136 lines
6.0 KiB
ReStructuredText
136 lines
6.0 KiB
ReStructuredText
==================
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HugeTLB Controller
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==================
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HugeTLB controller can be created by first mounting the cgroup filesystem.
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# mount -t cgroup -o hugetlb none /sys/fs/cgroup
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With the above step, the initial or the parent HugeTLB group becomes
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visible at /sys/fs/cgroup. At bootup, this group includes all the tasks in
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the system. /sys/fs/cgroup/tasks lists the tasks in this cgroup.
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New groups can be created under the parent group /sys/fs/cgroup::
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# cd /sys/fs/cgroup
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# mkdir g1
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# echo $$ > g1/tasks
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The above steps create a new group g1 and move the current shell
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process (bash) into it.
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Brief summary of control files::
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hugetlb.<hugepagesize>.rsvd.limit_in_bytes # set/show limit of "hugepagesize" hugetlb reservations
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hugetlb.<hugepagesize>.rsvd.max_usage_in_bytes # show max "hugepagesize" hugetlb reservations and no-reserve faults
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hugetlb.<hugepagesize>.rsvd.usage_in_bytes # show current reservations and no-reserve faults for "hugepagesize" hugetlb
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hugetlb.<hugepagesize>.rsvd.failcnt # show the number of allocation failure due to HugeTLB reservation limit
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hugetlb.<hugepagesize>.limit_in_bytes # set/show limit of "hugepagesize" hugetlb faults
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hugetlb.<hugepagesize>.max_usage_in_bytes # show max "hugepagesize" hugetlb usage recorded
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hugetlb.<hugepagesize>.usage_in_bytes # show current usage for "hugepagesize" hugetlb
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hugetlb.<hugepagesize>.failcnt # show the number of allocation failure due to HugeTLB usage limit
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hugetlb.<hugepagesize>.numa_stat # show the numa information of the hugetlb memory charged to this cgroup
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For a system supporting three hugepage sizes (64k, 32M and 1G), the control
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files include::
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hugetlb.1GB.limit_in_bytes
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hugetlb.1GB.max_usage_in_bytes
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hugetlb.1GB.numa_stat
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hugetlb.1GB.usage_in_bytes
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hugetlb.1GB.failcnt
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hugetlb.1GB.rsvd.limit_in_bytes
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hugetlb.1GB.rsvd.max_usage_in_bytes
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hugetlb.1GB.rsvd.usage_in_bytes
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hugetlb.1GB.rsvd.failcnt
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hugetlb.64KB.limit_in_bytes
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hugetlb.64KB.max_usage_in_bytes
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hugetlb.64KB.numa_stat
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hugetlb.64KB.usage_in_bytes
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hugetlb.64KB.failcnt
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hugetlb.64KB.rsvd.limit_in_bytes
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hugetlb.64KB.rsvd.max_usage_in_bytes
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hugetlb.64KB.rsvd.usage_in_bytes
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hugetlb.64KB.rsvd.failcnt
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hugetlb.32MB.limit_in_bytes
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hugetlb.32MB.max_usage_in_bytes
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hugetlb.32MB.numa_stat
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hugetlb.32MB.usage_in_bytes
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hugetlb.32MB.failcnt
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hugetlb.32MB.rsvd.limit_in_bytes
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hugetlb.32MB.rsvd.max_usage_in_bytes
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hugetlb.32MB.rsvd.usage_in_bytes
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hugetlb.32MB.rsvd.failcnt
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1. Page fault accounting
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hugetlb.<hugepagesize>.limit_in_bytes
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hugetlb.<hugepagesize>.max_usage_in_bytes
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hugetlb.<hugepagesize>.usage_in_bytes
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hugetlb.<hugepagesize>.failcnt
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The HugeTLB controller allows users to limit the HugeTLB usage (page fault) per
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control group and enforces the limit during page fault. Since HugeTLB
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doesn't support page reclaim, enforcing the limit at page fault time implies
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that, the application will get SIGBUS signal if it tries to fault in HugeTLB
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pages beyond its limit. Therefore the application needs to know exactly how many
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HugeTLB pages it uses before hand, and the sysadmin needs to make sure that
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there are enough available on the machine for all the users to avoid processes
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getting SIGBUS.
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2. Reservation accounting
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hugetlb.<hugepagesize>.rsvd.limit_in_bytes
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hugetlb.<hugepagesize>.rsvd.max_usage_in_bytes
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hugetlb.<hugepagesize>.rsvd.usage_in_bytes
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hugetlb.<hugepagesize>.rsvd.failcnt
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The HugeTLB controller allows to limit the HugeTLB reservations per control
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group and enforces the controller limit at reservation time and at the fault of
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HugeTLB memory for which no reservation exists. Since reservation limits are
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enforced at reservation time (on mmap or shget), reservation limits never causes
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the application to get SIGBUS signal if the memory was reserved before hand. For
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MAP_NORESERVE allocations, the reservation limit behaves the same as the fault
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limit, enforcing memory usage at fault time and causing the application to
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receive a SIGBUS if it's crossing its limit.
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Reservation limits are superior to page fault limits described above, since
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reservation limits are enforced at reservation time (on mmap or shget), and
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never causes the application to get SIGBUS signal if the memory was reserved
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before hand. This allows for easier fallback to alternatives such as
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non-HugeTLB memory for example. In the case of page fault accounting, it's very
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hard to avoid processes getting SIGBUS since the sysadmin needs precisely know
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the HugeTLB usage of all the tasks in the system and make sure there is enough
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pages to satisfy all requests. Avoiding tasks getting SIGBUS on overcommited
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systems is practically impossible with page fault accounting.
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3. Caveats with shared memory
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For shared HugeTLB memory, both HugeTLB reservation and page faults are charged
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to the first task that causes the memory to be reserved or faulted, and all
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subsequent uses of this reserved or faulted memory is done without charging.
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Shared HugeTLB memory is only uncharged when it is unreserved or deallocated.
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This is usually when the HugeTLB file is deleted, and not when the task that
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caused the reservation or fault has exited.
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4. Caveats with HugeTLB cgroup offline.
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When a HugeTLB cgroup goes offline with some reservations or faults still
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charged to it, the behavior is as follows:
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- The fault charges are charged to the parent HugeTLB cgroup (reparented),
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- the reservation charges remain on the offline HugeTLB cgroup.
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This means that if a HugeTLB cgroup gets offlined while there is still HugeTLB
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reservations charged to it, that cgroup persists as a zombie until all HugeTLB
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reservations are uncharged. HugeTLB reservations behave in this manner to match
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the memory controller whose cgroups also persist as zombie until all charged
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memory is uncharged. Also, the tracking of HugeTLB reservations is a bit more
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complex compared to the tracking of HugeTLB faults, so it is significantly
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harder to reparent reservations at offline time.
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