libvirt users - Aug 2022 - Memory locking limit and zero-copy migrations

On Fri, Aug 19, 2022 at 4:08 AM Milan Zamazal <mzamazal at redhat.com>
wrote:
> > 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.
>
> No, I didn't know it, thank you for pointing this out.  Indeed, 2. is
> what both the libvirt and kernel documentation seem to say, although not
> so clearly.
>
> But when I add <memtune> with <hard_limit> to the domain XML
and then
> start the VM, I can see the limit shown by `prlimit -l' is increased
> accordingly.  This is good for my use case, but does it match what you
> say about the two concepts?
memtune->hard_limit(hard limit of in-use memory) actually takes effect via
cgroup,
you can check the value by:
# virsh memtune uefi1
hard_limit     : 134217728
soft_limit     : unlimited
swap_hard_limit: unlimited
# cat
/sys/fs/cgroup/memory/machine.slice/machine-qemu\\x2d6\\x2duefi1.scope/libvirt/memory.limit_in_bytes

137438953472

When vm starts with memtune->hard_limit set in domain XML, memlock hard
limit(
hard_limit of locked memory, shown by 'prlimit -l')will be set to the
value
of
memtune->hard_limit. This's probably because memlock hard limit must be
less
than memtune->hard_limit.
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Fangge Jin <fjin at redhat.com> writes:
> On Fri, Aug 19, 2022 at 4:08 AM Milan Zamazal <mzamazal at
redhat.com> wrote:
>
>> > 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.
>>
>> No, I didn't know it, thank you for pointing this out.  Indeed, 2.
is
>> what both the libvirt and kernel documentation seem to say, although
not
>> so clearly.
>>
>> But when I add <memtune> with <hard_limit> to the domain
XML and then
>> start the VM, I can see the limit shown by `prlimit -l' is
increased
>> accordingly.  This is good for my use case, but does it match what you
>> say about the two concepts?
>
> memtune->hard_limit(hard limit of in-use memory) actually takes effect
via
> cgroup,
> you can check the value by:
> # virsh memtune uefi1
> hard_limit     : 134217728
> soft_limit     : unlimited
> swap_hard_limit: unlimited
> # cat
>
/sys/fs/cgroup/memory/machine.slice/machine-qemu\\x2d6\\x2duefi1.scope/libvirt/memory.limit_in_bytes
>
> 137438953472
>
> When vm starts with memtune->hard_limit set in domain XML, memlock
> hard limit( hard_limit of locked memory, shown by 'prlimit -l')will
be
> set to the value of memtune->hard_limit. This's probably because
> memlock hard limit must be less than memtune->hard_limit.
Well, increasing the memlock limit to keep it within memtune->hard_limit
wouldn't make much sense, but thank you for confirming that setting
memtune->hard_limit adjusts both the limits to the requested value.