Re: [Qemu-devel] [v2 RESEND 0/4] Fix long vm downtime during live migration

[Amit Shah]

On (Mon) 02 Nov 2015 [15:36:59], Liang Li wrote:
> The patch 3ea3b7fa9af067982f34b of kvm introduces a lazy collapsing
> of small sptes into large sptes mechanism, which intend to solve the
> performance drop issue if live migration fails or is canceled. The
> rmap will be scanned in the KVM_SET_USER_MEMORY_REGION ioctl context
> when dirty logging is stopped so as to drop the small sptes, scanning
> the rmap and drop the small sptes is a time consuming operation which
> will take dozens of milliseconds, the actual time depends on VM's
> memory size. For a VM with 8GB RAM, it will take about 30ms.
> 
> The current QEMU code stop the dirty logging during the pause and
> copy stage by calling the migration_end() function. Now migration_end()
> is a time consuming operation because it calls
> memroy_global_dirty_log_stop(), which will trigger the scanning of rmap
> and dropping small sptes operation. So call migration_end() before all
> the vmsate data has already been transferred to the destination will
> prolong VM downtime.
> 
> migration_end() should be deferred after all the data has been
> transferred to the destination. blk_mig_cleanup() can be deferred too.

Reviewed-by: Amit Shah <address@hidden>

Thanks for adding to the commit message, that helped.


                Amit