Re: [Qemu-devel] [RFC PATCH v2 06/12] mc: introduce state machine changes for MC

[Michael R. Hines]

On 02/19/2014 09:00 AM, Li Guang wrote:
> Hi,
>
> address@hidden wrote:
> > From: "Michael R. Hines"<address@hidden>
> >
> > This patch sets up the initial changes to the migration state
> > machine and prototypes to be used by the checkpointing code
> > to interact with the state machine so that we can later handle
> > failure and recovery scenarios.
> >
> > Signed-off-by: Michael R. Hines<address@hidden>
> > ---
> >   arch_init.c                   | 29 ++++++++++++++++++++++++-----
> >   include/migration/migration.h |  2 ++
> >   migration.c                   | 37 
> > +++++++++++++++++++++----------------
> >   3 files changed, 47 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch_init.c b/arch_init.c
> > index db75120..e9d4d9e 100644
> > --- a/arch_init.c
> > +++ b/arch_init.c
> > @@ -658,13 +658,13 @@ static void ram_migration_cancel(void *opaque)
> >       migration_end();
> >   }
> >
> > -static void reset_ram_globals(void)
> > +static void reset_ram_globals(bool reset_bulk_stage)
> >   {
> >       last_seen_block = NULL;
> >       last_sent_block = NULL;
> >       last_offset = 0;
> >       last_version = ram_list.version;
> > -    ram_bulk_stage = true;
> > +    ram_bulk_stage = reset_bulk_stage;
> >   }
>
> here is a chance that ram_save_block will never break while loop
> if loat_seen_block be reset for mc when there are no dirty pages
> to be migrated.
>
> Thanks!

I see. Question:

While running the code, I have never seen a case where there are no
dirty pages. Have you seen this before? And even if there are no dirty
pages during a single 100 millisecond checkpoint interval (the default
which can be changed) - the probability that there will continue to be
no dirty pages during the next checkpoint interval will be very low,
right?

If there are no dirty pages during a checkpoint, should not
ram_save_block() keep waiting? The virtual machine is still running
during MC - the MC code does not stop the virtual machine, so if there
are no dirty pages, isn't it safe to just let ram_save_block() keep looping
until there are dirty pages available?

- Michael