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 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.