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Re: Re: [RFC][PATCH 2/7] RSS controller core [message #11083 is a reply to message #10902] Tue, 13 March 2007 10:06 Go to previous messageGo to previous message
dev is currently offline  dev
Messages: 1693
Registered: September 2005
Location: Moscow
Senior Member

Andrew Morton wrote:
>>>> - shared mappings of 'shared' files (binaries
>>>> and libraries) to allow for reduced memory
>>>> footprint when N identical guests are running
>>>
>>>So, it sounds like this can be phrased as a requirement like:
>>>
>>> "Guests must be able to share pages."
>>>
>>>Can you give us an idea why this is so?
>>
>>sure, one reason for this is that guests tend to
>>be similar (or almost identical) which results
>>in quite a lot of 'shared' libraries and executables
>>which would otherwise get cached for each guest and
>>would also be mapped for each guest separately
>
>
> nooooooo. What you're saying there amounts to text replication. There is
> no proposal here to create duplicated copies of pagecache pages: the VM
> just doesn't support that (Nick has soe protopatches which do this as a
> possible NUMA optimisation).
>
> So these mmapped pages will contiue to be shared across all guests. The
> problem boils down to "which guest(s) get charged for each shared page".
>
> A simple and obvious and easy-to-implement answer is "the guest which paged
> it in". I think we should firstly explain why that is insufficient.
I guess by "paged it in" you essentially mean
"mapped the page into address space for the *first* time"?

i.e. how many times the same page mapped into 2 address spaces
in the same container should be accounted for?

We believe ONE. It is better due to:
- it allows better estimate how much RAM container uses.
- if one container mapped a single page 10,000 times,
it doesn't mean it is worse than a container which mapped only 200 pages
and that it should be killed in case of OOM.

Thanks,
Kirill
 
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