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manpages/virsh.rst: clarify numatune memory migration on Linux

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On Linux, changing the nodeset on 'numatune' does not imply that
the guest memory will be migrated on the spot to the new nodeset.
The memory migration is tied on guest usage of the memory pages,
and an idle guest will take longer to have its memory migrated
to the new nodeset.

This is a behavior explained in detail in the Linux kernel
documentation in Documentation/admin-guide/cgroup-v1/cpusets.rst.
The user doesn't need this level of detail though - just needs
his/her expectations under check. Running 'numastat' and hoping
for instant memory migration from the previous nodeset to the new
one is not viable.

There's also parts of the memory that are locked by QEMU in the
same place, e.g. when VFIO devices are present. Let's also
mention it as another factor that impacts the results the
user might expect from NUMA memory migration with numatune.

https://bugzilla.redhat.com/show_bug.cgi?id=1640869

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
This commit is contained in:
Daniel Henrique Barboza 2020-06-11 14:00:29 -03:00 committed by Andrea Bolognani
parent 8f58a40035
commit 3a58613b0c
1 changed files with 8 additions and 0 deletions
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@ -3395,6 +3395,14 @@ If *--live* is specified, set scheduler information of a running guest.
If *--config* is specified, affect the next boot of a persistent guest.
If *--current* is specified, affect the current guest state.
For running guests in Linux hosts, the changes made in the domain's
numa parameters does not imply that the guest memory will be moved to a
different nodeset immediately. The memory migration depends on the
guest activity, and the memory of an idle guest will remain in its
previous nodeset for longer. The presence of VFIO devices will also
lock parts of the guest memory in the same nodeset used to start the
guest, regardless of nodeset changes.
perf
----