Re: [Qemu-devel] migration: broken ram_save_pending

[Paolo Bonzini <pbonzini@redhat.com>]

Il 04/02/2014 13:16, Alexey Kardashevskiy ha scritto:
> 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? :)

I mean, my mistake. :)  I assumed 3000 ms = 3*10^9.

30 ms is too little, but 3000 ms is probably too much for a default.

>>> 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?

No, because new pages might be dirtied while ram_save_iterate() was running.

Paolo