On Fri, Aug 19, 2022 at 08:01:58 +0200, Milan Zamazal wrote:
Fangge Jin <fjin@redhat.com> writes:
On Fri, Aug 19, 2022 at 4:08 AM Milan Zamazal <mzamazal@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.
Right, whenever libvirt needs to set a memory locking limit for zero copy migration it uses memtune->hard_limit if it is set (RDMA migration does the same), otherwise the configured memory amount for the domain is used. Anyway, unless you have a specific reason for setting the hard_limit, it's usually better to leave it unset as the overall memory consumption of a domain including the memory consumed by QEMU is hard to predict and you would risk the domain to be killed unexpectedly by the kernel when the limit is reached. Jirka