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.