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

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 02/19/2014 09:00 AM, Li Guang wrote:
> Hi,
>
> mrhines@linux.vnet.ibm.com wrote:
>> From: "Michael R. Hines"<mrhines@us.ibm.com>
>>
>> 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<mrhines@us.ibm.com>
>> ---
>>   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