Re: Memory locking limit and zero-copy migrations

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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

Fangge Jin

On Wed, Aug 17, 2022 at 19:25 Milan Zamazal <mzamazal@xxxxxxxxxx> wrote:
Peter Krempa <pkrempa@xxxxxxxxxx> writes:

> On Wed, Aug 17, 2022 at 10:56:54 +0200, Milan Zamazal wrote:
>> Hi,
>> do I read libvirt sources right that when <memtune> is not used in the
>> libvirt domain then libvirt takes proper care about setting memory
>> locking limits when zero-copy is requested for a migration?
> Well yes, for a definition of "proper". In this instance qemu can lock
> up to the guest-visible memory size of memory for the migration, thus we
> set the lockable size to the guest memory size. This is a simple upper
> bound which is supposed to work in all scenarios. Qemu is also unlikely
> to ever use up all the allowed locking.

Great, thank you for confirmation.

>> I also wonder whether there are any other situations where memory limits
>> could be set by libvirt or QEMU automatically rather than having no
>> memory limits?  We had oVirt bugs in the past where certain VMs with
>> VFIO devices couldn't be started due to extra requirements on the amount
>> of locked memory and adding <hard_limit> to the domain apparently
>> helped.
> <hard_limit> is not only an amount of memory qemu can lock into ram, but
> an upper bound of all memory the qemu process can consume. This includes
> any qemu overhead e.g. used for the emulation layer.
> Guessing the correct size of overhead still has the same problems it had
> and libvirt is not going to be in the business of doing that.

To clarify, my point was not whether libvirt should, but whether libvirt
or any related component possibly does (or did in the past) impose
memory limits.  Because as I was looking around it seems there are no
real memory limits by default, at least in libvirt, but some limit had
been apparently hit in the reported bugs.

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