* Jay Zhou (jianjay.z...@huawei.com) wrote: > > On 2017/7/24 23:35, Dr. David Alan Gilbert wrote: > > * Jay Zhou (jianjay.z...@huawei.com) wrote: > > > Hi Dave, > > > > > > On 2017/7/21 17:49, Dr. David Alan Gilbert wrote: > > > > * Jay Zhou (jianjay.z...@huawei.com) wrote: > > > > > Qemu_savevm_state_cleanup() takes about 300ms in my ram migration > > > > > tests > > > > > with a 8U24G vm(20G is really occupied), the main cost comes from > > > > > KVM_SET_USER_MEMORY_REGION ioctl when mem.memory_size = 0 in > > > > > kvm_set_user_memory_region(). In kmod, the main cost is > > > > > kvm_zap_obsolete_pages(), which traverses the active_mmu_pages list to > > > > > zap the unsync sptes. > > > > > > > > Hi Jay, > > > > Is this actually increasing the real downtime when the guest isn't > > > > running, or is it just the reported time? I see that the s->downtime > > > > value is calculated right after where we currently call > > > > qemu_savevm_state_cleanup. > > > > > > It actually increased the real downtime, I used the "ping" command to > > > test. Reason is that the source side libvirt sends qmp to qemu to query > > > the status of migration, which needs the BQL. qemu_savevm_state_cleanup > > > is done with BQL, qemu can not handle the qmp if qemu_savevm_state_cleanup > > > has not finished. And the source side libvirt delays about 300ms to notify > > > the destination side libvirt to send the "cont" command to start the vm. > > > > > > I think the value of s->downtime is not accurate enough, maybe we could > > > move the calculation of end_time after qemu_savevm_state_cleanup has done. > > > > I'm copying in Paolo, Radim and Andrea- is there anyway we can make the > > teardown of KVMs dirty tracking not take so long? 300ms is a silly long time > > on only a small VM. > > > > > > I guess the biggest problem is that 300ms happens before we restart > > > > the guest on the source if a migration fails. > > > > > > 300ms happens even if a migration succeeds. > > > > Hmm, OK, this needs fixing then - it does explain a result I saw a while > > ago where the downtime was much bigger with libvirt than it was with > > qemu on it's own. > > > > > > > I think it can be optimized: > > > > > (1) source vm will be destroyed if the migration is successfully done, > > > > > so the resources will be cleanuped automatically by the system > > > > > (2) delay the cleanup if the migration failed > > > > > > > > I don't like putting it in qmp_cont; that shouldn't have migration magic > > > > in it. > > > > > > Yes, it is not a ideal place. :( > > > > > > > I guess we could put it in migrate_fd_cleanup perhaps? It gets called on > > > > a bh near the end - or could we just move it closer to the end of > > > > migration_thread? > > > > > > I have tested putting it in migrate_fd_cleanup, but the downtime is not > > > optimized. So I think it is the same to move it closer to the end of > > > migration_thread if it holds the BQL. > > > Could we put it in migrate_init? > > > > Your explanation above hints as to why migrate_fd_cleanup doesn't help; > > it's because we're still going to be doing it with the BQL taken. > > Yes, it is. > > > Can you tell me which version of libvirt you're using? > > I'm using 1.3.4 > > > I thought the newer ones were supposed to use events so they did't > > have to poll qemu. > > After checking the codes of the newest libvirt, I think it is the same > in the qemuMigrationWaitForCompletion function, which is used to poll > qemu every 50ms.
Checking with Jiri Denemark (added to cc), newer libvirt should use events when available - but that polling code is there to cope with older qemu's. So with a newer qemu, i think it should spot the COMPLETED event. Dave > Thanks, > Jay > > > If we move qemu_savevm_state_cleanup is it still safe? Are there > > some things we're supposed to do at that point which are wrong if > > we don't. > > > > I wonder about something like; take a mutex in > > memory_global_dirty_log_start, release it in > > memory_global_dirty_log_stop. Then make ram_save_cleanup start > > a new thread that does the call to memory_global_dirty_log_stop. > > > > Dave > > > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
