Hi, Recently I found that when doing migration on a VM with many Virtio NICs, a lot down time was consuming in vm_state_notify(). Further investigation shows major consumption is in function memory_region_del_eventfd(). When deletes an ioeventfd, in address space transactions commit, it begins with deleting all memory mappings in all address spaces, and add each memory mapping again. This is time consuming.
In my test, each ioeventfd deleing needs about 5ms, within which memory mapping rebuilding needs about 4ms. With many Nics and vmchannel in a VM doing migrating, there can be many ioeventfds deleting which increasing downtime remarkably. As far as I can see, memory mappings don't rely on ioeventfds, there is no need to destroy and rebuild them when manipulating ioeventfds. I think this is for decoupling consideration. Although this simplifies coding and design, it scarifies performance. So I'd want to know if you have any improve plan on it? Here's my assumption: in memory_region_del_eventfd() and memory_region_add_eventfd(), we mark that this is a pure ioeventfd operation, so in begin and region_nop memory listeners, we eliminate memory mapping destroy and rebuild, thus decrease migration down time and improve migration performance. Best regards, -Gonglei
