Re: [PATCH v4 1/1] migration: merge fragmented clear_dirty ioctls

[Peter Xu <peterx@redhat.com>]

On Thu, Dec 18, 2025 at 09:12:27AM -0300, Fabiano Rosas wrote:
> "Chuang Xu" <xuchuangxclwt@bytedance.com> writes:
> 
> > In our long-term experience in Bytedance, we've found that under
> > the same load, live migration of larger VMs with more devices is
> > often more difficult to converge (requiring a larger downtime limit).
> >
> > Through some testing and calculations, we conclude that bitmap sync time
> > affects the calculation of live migration bandwidth.
> >
> > When the addresses processed are not aligned, a large number of
> > clear_dirty ioctl occur (e.g. a 4MB misaligned memory can generate
> > 2048 clear_dirty ioctls from two different memory_listener),
> > which increases the time required for bitmap_sync and makes it
> > more difficult for dirty pages to converge.
> >
> > For a 64C256G vm with 8 vhost-user-net(32 queue per nic) and
> > 16 vhost-user-blk(4 queue per blk), the sync time is as high as *73ms*
> > (tested with 10GBps dirty rate, the sync time increases as the dirty
> > page rate increases), Here are each part of the sync time:
> >
> > - sync from kvm to ram_list: 2.5ms
> > - vhost_log_sync:3ms
> > - sync aligned memory from ram_list to RAMBlock: 5ms
> > - sync misaligned memory from ram_list to RAMBlock: 61ms
> >
> > Attempt to merge those fragmented clear_dirty ioctls, then syncing
> > misaligned memory from ram_list to RAMBlock takes only about 1ms,
> > and the total sync time is only *12ms*.
> >
> > Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>
> 
> Reviewed-by: Fabiano Rosas <farosas@suse.de>

Side note: I plan to fold a var, hence a fixup to be squashed:

diff --git a/migration/ram.c b/migration/ram.c
index 27ffa13763..c403fd73a6 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -992,10 +992,8 @@ static uint64_t physical_memory_sync_dirty_bitmap(RAMBlock *rb,
             memory_region_clear_dirty_bitmap(rb->mr, start, length);
         }
     } else {
-        ram_addr_t offset = rb->offset;
-
         num_dirty = physical_memory_test_and_clear_dirty(
-                        start + offset,
+                        start + rb->offset,
                         length,
                         DIRTY_MEMORY_MIGRATION,
                         dest);

Queued, thanks!

-- 
Peter Xu