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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> Link: https://lore.kernel.org/r/20251218114220.83354-1-xuchuangxclwt@bytedance.com [peterx: drop var "offset" in physical_memory_sync_dirty_bitmap] Signed-off-by: Peter Xu <peterx@redhat.com>