Re: [Qemu-devel] [PATCH] A small patch to introduce stop conditions to the live migration.

[Michael Roth]

On 09/14/2011 10:22 AM, Michael Roth wrote:
> On 09/14/2011 08:18 AM, Thomas Treutner wrote:
> > Currently, it is possible that a live migration never finishes, when
> > the dirty page rate is high compared to the scan/transfer rate. The
> > exact values for MAX_MEMORY_ITERATIONS and
> > MAX_TOTAL_MEMORY_TRANSFER_FACTOR are arguable, but there should be
> > *some* limit to force the final iteration of a live migration that
> > does not converge.
> >
> > ---
> > arch_init.c | 10 +++++++++-
> > 1 files changed, 9 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch_init.c b/arch_init.c
> > index 4486925..57fcb1e 100644
> > --- a/arch_init.c
> > +++ b/arch_init.c
> > @@ -89,6 +89,9 @@ const uint32_t arch_type = QEMU_ARCH;
> > #define RAM_SAVE_FLAG_EOS 0x10
> > #define RAM_SAVE_FLAG_CONTINUE 0x20
> >
> > +#define MAX_MEMORY_ITERATIONS 10
> > +#define MAX_TOTAL_MEMORY_TRANSFER_FACTOR 3
> > +
> > static int is_dup_page(uint8_t *page, uint8_t ch)
> > {
> > uint32_t val = ch<< 24 | ch<< 16 | ch<< 8 | ch;
> > @@ -107,6 +110,8 @@ static int is_dup_page(uint8_t *page, uint8_t ch)
> > static RAMBlock *last_block;
> > static ram_addr_t last_offset;
> >
> > +static int numberFullMemoryIterations = 0;
> > +
> > static int ram_save_block(QEMUFile *f)
> > {
> > RAMBlock *block = last_block;
> > @@ -158,7 +163,10 @@ static int ram_save_block(QEMUFile *f)
> > offset = 0;
> > block = QLIST_NEXT(block, next);
> > if (!block)
> > + {
> > + numberFullMemoryIterations++;
> > block = QLIST_FIRST(&ram_list.blocks);
> > + }
> > }
> >
> > current_addr = block->offset + offset;
> > @@ -295,7 +303,7 @@ int ram_save_live(Monitor *mon, QEMUFile *f, int
> > stage, void *opaque)
> >
> > expected_time = ram_save_remaining() * TARGET_PAGE_SIZE / bwidth;
> >
> > - return (stage == 2)&& (expected_time<= migrate_max_downtime());
> > + return (stage == 2)&& ((expected_time<= migrate_max_downtime() ||
> > (numberFullMemoryIterations == MAX_MEMORY_ITERATIONS) ||
> > (bytes_transferred>
> > (MAX_TOTAL_MEMORY_TRANSFER_FACTOR*ram_bytes_total()))));
> > }
> >
> > static inline void *host_from_stream_offset(QEMUFile *f,
>
> To me it seems like a simpler solution is to do something like:
>
> return (stage == 2) && current_time() + expected_time < migrate_deadline()
>
> where migrate_deadline() is the time that the migration began plus
> migrate_max_downtime().
>
> Currently, it looks like migrate_max_downtime() is being applied on a
> per-iteration basis rather than per-migration, which seems like a bug to
> me. Block migration seems to suffer from this as well...

Sorry, ignore this, that calculation's just for stage 3.