Re: [Qemu-devel] Block Migration and CPU throttling

[Peter Lieven]

Am 12.12.2017 um 18:05 schrieb Dr. David Alan Gilbert:
> * Peter Lieven (address@hidden) wrote:
> > Am 21.09.2017 um 14:36 schrieb Dr. David Alan Gilbert:
> > > * Peter Lieven (address@hidden) wrote:
> > > > Am 19.09.2017 um 16:41 schrieb Dr. David Alan Gilbert:
> > > > > * Peter Lieven (address@hidden) wrote:
> > > > > > Am 19.09.2017 um 16:38 schrieb Dr. David Alan Gilbert:
> > > > > > > * Peter Lieven (address@hidden) wrote:
> > > > > > > > Hi,
> > > > > > > >
> > > > > > > > I just noticed that CPU throttling and Block Migration don't work together very 
> > > > > > > > well.
> > > > > > > > During block migration the throttling heuristic detects that we obviously make 
> > > > > > > > no progress
> > > > > > > > in ram transfer. But the reason is the running block migration and not a too 
> > > > > > > > high dirty pages rate.
> > > > > > > >
> > > > > > > > The result is that any VM is throttled by 99% during block migration.
> > > > > > > Hmm that's unfortunate; do you have a bandwidth set lower than your
> > > > > > > actual network connection? I'm just wondering if it's actually going
> > > > > > > between the block and RAM iterative sections or getting stuck in ne.
> > > > > > It happens also if source and dest are on the same machine and speed is set to 
> > > > > > 100G.
> > > > > But does it happen if they're not and the speed is set low?
> > > > Yes, it does. I noticed it in our test environment between different nodes with 
> > > > a 10G
> > > > link in between. But its totally clear why it happens. During block migration 
> > > > we transfer
> > > > all dirty memory pages in each round (if there is moderate memory load), but 
> > > > all dirty
> > > > pages are obviously more than 50% of the transferred ram in that round.
> > > > It is more exactly 100%. But the current logic triggers on this condition.
> > > >
> > > > I think I will go forward and send a patch which disables auto converge during
> > > > block migration bulk stage.
> > > Yes, that's fair;  it probably would also make sense to throttle the RAM
> > > migration during the block migration bulk stage, since the chances are
> > > it's not going to get far.  (I think in the nbd setup, the main
> > > migration process isn't started until the end of bulk).
> > Catching up with the idea of delaying ram migration until block bulk has 
> > completed.
> > What do you think is the easiest way to achieve this?
> <excavates inbox, and notices I never replied>
>
> I think the answer depends whether we think this is a 'special' or we
> need a new general purpose mechanism.
>
> If it was really general then we'd probably want to split the iterative
> stage in two somehow, and only do RAM in the second half.
>
> But I'm not sure it's worth it; I suspect the easiest way is:
>
>     a) Add a counter in migration/ram.c or in the RAM state somewhere
>     b) Make ram_save_inhibit increment the counter
>     c) Check the counter at the head of ram_save_iterate and just exit
>       if it's none 0
>     d) Call ram_save_inhibit from block_save_setup
>     e) Then release it when you've finished the bulk stage
>
> Make sure you still count the RAM in the pending totals, otherwise
> migration might think it's finished a bit early.

Is there any culprit I don't see or is it as easy as this?

diff --git a/migration/ram.c b/migration/ram.c
index cb1950f..c67bcf1 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2255,6 +2255,13 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
     int64_t t0;
     int done = 0;

+    if (blk_mig_bulk_active()) {
+        /* Avoid transferring RAM during bulk phase of block migration as
+         * the bulk phase will usually take a lot of time and transferring
+         * RAM updates again and again is pointless. */
+        goto out;
+    }
+
     rcu_read_lock();
     if (ram_list.version != rs->last_version) {
         ram_state_reset(rs);
@@ -2301,6 +2308,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
      */
     ram_control_after_iterate(f, RAM_CONTROL_ROUND);

+out:
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
     ram_counters.transferred += 8;


Peter