Re: [PATCH 3/6] KVM: Dirty memory tracking for performant checkpointing and improved live migration

[Kai Huang <kaih.linux@gmail.com>]

On Thu, May 5, 2016 at 6:57 AM, Radim Krčmář <rkrcmar@redhat.com> 
wrote:
> 2016-05-04 18:33+0000, Cao, Lei:
>>  On 5/4/2016 1:15 PM, Cao, Lei wrote:
>>>  On 5/4/2016 9:13 AM, Radim Krčmář wrote:
>>>>  Good designs so far seem to be:
>>>>   memslot -> lockless radix tree
>>>>  and
>>>>   vcpu -> memslot -> list  (memslot -> vcpu -> list)
>>>> 
>>> 
>>>  There is no need for lookup, the dirty log is fetched in sequence, 
>>> so why use
>>>  radix tree with added complexity but no benefit?
>>> 
>>>  List can be designed to be lockless, so memslot -> lockless fixed 
>>> list?
>> 
>>  Never mind, lookup is needed to avoid duplicates. We can use 
>> list+bitmap, but
>>  it's obviously not as efficient as radix tree.
> 
> Are duplicates a significant problem?
> 
> (The dirtied page is marked as dirty, so we should have zero to very 
> few
>  duplicates, depending on how dirtying and vm-exit on write to clean 
> page
>  cooperate.  Duplicates don't introduce any bugs and we could also 
> check
>  last few entries in the list to weed out most likely cases.)

I don't think duplicated pages are significant problem. The point is we 
don't lose pages.

I actually don't quite follow why there will be duplicated pages. Is it 
because lockless thing?

Thanks,
-Kai

> 
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