commit 8b2395970aa3beab91b98dda89c7ed471e65ad25
Author: Peter Xu <peterx@redhat.com>
Date:   Tue Oct 10 18:19:22 2023 -0400    migration: Allow user to specify available switchover bandwidth
    
    Migration bandwidth is a very important value to live migration.  It's
    because it's one of the major factors that we'll make decision on when to
    switchover to destination in a precopy process.
    
    This value is currently estimated by QEMU during the whole live migration
    process by monitoring how fast we were sending the data.  This can be the
    most accurate bandwidth if in the ideal world, where we're always feeding
    unlimited data to the migration channel, and then it'll be limited to the
    bandwidth that is available.
    
    However in reality it may be very different, e.g., over a 10Gbps network we
    can see query-migrate showing migration bandwidth of only a few tens of
    MB/s just because there are plenty of other things the migration thread
    might be doing.  For example, the migration thread can be busy scanning
    zero pages, or it can be fetching dirty bitmap from other external dirty
    sources (like vhost or KVM).  It means we may not be pushing data as much
    as possible to migration channel, so the bandwidth estimated from "how many
    data we sent in the channel" can be dramatically inaccurate sometimes.
    
    With that, the decision to switchover will be affected, by assuming that we
    may not be able to switchover at all with such a low bandwidth, but in
    reality we can.
    
    The migration may not even converge at all with the downtime specified,
    with that wrong estimation of bandwidth, keeping iterations forever with a
    low estimation of bandwidth.
    
    The issue is QEMU itself may not be able to avoid those uncertainties on
    measuing the real "available migration bandwidth".  At least not something
    I can think of so far.
    
    One way to fix this is when the user is fully aware of the available
    bandwidth, then we can allow the user to help providing an accurate value.
    
    For example, if the user has a dedicated channel of 10Gbps for migration
    for this specific VM, the user can specify this bandwidth so QEMU can
    always do the calculation based on this fact, trusting the user as long as
    specified.  It may not be the exact bandwidth when switching over (in which
    case qemu will push migration data as fast as possible), but much better
    than QEMU trying to wildly guess, especially when very wrong.      
    
A new parameter "avail-switchover-bandwidth" is introduced just for this.
    So when the user specified this parameter, instead of trusting the
    estimated value from QEMU itself (based on the QEMUFile send speed), it
    trusts the user more by using this value to decide when to switchover,
    assuming that we'll have such bandwidth available then.
    
    Note that specifying this value will not throttle the bandwidth for
    switchover yet, so QEMU will always use the full bandwidth possible for
    sending switchover data, assuming that should always be the most important
    way to use the network at that time.
    
    This can resolve issues like "unconvergence migration" which is caused by
    hilarious low "migration bandwidth" detected for whatever reason.
    
    Reported-by: Zhiyi Guo <zhguo@redhat.com>
    Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
    Reviewed-by: Juan Quintela <quintela@redhat.com>
    Signed-off-by: Peter Xu <peterx@redhat.com>
    Signed-off-by: Juan Quintela <quintela@redhat.com>
    Message-ID: <20231010221922.40638-1-peterx@redhat.com>