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. Regards, Milan > BR, > 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. >> >>
