Re: [Qemu-devel] [PATCH v2] migration: skip sending ram pages released by virtio-balloon driver.

[Jitendra Kolhe]

On 4/13/2016 5:06 PM, Michael S. Tsirkin wrote:
> On Wed, Apr 13, 2016 at 12:15:38PM +0100, Dr. David Alan Gilbert wrote:
>> * Michael S. Tsirkin (address@hidden) wrote:
>>> On Wed, Apr 13, 2016 at 04:24:55PM +0530, Jitendra Kolhe wrote:
>>>> Can we extend support for post-copy in a different patch set?
>>>
>>> If the optimization does not *help* on some paths,
>>> that's fine. The issue is with adding extra code
>>> special-casing protocols:
>>>
>>> +    if (migrate_postcopy_ram()) {
>>> +        balloon_bitmap_disable_state = BALLOON_BITMAP_DISABLE_PERMANENT;
>>> +    }
>>>
>>> Generally when one sees that patchset breaks XYZ...
>>> the easy solution is "check for XYZ
>>> and disable the optimization". But do this enough times
>>> and the codebase becomes impossible to reason about.
>>>     why did migration become slower? oh it enabled
>>>     optimization A and that conflicts with optimization B ...

Ok, I understand the confusion caused by such checks. Will remove 
the migration_postcopy checks from the patch and make sure things 
don’t break in postcopy path.

>>
>> Hang on; this is getting all very complicated; I wouldn't start tieing
>> this thing up with postcopy yet.  Lets try and keep this simple for starters.
>>
>> Dave
> 
> Absolutely, but I don't understand why is this implemented in such
> a complex way.
> 1. keep track of pages in balloon in a bitmap
> 2. put bitmap in a ram block
> 3. check that before sending page. if there - it's a zero page
> 

Thanks for the suggestion, putting bitmap in ramblock does remove the
code for migrating and dirty page tracking of balloon bitmap pages itself.
In case, the feature is turned-off, the balloon bitmap ramblock is resized
to 0 to make sure that the bitmap is not migrated to reduce the overhead.

> All the complexity is to avoid migrating an extra bit per page
> and it seems like a premature optimization to me at this stage.
> 

Major benefit of the optimization is to avoid zero page scan for
ballooned out pages. Skipping migrating extra bit per ballooned out 
page is just an add-on benefit without any overhead as long as it 
doesn’t break any protocol. 

Thanks,
- Jitendra

>>>
>>>
>>>> and use 
>>>> current patch set to support other remaining protocols?
>>>
>>> Even disregarding postcopy, I think there were
>>> comments that need to be addressed.
>>>
>>> -- 
>>> MST
>> --
>> Dr. David Alan Gilbert / address@hidden / Manchester, UK