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> Lastly, qemu has overhead that varies depending on what you're doing in the
> guest.  In particular, there are various IO queues that can consume
> significant amounts of memory.  The company that I work for put in a good
> bit of effort engineering things so that they work more reliably, and part
> of that was determining how much memory to reserve for the host.
>
> Chris

Hi, I work with Jakub (the op of this thread) and here is my two
cents: I think what is critical to realize is that KVM virtual
machines can have substantial memory overhead of up to 25% of memory,
allocated to KVM virtual machine itself. This overhead memory is not
considered in nova code when calculating if the instance being
provisioned actually fits into host's available resources (only the
memory, configured in instance's flavor is considered). And this is
especially being a problem when CPU pinning is used as the memory
allocation is bounded by limits of specific NUMA node (due to the
strict memory allocation mode). This renders the global reservation
parameter reserved_host_memory_mb useless as it doesn't take NUMA into
account.

This KVM virtual machine overhead is what is causing the OOMs in our
infrastructure and that's what we need to fix.

Regards,
Prema