Re: Memory locking limit and zero-copy migrations

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On Thu, Aug 18, 2022 at 2:46 PM Milan Zamazal <mzamazal@xxxxxxxxxx> wrote:
Fangge Jin <fjin@xxxxxxxxxx> writes:

> I can share some test results with you:
> 1. If no memtune->hard_limit is set when start a vm, the default memlock
> hard limit is 64MB
> 2. If memtune->hard_limit is set when start a vm, memlock hard limit will
> be set to the value of memtune->hard_limit
> 3. If memtune->hard_limit is updated at run-time, memlock hard limit won't
> be changed accordingly
> And some additional knowledge:
> 1. memlock hard limit can be shown by ‘prlimit -p <pid-of-qemu> -l’
> 2. The default value of memlock hard limit can be changed by setting
> LimitMEMLOCK in /usr/lib/systemd/system/virtqemud.service

Ah, that explains it to me, thank you.  And since in the default case
the systemd limit is not reported in <memtune> of a running VM, I assume
libvirt takes it as "not set" and sets the higher limit when setting up
a zero-copy migration.  Good.
Not sure whether you already know this, but I had a hard time differentiating the two concepts:
1. memlock hard limit(shown by prlimit): the hard limit for locked host memory
2. memtune hard limit(memtune->hard_limit): the hard limit for in-use host memory, this memory can be swapped out.


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