative means 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 it only works for anonymous memory mappings but in the
|
|
future it can expand over the pagecache layer starting with tmpfs.
|
|
|
|
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) and 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.
|
|
|
|
== Design ==
|
|
|
|
- "graceful fallback": mm components which don't have transparent
|
|
hugepage knowledge fall back to breaking a transparent hugepage and
|
|
working on the regular pages and their respective regular pmd/pte
|
|
mappings
|
|
|
|
- if a hugepage allocation fails because of memory fragmentation,
|
|
regular pages should be gracefully allocated instead and mixed in
|
|
the same vma without any failure or significant delay and without
|
|
userland noticing
|
|
|
|
- if some task quits and more hugepages become available (either
|
|
immediately in the buddy or through the VM), guest physical memory
|
|
backed by regular pages should be relocated on hugepages
|
|
automatically (with khugepaged)
|
|
|
|
- it doesn't require memory reservation and in turn it uses hugepages
|
|
whenever possible (the only possible reservation here is kernelcore=
|
|
to avoid unmovable pages to fragment all the memory but such a tweak
|
|
is not specific to transparent hugepage support and it's a generic
|
|
feature that applies to all dynamic high order allocations in the
|
|
kernel)
|
|
|
|
- this initial support only offers the feature in the anonymous memory
|
|
regions but it'd be ideal to move it to tmpfs and the pagecache
|
|
later
|
|
|
|
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 ==
|
|
|
|
Transparent Hugepage Support can be entirely disabled (mostly for
|
|
debugging purposes) or only enabled inside MADV_HUGEPAGE regions (to
|
|
avoid the risk of consuming more memory resources) or enabled system
|
|
wide. This can be achieved with one of:
|
|
|
|
echo always >/sys/kernel/mm/transparent_hugepage/enabled
|
|
echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
|
|
echo never >/sys/kernel/mm/transparent_hugepage/enabled
|
|
|
|
It's also possible to limit defrag efforts in the VM to generate
|
|
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 madvise >/sys/kernel/mm/transparent_hugepage/defrag
|
|
echo never >/sys/kernel/mm/transparent_hugepage/defrag
|
|
|
|
By default kernel tries to use huge zero page on read page fault.
|
|
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
|
|
|
|
khugepaged will be automatically started when
|
|
transparent_hugepage/enabled is set to "always" or "madvise, and it'll
|
|
be automatically shutdown if it's set to "never".
|
|
|
|
khugepaged runs usually at low frequency so while one may not want to
|
|
invoke defrag algorithms synchronously during the page faults, it
|
|
should be worth invoking defrag at least in khugepaged. However it's
|
|
also possible to disable defrag in khugepaged by writing 0 or enable
|
|
defrag in khugepaged by writing 1:
|
|
|
|
echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
|
|
echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
|
|
|
|
You can also control how many pages khugepaged should scan at each
|
|
pass:
|
|
|
|
/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
|
|
|
|
and how many milliseconds to wait in khugepaged between each pass (you
|
|
can set this to 0 to run khugepaged at 100% utilization of one core):
|
|
|
|
/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
|
|
|
|
and how many milliseconds to wait in khugepaged if there's an hugepage
|
|
allocation failure to throttle the next allocation attempt.
|
|
|
|
/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
|
|
|
|
The khugepaged progress can be seen in the number of pages collapsed:
|
|
|
|
/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
|
|
|
|
for each pass:
|
|
|
|
/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
|
|
|
|
== 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"
|
|
(without "") to the kernel command line.
|
|
|
|
== Need of application restart ==
|
|
|
|
The transparent_hugepage/enabled values only affect future
|
|
behavior. So to make them effective you need to restart any
|
|
application that could have been using hugepages. This also applies to
|
|
the regions registered in khugepaged.
|
|
|
|
== Monitoring usage ==
|
|
|
|
The number of transparent huge pages currently used by the system is
|
|
available by reading the AnonHugePages field in /proc/meminfo. To
|
|
identify what applications are using transparent huge pages, it is
|
|
necessary to read /proc/PID/smaps and count the AnonHugePages fields
|
|
for each mapping. Note that reading the smaps file is expensive and
|
|
reading it frequently will incur overhead.
|
|
|
|
There are a number of counters in /proc/vmstat that may be used to
|
|
monitor how successfully the system is providing huge pages for use.
|
|
|
|
thp_fault_alloc is incremented every time a huge page is successfully
|
|
allocated to handle a page fault. This applies to both the
|
|
first time a page is faulted and for COW faults.
|
|
|
|
thp_collapse_alloc is incremented by khugepaged when it has found
|
|
a range of pages to collapse into one huge page and has
|
|
successfully allocated a new huge page to store the data.
|
|
|
|
thp_fault_fallback is incremented if a page fault fails to allocate
|
|
a huge page and instead falls back to using small pages.
|
|
|
|
thp_collapse_alloc_failed is incremented if khugepaged found a range
|
|
of pages that should be collapsed into one huge page but failed
|
|
the allocation.
|
|
|
|
thp_split 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.
|
|
|
|
thp_zero_page_alloc is incremented every time a huge zero page is
|
|
successfully allocated. It includes allocations which where
|
|
dropped due race with other allocation. Note, it doesn't count
|
|
every map of the huge zero page, only its allocation.
|
|
|
|
thp_zero_page_alloc_failed is incremented if kernel fails to allocate
|
|
huge zero page and falls back to using small pages.
|
|
|
|
As the system ages, allocating huge pages may be expensive as the
|
|
system uses memory compaction to copy data around memory to free a
|
|
huge page for use. There are some counters in /proc/vmstat to help
|
|
monitor this overhead.
|
|
|
|
compact_stall is incremented every time a process stalls to run
|
|
memory compaction so that a huge page is free for use.
|
|
|
|
compact_success is incremented if the system compacted memory and
|
|
freed a huge page for use.
|
|
|
|
compact_fail is incremented if the system tries to compact memory
|
|
but failed.
|
|
|
|
compact_pages_moved is incremented each time a page is moved. If
|
|
this value is increasing rapidly, it implies that the system
|
|
is copying a lot of data to satisfy the huge page allocation.
|
|
It is possible that the cost of copying exceeds any savings
|
|
from reduced TLB misses.
|
|
|
|
compact_pagemigrate_failed is incremented when the underlying mechanism
|
|
for moving a page failed.
|
|
|
|
compact_blocks_moved is incremented each time memory compaction examines
|
|
a huge page aligned range of pages.
|
|
|
|
It is possible to establish how long the stalls were using the function
|
|
tracer to record how long was spent in __alloc_pages_nodemask and
|
|
using the mm_page_alloc tracepoint to identify which allocations were
|
|
for huge pages.
|
|
|
|
== get_user_pages and follow_page ==
|
|
|
|
get_user_pages and follow_page if run on a hugepage, will return the
|
|
head or tail pages as usual (exactly as they would do on
|
|
hugetlbfs). Most gup users will only care about the actual physical
|
|
address of the page and its temporary pinning to release after the I/O
|
|
is complete, so they won't ever notice the fact the page is huge. But
|
|
if any driver is going to mangle over the page structure of the tail
|
|
page (like for checking page->mapping or other bits that are relevant
|
|
for the head page and not the tail page), it should be updated to jump
|
|
to check head page instead (while serializing properly against
|
|
split_huge_page() to avoid the head and tail pages to disappear from
|
|
under it, see the futex code to see an example of that, hugetlbfs also
|
|
needed special handling in futex code for similar reasons).
|
|
|
|
NOTE: these aren't new constraints to the GUP API, and they match the
|
|
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
|
|
follow_page, so that it will split the hugepages before returning
|
|
them. Migration for example passes FOLL_SPLIT as parameter to
|
|
follow_page because it's not hugepage aware and in fact it can't work
|
|
at all on hugetlbfs (but it instead works fine on transparent
|
|
hugepages thanks to FOLL_SPLIT). migration simply can't deal with
|
|
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 ==
|
|
|
|
To be guaranteed that the kernel will map a 2M page immediately in any
|
|
memory region, the mmap region has to be hugepage naturally
|
|
aligned. posix_memalign() can provide that guarantee.
|
|
|
|
== Hugetlbfs ==
|
|
|
|
You can use hugetlbfs on a kernel that has transparent hugepage
|
|
support enabled just fine as always. No difference can be noted in
|
|
hugetlbfs other than there will be less overall fragmentation. All
|
|
usual features belonging to hugetlbfs are preserved and
|
|
unaffected. libhugetlbfs will also work fine as usual.
|
|
|
|
== Graceful fallback ==
|
|
|
|
Code walking pagetables but unware about huge pmds can simply call
|
|
split_huge_page_pmd(vma, addr, pmd) where the pmd is the one returned by
|
|
pmd_offset. It's trivial to make the code transparent hugepage aware
|
|
by just grepping for "pmd_offset" and adding split_huge_page_pmd where
|
|
missing after pmd_offset returns the pmd. Thanks to the graceful
|
|
fallback design, with a one liner change, you can avoid to write
|
|
hundred if not thousand of lines of complex code to make your code
|
|
hugepage aware.
|
|
|
|
If you're not walking pagetables but you run into a physical hugepage
|
|
but you can't handle it natively in your code, you can split it by
|
|
calling split_huge_page(page). This is what the Linux VM does before
|
|
it tries to swapout the hugepage for example.
|
|
|
|
Example to make mremap.c transparent hugepage aware with a one liner
|
|
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;
|
|
|
|
pmd = pmd_offset(pud, addr);
|
|
+ split_huge_page_pmd(vma, addr, pmd);
|
|
if (pmd_none_or_clear_bad(pmd))
|
|
return NULL;
|
|
|
|
== Locking in hugepage aware code ==
|
|
|
|
We want as much code as possible hugepage aware, as calling
|
|
split_huge_page() or split_huge_page_pmd() has a cost.
|
|
|
|
To make pagetable walks huge pmd aware, all you need to do is to call
|
|
pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
|
|
mmap_sem in read (or write) mode to be sure an huge pmd cannot be
|
|
created from under you by khugepaged (khugepaged collapse_huge_page
|
|
takes the mmap_sem in write mode in addition to the anon_vma lock). If
|
|
pmd_trans_huge returns false, you just fallback in the old code
|
|
paths. If instead pmd_trans_huge returns true, you have to take the
|
|
mm->page_table_lock and re-run pmd_trans_huge. Taking the
|
|
page_table_lock will prevent the huge pmd to be converted into a
|
|
regular pmd from under you (split_huge_page can run in parallel to the
|
|
pagetable walk). If the second pmd_trans_huge returns false, you
|
|
should just drop the page_table_lock and fallback to the old code as
|
|
before. Otherwise you should run pmd_trans_splitting on the pmd. In
|
|
case pmd_trans_splitting returns true, it means split_huge_page is
|
|
already in the middle of splitting the page. So if pmd_trans_splitting
|
|
returns true it's enough to drop the page_table_lock and call
|
|
wait_split_huge_page and then fallback the old code paths. You are
|
|
guaranteed by the time wait_split_huge_page returns, the pmd isn't
|
|
huge anymore. If pmd_trans_splitting returns false, you can proceed to
|
|
process the huge pmd and the hugepage natively. Once finished you can
|
|
drop the page_table_lock.
|
|
|
|
== compound_lock, get_user_pages and put_page ==
|
|
|
|
split_huge_page internally has to distribute the refcounts in the head
|
|
page to the tail pages before clearing all PG_head/tail bits from the
|
|
page structures. It can do that easily for refcounts taken by huge pmd
|
|
mappings. But the GUI API as created by hugetlbfs (that returns head
|
|
and tail pages if running get_user_pages on an address backed by any
|
|
hugepage), requires the refcount to be accounted on the tail pages and
|
|
not only in the head pages, if we want to be able to run
|
|
split_huge_page while there are gup pins established on any tail
|
|
page. Failure to be able to run split_huge_page if there's any gup pin
|
|
on any tail page, would mean having to split all hugepages upfront in
|
|
get_user_pages which is unacceptable as too many gup users are
|
|
performance critical and they must work natively on hugepages like
|
|
they work natively on hugetlbfs already (hugetlbfs is simpler because
|
|
hugetlbfs pages cannot be split so there wouldn't be requirement of
|
|
accounting the pins on the tail pages for hugetlbfs). If we wouldn't
|
|
account the gup refcounts on the tail pages during gup, we won't know
|
|
anymore which tail page is pinned by gup and which is not while we run
|
|
split_huge_page. But we still have to add the gup pin to the head page
|
|
too, to know when we can free the compound page in case it's never
|
|
split during its lifetime. That requires changing not just
|
|
get_page, but put_page as well so that when put_page runs on a tail
|
|
page (and only on a tail page) it will find its respective head page,
|
|
and then it will decrease the head page refcount in addition to the
|
|
tail page refcount. To obtain a head page reliably and to decrease its
|
|
refcount without race conditions, put_page has to serialize against
|
|
__split_huge_page_refcount using a special per-page lock called
|
|
compound_lock.
|