Re: [libvirt-users] starting a domain only when you have enough resources

On Tue, Jan 26, 2016 at 1:51 PM, Michal Privoznik <mprivozn(a)redhat.com&gt; wrote: > On 26.01.2016 14:35, Andrei Perietanu wrote: >> On Tue, Jan 26, 2016 at 12:39 PM, Michal Privoznik <mprivozn(a)redhat.com&gt; >> wrote: >> >>> On 26.01.2016 12:30, Andrei Perietanu wrote: >>>> Hi all, >>>> >>>> I am running KVM on a 3.18 kernel. The system runs and Atom processor >>> with >>>> 2Gb RAM. >>>> >>>> Using KVM you obviously can over allocate your resources: say you have > 4 >>>> guests each configured with 1GB ram. Running all four at the same time, >>>> depending on the workload, can crash the system - I get a kernel trace >>> when >>>> this happens. >>>> >>>> But let's consider a simpler case: one guest with 1.5 Gb RAM, ubuntu >>> 14.03. >>>> During the installation the system will again crash. >>>> The memory statistics (top or proc/meminfo) will show that the > FreeMemory >>>> goes down to 12Mb when this happens - which kind of makes sense >>> considering >>>> the host will require some RAM to run. >>>> >>>> But the question is: does libvirt offer any way to prevent this from >>>> happening? >>>> >>>> Some way of not allowing the user to start a guest unless you have > enough >>>> free memory. I know how much ram each guest has configured but that is >>> not >>>> enough. I need to know how much the system has available, and just >>> reading >>>> the free memory statistic does not help much since that is only a >>> snapshot >>>> - when running a guest you can have 1gb free now, and 10 mb free 2 min >>>> later. >>>> >>>> Any ideas? >>> >>> There is one option I see, use -mem-prealloc. Either you can passthrough >>> it onto qemu commandline [1] or use locked memoryBacking [2]. I advocate >>> for the latter though. Not only it will allocate all the memory at qemu >>> startup it will also lock it so it won't get swapped off. >>> >>> Michal >>> >>> 1: http://libvirt.org/drvqemu.html#qemucommand >>> 2: http://libvirt.org/formatdomain.html#elementsMemoryBacking >>> >> >> I tried memoryBacking: I added this to the domain xml: >> <memoryBacking> >> <locked/> >> </memoryBacking> >> >> And got an error when attempting to start the vm: memory locking no >> supported by QEMU binary. >> >> Aside from that I don't really understand how this helps solve my issue - >> if you don't mind going a bit into details I'd appreciate it. >> > > Well, it will make qemu to allocate all its memory on the start. So > either the allocation would be successful and domain will run or it > won't and qemu will die immediately. > > On the other hand, your kernel is heavily broken too. Instead of > crashing at OOM it should kill a process (usually the one that's > consuming the most memory => qemu in this case). > > Michal > Sorry, I said crashing; I meant it spits out a kernel trace. You can still use the system after it happens. I just want to prevent from happening in the first place.