Re: [Qemu-devel] migration: broken ram_save_pending

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 02/04/2014 11:07 PM, Paolo Bonzini wrote:
> Il 04/02/2014 12:59, Alexey Kardashevskiy ha scritto:
>>> > With the default throttling of 32 MiB/s, bandwidth must be something like
>>> > 33000 (expressed in bytes/ms) with the default settings, and then
>>> max_size
>>> > should be 33000*3*10^9 / 10^6 = 6000000.  Where is my computation wrong?
>>
>> migrate_max_downtime() = 30000000 = 3*10^7.
> 
> Oops, that's the mistake.

Make a patch? :)


>> When the migration is in iterating stage, bandwidth is a speed in last
>> 100ms which is usually 5 blocks 250KB each so it is
>> 1250000/100=12500bytes/s and max_size=12500*30000000/10^6=375000 which is
>> less than the last chunk is.
>>
>>
> 
> Perhaps our default maximum downtime is too low.  30 ms doesn't seem
> achievable in practice with 32 MiB/s bandwidth.  Just making it 300 ms or
> so should fix your problem.

Well, it will fix it in my particular case but in a long run this does not
feel like a fix - there should be a way for migration_thread() to know that
ram_save_iterate() sent all dirty pages it had to send, no?



-- 
Alexey