Re: KSM Algorithm

[Izik Eidus <izik@qumranet.com>]

ציטוט Sukanto Ghosh:
>>> Doesn't KSM notifies KVM about the shared pages so that KVM can update
>>> its sptes accordingly or is it done by KSM itself ?
>>>
>>>       
>> when using kvm, mmu notifiers is a must for ksm, (mmu notifiers update kvm
>> about the changes in the host page table)
>>
>>     
>>> What about KSM being used by someone other than KVM ? Does KSM updates
>>> the ptes of the shared pages itself ?
>>>
>>>       
>> it can be used by any application, it update the ptes of the host, what you
>> mean by the shared pages itself?
>>     
>
> by "itself" I meant 'ksm'. So ksm will take care of updating the ptes
> of the shared guest pages.
>   

yes

>
>   
>>>       
>>>>> Where are the shared pages kept ? In kernel memory ? Aren't the shared
>>>>> pages always pinned in RAM (due to same reasons for pinned shadowed
>>>>> pages) ?
>>>>>
>>>>>
>>>>>           
>>>> in the version that was sent to the list it was kernel memory (meaning
>>>> shared pages are not swappable
>>>> (just the pages that are shared not the pages that we scan, when they are
>>>> split beacuse of copy on write it become swappable again
>>>> new version that i will send soon the pages are normal
>>>> anonymous/userspace
>>>> memory that is swappable
>>>>
>>>>
>>>>         
>>> What is the strategy/method which you will use to represent the shared
>>> memory as normal anonymous/userspace memory ? Then, these pages must
>>> be staying at one of the guest's address-space (whose pages have been
>>>  shared) ?
>>>
>>>       
>> it possible due to another patch that i need to send, that allow modules
>> register new reverse mapping call backs...,
>>
>>     
> ok
>
> but in whose address-space will these shared pages stay ?
>
>   

if we have 5 applications sharing the same page, we will have the page 
stay in 5 diffrent address-spaces...

>   
>>>       
>>>>> How much is the overhead involved due to this ?
>>>>>
>>>>>
>>>>>           
>>>> depeand on the speed you tell it to scan, but the overhead is about ~5%
>>>> for
>>>> common cases
>>>>
>>>>         
>>> Can I know the typical values of the parameters : sleep time, no. of
>>> pages to scan, which you used to arrive at the above mentioned
>>> overhead.
>>>
>>>       
>> i think it is about 256 2000 (256 pages per 2000 usleep)
>> (note that most of the cpu is acutlay taken by the copying of the pages when
>> a new shared page is created, i have possible way to change it and reduce
>> the cpu even more
>> but i dont know if i want to do it) (it add more IFs to the fast path inside
>> the mm)
>>
>>
>>     
>>>       
>>     
>
> Thanks :)
>
>