Re: Memory locking limit and zero-copy migrations

Fangge Jin <fjin(a)redhat.com&gt; writes: > On Fri, Aug 19, 2022 at 4:08 AM Milan Zamazal <mzamazal(a)redhat.com&gt; 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.