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

[Stefan Hajnoczi]

On Mon, Nov 02, 2015 at 03:36:59PM +0800, 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.
> 
> Effect of this patch
> ====================
> For a VM with 8G RAM, this patch can reduce the VM downtime about 30 ms.
> 
> You can follow these steps to see the effect of this patch.
> 
> 1. Start a VM with the command:
>   ./qemu-system-x86_64 -enable-kvm -smp 4 -m 8192 -monitor stdio\
>       /share/rhel6u5.qcow
>    in the source host and 
>   ./qemu-system-x86_64 -enable-kvm -smp 4 -m 8192 -monitor stdio\
>       /share/rhel6u5.qcow -incoming tcp:0:4444
>    in the destination host.
> 2. In the source side qemu monitor:
>   (qemu) migrate_set_speed 0
>   (qemu) migrate_set_downtime 0.01
>   (qemu) migrate -d tcp:($DST_HOST_IP):4444
>   (qemu) info migrate
> 
> The actual VM downtime in my environment:
> =====================================
> |without this patch| with this patch|
> |-----------------------------------|
> |      35ms        |     4ms        |
> =====================================
> 
> 
> Changes:
>   * Remove qemu_savevm_sate_cancel() in migrate_fd_cleanup().
>   * Add 2 more patches for code cleanup.
>   * Add more details in the commit message.
> 
> Liang Li (4):
>   migration: defer migration_end & blk_mig_cleanup
>   migration: rename qemu_savevm_state_cancel
>   migration: rename cancel to cleanup in SaveVMHandles
>   migration: code clean up
> 
>  include/migration/vmstate.h |  2 +-
>  include/sysemu/sysemu.h     |  2 +-
>  migration/block.c           | 10 ++--------
>  migration/migration.c       | 13 ++++++-------
>  migration/ram.c             | 10 ++--------
>  migration/savevm.c          | 10 +++++-----
>  trace-events                |  2 +-
>  7 files changed, 18 insertions(+), 31 deletions(-)

I haven't reviewed in detail, but if migration folks are happy then it's
fine by me:

Acked-by: Stefan Hajnoczi <address@hidden>

signature.asc
Description: PGP signature