Re: [Qemu-devel] Stalls on Live Migration of VMs with a lot of memory

[Peter Lieven]

On 04.01.2012 12:28, Paolo Bonzini wrote:
> On 01/04/2012 12:22 PM, Peter Lieven wrote:
> > > There were patches to move RAM migration to a separate thread. The
> > > problem is that they broke block migration.
> > >
> > > However, asynchronous NBD is in and streaming will follow suit soon.
> > > As soon as we have those two features, we might as well remove the
> > > block migration code.
> >
> > ok, so its a matter of time, right?
>
> Well, there are other solutions of varying complexity in the works, 
> that might remove the need for the migration thread or at least reduce 
> the problem (post-copy migration, XBRLE, vectorized hot loops).  But 
> yes, we are aware of the problem and we should solve it in one way or 
> the other.
i have read all these approached and they seem all promising.
> > would it make sense to patch ram_save_block to always process a full ram
> > block?
>
> If I understand the proposal, then migration would hardly be live 
> anymore.  The biggest RAM block in a 32G machine is, well, 32G big. 
> Other RAM blocks are for the VRAM and for some BIOS data, but they are 
> very small in proportion.
ok, then i misunderstood the ram blocks thing. i thought the guest ram 
would consist of a collection of ram blocks.
then let me describe it differntly. would it make sense to process 
bigger portions of memory (e.g. 1M) in stage 2 to reduce the number of 
calls to cpu_physical_memory_reset_dirty and instead run it on bigger 
portions of memory. we might loose a few dirty pages but they will be 
tracked in the next iteration in stage 2 or in stage 3 at least. what 
would be necessary is that nobody marks a page dirty
while i copy the dirty information for the portion of memory i want to 
process.
> > - in stage 3 the vm is stopped, right? so there can't be any more dirty
> > blocks after scanning the whole memory once?
>
> No, stage 3 is entered when there are very few dirty memory pages 
> remaining.  This may happen after scanning the whole memory many 
> times.  It may even never happen if migration does not converge 
> because of low bandwidth or too strict downtime requirements.
ok, is there a chance that i lose one final page if it is modified just 
after i walked over it and i found no other page dirty (so bytes_sent = 0).

Peter
> Paolo