linux-kernel - Re: [Devel] Re: [RFC][PATCH 2/7] RSS controller core

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Date:	Tue, 13 Mar 2007 13:19:53 +0300
From:	Kirill Korotaev <dev@...ru>
To:	akpm@...ux-foundation.org
CC:	Herbert Poetzl <herbert@...hfloor.at>, containers@...ts.osdl.org,
	hansendc@...ibm.com, linux-kernel@...r.kernel.org, devel@...nvz.org
Subject: Re: [Devel] Re: [RFC][PATCH 2/7] RSS controller core

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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