RHEL8 - The pitfalls of using zone_reclaim_mode in a multi-node server

Solution In Progress - Updated -

Issue

There were changes between RHEL7 and RHEL8 which, under certain conditions, can severely impact the performance of an application/server by using vm.zone_reclaim_mode=n (any value other than 0) on a multi node server.

The impact may be seen in areas such as:
- Application performance issue
- Slow IO while free memory is available
- IO performance drops after time

In these examples, the performance decline is due to memory reclamation in a Memory Zone or Node that has become full even though there appears to be plenty of free memory overall. Unless absolutely necessary do not enable zone/node reclamation in RHEL8.

Environment

  • Red Hat Enterprise Linux 8
  • Application pinned to a NUMA Node (eg: with numactl or libnuma)
  • Async file write(s) larger than the memory in the NUMA Node
  • Kernel tunble vm.zone_reclaim_mode greater than zero

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