==================
HugeTLB Controller
==================

HugeTLB controller can be created by first mounting the cgroup filesystem.

# mount -t cgroup -o hugetlb none /sys/fs/cgroup

With the above step, the initial or the parent HugeTLB group becomes
visible at /sys/fs/cgroup. At bootup, this group includes all the tasks in
the system. /sys/fs/cgroup/tasks lists the tasks in this cgroup.

New groups can be created under the parent group /sys/fs/cgroup::

  # cd /sys/fs/cgroup
  # mkdir g1
  # echo $$ > g1/tasks

The above steps create a new group g1 and move the current shell
process (bash) into it.

Brief summary of control files::

 hugetlb.<hugepagesize>.rsvd.limit_in_bytes            # set/show limit of "hugepagesize" hugetlb reservations
 hugetlb.<hugepagesize>.rsvd.max_usage_in_bytes        # show max "hugepagesize" hugetlb reservations and no-reserve faults
 hugetlb.<hugepagesize>.rsvd.usage_in_bytes            # show current reservations and no-reserve faults for "hugepagesize" hugetlb
 hugetlb.<hugepagesize>.rsvd.failcnt                   # show the number of allocation failure due to HugeTLB reservation limit
 hugetlb.<hugepagesize>.limit_in_bytes                 # set/show limit of "hugepagesize" hugetlb faults
 hugetlb.<hugepagesize>.max_usage_in_bytes             # show max "hugepagesize" hugetlb  usage recorded
 hugetlb.<hugepagesize>.usage_in_bytes                 # show current usage for "hugepagesize" hugetlb
 hugetlb.<hugepagesize>.failcnt                        # show the number of allocation failure due to HugeTLB usage limit
 hugetlb.<hugepagesize>.numa_stat                      # show the numa information of the hugetlb memory charged to this cgroup

For a system supporting three hugepage sizes (64k, 32M and 1G), the control
files include::

  hugetlb.1GB.limit_in_bytes
  hugetlb.1GB.max_usage_in_bytes
  hugetlb.1GB.numa_stat
  hugetlb.1GB.usage_in_bytes
  hugetlb.1GB.failcnt
  hugetlb.1GB.rsvd.limit_in_bytes
  hugetlb.1GB.rsvd.max_usage_in_bytes
  hugetlb.1GB.rsvd.usage_in_bytes
  hugetlb.1GB.rsvd.failcnt
  hugetlb.64KB.limit_in_bytes
  hugetlb.64KB.max_usage_in_bytes
  hugetlb.64KB.numa_stat
  hugetlb.64KB.usage_in_bytes
  hugetlb.64KB.failcnt
  hugetlb.64KB.rsvd.limit_in_bytes
  hugetlb.64KB.rsvd.max_usage_in_bytes
  hugetlb.64KB.rsvd.usage_in_bytes
  hugetlb.64KB.rsvd.failcnt
  hugetlb.32MB.limit_in_bytes
  hugetlb.32MB.max_usage_in_bytes
  hugetlb.32MB.numa_stat
  hugetlb.32MB.usage_in_bytes
  hugetlb.32MB.failcnt
  hugetlb.32MB.rsvd.limit_in_bytes
  hugetlb.32MB.rsvd.max_usage_in_bytes
  hugetlb.32MB.rsvd.usage_in_bytes
  hugetlb.32MB.rsvd.failcnt


1. Page fault accounting

::

  hugetlb.<hugepagesize>.limit_in_bytes
  hugetlb.<hugepagesize>.max_usage_in_bytes
  hugetlb.<hugepagesize>.usage_in_bytes
  hugetlb.<hugepagesize>.failcnt

The HugeTLB controller allows users to limit the HugeTLB usage (page fault) per
control group and enforces the limit during page fault. Since HugeTLB
doesn't support page reclaim, enforcing the limit at page fault time implies
that, the application will get SIGBUS signal if it tries to fault in HugeTLB
pages beyond its limit. Therefore the application needs to know exactly how many
HugeTLB pages it uses beforehand, and the sysadmin needs to make sure that
there are enough available on the machine for all the users to avoid processes
getting SIGBUS.


2. Reservation accounting

::

  hugetlb.<hugepagesize>.rsvd.limit_in_bytes
  hugetlb.<hugepagesize>.rsvd.max_usage_in_bytes
  hugetlb.<hugepagesize>.rsvd.usage_in_bytes
  hugetlb.<hugepagesize>.rsvd.failcnt

The HugeTLB controller allows limiting the HugeTLB reservations per control
group and enforces the controller limit at reservation time and at the fault of
HugeTLB memory for which no reservation exists. Since reservation limits are
enforced at reservation time (on mmap or shget), reservation limits never cause
the application to get SIGBUS signal if the memory was reserved beforehand. For
MAP_NORESERVE allocations, the reservation limit behaves the same as the fault
limit, enforcing memory usage at fault time and causing the application to
receive a SIGBUS if it's crossing its limit.

Reservation limits are superior to page fault limits described above, since
reservation limits are enforced at reservation time (on mmap or shget), and
never cause the application to get SIGBUS signal if the memory was reserved
beforehand. This allows for easier fallback to alternatives such as
non-HugeTLB memory for example. In the case of page fault accounting, it's very
hard to avoid processes getting SIGBUS since the sysadmin needs to precisely know
the HugeTLB usage of all the tasks in the system and make sure there are enough
pages to satisfy all requests. Avoiding tasks getting SIGBUS on overcommitted
systems is practically impossible with page fault accounting.


3. Caveats with shared memory

For shared HugeTLB memory, both HugeTLB reservation and page faults are charged
to the first task that causes the memory to be reserved or faulted, and all
subsequent uses of this reserved or faulted memory is done without charging.

Shared HugeTLB memory is only uncharged when it is unreserved or deallocated.
This is usually when the HugeTLB file is deleted, and not when the task that
caused the reservation or fault has exited.


4. Caveats with HugeTLB cgroup offline.

When a HugeTLB cgroup goes offline with some reservations or faults still
charged to it, the behavior is as follows:

- The fault charges are charged to the parent HugeTLB cgroup (reparented),
- the reservation charges remain on the offline HugeTLB cgroup.

This means that if a HugeTLB cgroup gets offlined while there is still HugeTLB
reservations charged to it, that cgroup persists as a zombie until all HugeTLB
reservations are uncharged. HugeTLB reservations behave in this manner to match
the memory controller whose cgroups also persist as zombie until all charged
memory is uncharged. Also, the tracking of HugeTLB reservations is a bit more
complex compared to the tracking of HugeTLB faults, so it is significantly
harder to reparent reservations at offline time.
