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Transcendent memory

Transcendent memory

Posted Jul 8, 2009 20:16 UTC (Wed) by johill (subscriber, #25196)
Parent article: Transcendent memory

What's the use case for shared anyway? Copying data between the different guests?


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Transcendent memory

Posted Jul 8, 2009 20:57 UTC (Wed) by elanthis (guest, #6227) [Link]

Sharing data between the guests. If both guests have an identical binary loaded up from their identical page cache of their identical read only /usr, why copy it into memory twice and waste the space and incur the cache/copying/performance overhead?

Transcendent memory

Posted Jul 9, 2009 0:44 UTC (Thu) by johill (subscriber, #25196) [Link]

I don't see how that's possible with this API though. Or rather, how the API has any influence on it.

See, if the host were to hash the pages I gave it (from the guest) with this API, and memcmp() them, it could with very little effort tell me it's stored it even though it just made a note that my ID(s) point to that particular page, and refcounted the page. This is easy since a page, once pushed from the guest to the host, is immutable.

The way the article was written though it seems that with the shared stuff, different hosts could access the same page frame by the same ID. So guest A could push out a page and guest B could retrieve it with that same ID? Where's the use in that?

Transcendent memory

Posted Jul 9, 2009 0:59 UTC (Thu) by ncm (subscriber, #165) [Link]

If the client were responsible for hashing the page contents to produce the UUID, then the kernel wouldn't have to do it; it could rely on the UUID itself to identify sharable pages automatically.

Transcendent memory

Posted Jul 9, 2009 14:35 UTC (Thu) by johill (subscriber, #25196) [Link]

That
  1. doesn't work – the host would still have to verify there are no hash collisions
  2. doesn't really make a difference afaict?

Transcendent memory

Posted Jul 11, 2009 16:04 UTC (Sat) by MarkWilliamson (subscriber, #30166) [Link]

How about for implementing a shared in-memory filesystem that multiple guests can retrieve information from? The filesystem itself might have been created by a third-party "trusted" VM or the hypervisor itself.


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