Re: [Qemu-devel] Do memory mappings need be rebuilt when deleting ioeventfds?

[Paolo Bonzini <pbonzini@redhat.com>]

Il 04/03/2014 11:49, Gonglei (Arei) ha scritto:
> 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.

Precisely, what you suggest won't work because you can have 
memory_region_add/del_eventfd as a part of a larger transaction.  But 
overall this is a good idea.

You can split memory_region_update_pending in two parts, like 
memory_region_update_pending and memory_region_ioeventfd_change_pending. 
  Then, if memory_region_update_pending is false but 
memory_region_ioeventfd_change_pending is true, you can activate an 
optimized path.

Paolo