Re: [Qemu-devel] About QEMU BQL and dirty log switch in Migration

[Yang Hongyang]

Hi Paolo, Dave,

On 2017/4/26 23:46, Paolo Bonzini wrote:
> 
> 
> On 24/04/2017 18:42, Dr. David Alan Gilbert wrote:
>> I suppose there's a few questions;
>>   a) Do we actually need the BQL - and if so why

Enable/disable dirty log tracking are operations on memory regions.
That's why they need to be in BQL I think.

>>   b) What actually takes 13s?  It's probably worth figuring
>> out where it goes,  the whole bitmap is only 1GB isn't it
>> even on a 4TB machine, and even the simplest way to fill
>> that takes way less than 13s.

I found two time consuming operations in KVM module,

- one is kvm_mmu_slot_apply_flags(), when enable dirty log tracking
kvm_vm_ioctl_set_memory_region
  |->kvm_set_memory_region
     |->__kvm_set_memory_region
        |->kvm_arch_commit_memory_region
           |->kvm_mmu_slot_apply_flags
              ...

- the other is kvm_mmu_zap_collapsible_sptes(), when disable dirty log tracking
kvm_vm_ioctl_set_memory_region
  |->kvm_set_memory_region
     |->__kvm_set_memory_region
        |->kvm_arch_commit_memory_region
           |->kvm_mmu_zap_collapsible_sptes
              ...

Any ideas that could optimize the time spending for these operations?

> 
> It's more likely that it is the migration_bitmap_sync immediately after
> that.

It's not, it's enable/disable dirty log tracking that costs time.

> 
> I think it is possible to move migration_bitmap_sync outside BQL.  It
> should be simpler to evaluate that after Juan's cleanups go in.
> 
> Paolo
> 

-- 
Thanks,
Yang