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Re: [Qemu-devel] About QEMU BQL and dirty log switch in Migration
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] About QEMU BQL and dirty log switch in Migration |
Date: |
Mon, 24 Apr 2017 17:42:44 +0100 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
* Yang Hongyang (address@hidden) wrote:
>
>
> On 2017/4/24 20:06, Juan Quintela wrote:
> > Yang Hongyang <address@hidden> 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 / address@hidden / Manchester, UK