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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Yang Hongyang (yanghongyang@huawei.com) wrote:
> 
> 
> On 2017/4/24 20:06, Juan Quintela wrote:
> > Yang Hongyang <yanghongyang@huawei.com> wrote:
> >> Hi all,
> >>
> >> We found dirty log switch costs more then 13 seconds while migrating
> >> a 4T memory guest, and dirty log switch is currently protected by QEMU
> >> BQL. This causes guest freeze for a long time when switching dirty log on,
> >> and the migration downtime is unacceptable.
> >> Are there any chance to optimize the time cost for dirty log switch operation?
> >> Or move the time consuming operation out of the QEMU BQL?
> > 
> > Hi
> > 
> > Could you specify what do you mean by dirty log switch?
> > The one inside kvm?
> > The merge between kvm one and migration bitmap?
> 
> The call of the following functions:
> memory_global_dirty_log_start/stop();

I suppose there's a few questions;
  a) Do we actually need the BQL - and if so why
  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.

Dave

> 
> > 
> > Thanks, Juan.
> > 
> 
> -- 
> Thanks,
> Yang
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK