On 05/17/2018 06:50 AM, Tiwei Bie wrote:
On Thu, May 17, 2018 at 06:06:34AM +0300, Michael S. Tsirkin wrote:
On Wed, May 16, 2018 at 06:54:23PM +0200, Maxime Coquelin wrote:
This patch caches all dirty pages logging until the used ring index
is updated.
The goal of this optimization is to fix a performance regression
introduced when the vhost library started to use atomic operations
to set bits in the shared dirty log map. While the fix was valid
as previous implementation wasn't safe against concurent accesses,
contention was induced.
With this patch, during migration, we have:
1. Less atomic operations as only a single atomic OR operation
per 32 or 64 (depending on CPU) pages.
2. Less atomic operations as during a burst, the same page will
be marked dirty only once.
3. Less write memory barriers.
Fixes: 897f13a1f726 ("vhost: make page logging atomic")
Cc: sta...@dpdk.org
Cc: Tiwei Bie <tiwei....@intel.com>
Suggested-by: Michael S. Tsirkin <m...@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
Deferring updates until GET_BASE would also be possible,
but would increase the chance that a disconnect causes
ring to become inconsistent.
Yeah. The sync of the updates from vhost backend will
be deferred a lot. Another issue is that, it probably
will increase the downtime, as there will be more pages
to sync after the old device is stopped and before the
new device is started.
I'm not sure whether there is a chance of that with this
patch (in case of a crash after used idx updated but
before dirty log update of the used idx), but
at least it's not bigger than before this patch.
The used idx update and the corresponding logging
are two operations instead of one atomic operation.
So theoretically, it should be possible.
I got your point now. Maybe we should add a barrier
between cache sync and the used idx update to ensure
that all dirty pages are logged before they can be
seen by the guest.
Good idea, I'll add a carrier at the end of the sync.
Thanks,
Maxime
Best regards,
Tiwei Bie
