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

[Michael Roth <mdroth@linux.vnet.ibm.com>]

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...