Re: [libvirt] question about rdma migration

Hi Roy, On 02/09/2016 03:57 AM, Roy Shterman wrote: > Hi, > > I tried to understand the rdma-migration in qemu code and i have two > questions about it: > > 1. I'm working with qemu-kvm using libvirt and i'm getting > > MEMLOCK max locked-in-memory address space 65536 65536 bytes > > in qemu process so I don't understand how can you use rdma-pin-all with > such low MEMLOCK. > > I found a solution in libvirt to lock all vm memory in advance and to > enlarge MEMLOCK. > It uses memoryBacking locking and memory tuning hard_limit of vm memory > but I couldn't find a usage of this in rdma-migration code. > > You're absolutey right, the RDMA migration code itself doesn't set this lock limit explicitly because there are system-wide restrictions in both appArmour, /etc/security, as well as SELINUX that restrict applications from arbitrarily setting their maximum memory lock limits. The other problem is CGROUPS: If someone sets a cgroup control for maximum memory and forgets about that mlock() limits, then there will be a conflict. So, libvirt must have a policy to deal with all of these possibilities, not just handle a special case for RDMA migration. The only way "simple" way (without patching the problems above) to apply a higher lock limit to QEMU is to set the ulimit for libvirt (or for QEMU if starting QEMU manually) in your environment or the command line with $ ulimit # before attempting the migration, then the RDMA subsystem will be able to lock the memory successfully. The other option is to use /etc/security/limits.conf and set the option for a specific libvirt process user and make sure your libvirt/qemu are not running as root. QEMU itself also has a "mlock" option built into the command line, but it also suffers from the same problem --- you have to find a way (currently) to increase the limit before using the option. 2. Do you have any comparison of IOPS and bandwidth between TCP migration > and rdma migration? > > Yes, lots of comparisons. http://wiki.qemu.org/Features/RDMALiveMigration http://www.canturkisci.com/ETC/papers/IBMJRD2011/preprint.pdf Regards, > Roy > > >