[Bug 1950186] Re: Nova doesn't account for hugepages when scheduling VMs

[James Page]

Another suggestion was to limit the 'max_unit' value for hypervisors
with this memory configuration to the total memory - the hugepage
configured memory - this means that the maximum footprint for a single
VM is limited.

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https://bugs.launchpad.net/bugs/1950186

Title:
  Nova doesn't account for hugepages when scheduling VMs

Status in nova package in Ubuntu:
  New

Bug description:
  Description
  ===========

  When hugepages are enabled on the host it's possible to schedule VMs
  using more RAM than available.

  On the node with memory usage presented below it was possible to
  schedule 6 instances using a total of 140G of memory and a non-
  hugepages-enabled flavor. The same machine has 188G of memory in
  total, of which 64G were reserved for hugepages. Additional ~4G were
  used for housekeeping, OpenStack control plane, etc. This resulted in
  overcommitment of roughly 20G.

  After running memory intensive operations on the VMs, some of them got
  OOM killed.

  $ cat /proc/meminfo  | egrep "^(Mem|Huge)" # on the compute node
  MemTotal:       197784792 kB
  MemFree:        115005288 kB
  MemAvailable:   116745612 kB
  HugePages_Total:      64
  HugePages_Free:       64
  HugePages_Rsvd:        0
  HugePages_Surp:        0
  Hugepagesize:    1048576 kB
  Hugetlb:        67108864 kB

  $ os hypervisor show copmute1 -c memory_mb -c memory_mb_used -c free_ram_mb
  +----------------+--------+
  | Field          | Value  |
  +----------------+--------+
  | free_ram_mb    | 29309  |
  | memory_mb      | 193149 |
  | memory_mb_used | 163840 |
  +----------------+--------+

  $ os host show compute1
  +----------+----------------------------------+-----+-----------+---------+
  | Host     | Project                          | CPU | Memory MB | Disk GB |
  +----------+----------------------------------+-----+-----------+---------+
  | compute1 | (total)                          |   0 |    193149 |     893 |
  | compute1 | (used_now)                       |  72 |    163840 |     460 |
  | compute1 | (used_max)                       |  72 |    147456 |     460 |
  | compute1 | some_project_id_was_here         |   2 |      4096 |      40 |
  | compute1 | another_anonymized_id_here       |  70 |    143360 |     420 |
  +----------+----------------------------------+-----+-----------+---------+

  $ os resource provider inventory list uuid_of_compute1_node
  +----------------+------------------+----------+----------+----------+-----------+--------+
  | resource_class | allocation_ratio | min_unit | max_unit | reserved | step_size |  total |
  +----------------+------------------+----------+----------+----------+-----------+--------+
  | MEMORY_MB      |              1.0 |        1 |   193149 |    16384 |         1 | 193149 |
  | DISK_GB        |              1.0 |        1 |      893 |        0 |         1 |    893 |
  | PCPU           |              1.0 |        1 |       72 |        0 |         1 |     72 |
  +----------------+------------------+----------+----------+----------+-----------+--------+

  Steps to reproduce
  ==================

  1. Reserve a large part of memory for hugepages on the hypervisor.
  2. Create VMs using a flavor that uses a lot of memory that isn't backed by hugepages.
  3. Start memory intensive operations on the VMs, e.g.:
  stress-ng --vm-bytes $(awk '/MemAvailable/{printf "%d", $2 * 0.98;}' < /proc/meminfo)k --vm-keep -m 1

  Expected result
  ===============

  Nova should not allow overcommitment and should be able to
  differentiate between hugepages and "normal" memory.

  Actual result
  =============
  Overcommitment resulting in OOM kills.

  Environment
  ===========
  nova-api-metadata 2:21.2.1-0ubuntu1~cloud0
  nova-common 2:21.2.1-0ubuntu1~cloud0
  nova-compute 2:21.2.1-0ubuntu1~cloud0
  nova-compute-kvm 2:21.2.1-0ubuntu1~cloud0
  nova-compute-libvirt 2:21.2.1-0ubuntu1~cloud0
  python3-nova 2:21.2.1-0ubuntu1~cloud0
  python3-novaclient 2:17.0.0-0ubuntu1~cloud0

  OS: Ubuntu 18.04.5 LTS
  Hypervisor: libvirt + KVM

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