On 05/16/2018 08:10 AM, Tiwei Bie wrote:
On Tue, May 15, 2018 at 03:50:54PM +0200, Maxime Coquelin wrote:
Hi Tiwei,
I just see I missed to reply to your comment on my commit message:
On 05/03/2018 01:56 PM, Tiwei Bie wrote:
On Mon, Apr 30, 2018 at 05:59:54PM +0200, Maxime Coquelin wrote:
This patch caches all dirty pages logging until the used ring index
is updated. These dirty pages won't be accessed by the guest as
long as the host doesn't give them back to it by updating the
index.
Below sentence in above commit message isn't the reason why
we can cache the dirty page logging. Right?
"""
These dirty pages won't be accessed by the guest as
long as the host doesn't give them back to it by updating the
index.
That's my understanding.
As long as the used index is not updated, the guest will not process
the descs.
If the migration converges between the time the descs are written,
and the time the used index is updated on source side. Then the guest
running on destination will not see the descriptors as used but as
available, and so will be overwritten by the vhost backend on
destination.
If my understanding is correct, theoretically the vhost
backend can cache all the dirty page loggings before it
responds to the GET_VRING_BASE messages. Below are the
steps how QEMU live migration works (w/o postcopy):
1. Syncing dirty pages between src and dst;
2. The dirty page sync converges;
3. The src QEMU sends GET_VRING_BASE to vhost backend;
4. Vhost backend still has a chance to log some dirty
pages before responding the GET_VRING_BASE messages;
5. The src QEMU receives GET_VRING_BASE response (which
means the device has stopped);
6. QEMU sync the remaining dirty pages;
7. QEMU on the dst starts running.
Thanks for the clarification.
(The steps 3~6 are the downtime which we want to minimize)
Right, we want to minimize the downtime, but in the same time,
be able to converge.
So I think the words in commit log isn't really related
to why we can cache the dirty page loggings.
I'll try to improve it in v3.
Thanks,
Maxime
Best regards,
Tiwei Bie
Regards,
Maxime
