Re: Seamlessly sharing identical memory pages among domains

[Niraj Tolia <ntolia@hotmail.com>]

While not exactly related, this paper 
<http://www.cs.rice.edu/~ssiyer/r/superpages/osdi02superpages/> (Section 
4.7) presented at OSDI'02 showed that there was significant overhead 
when trying to detect dirty pages using SHA-1 hash digests. It would be 
interesting to see if this applied to Xen.

Niraj
-- 
http://www.andrew.cmu.edu/~ntolia

Ian Pratt wrote:
>>I'd like to know if XEN is or will be implementing any mechanism for
>>transparently sharing pages with identical content among domains, to
>>minimize memory consumption, similar to what is implemented in VMWare's
>>ESX server (chapter 4 of the paper whose link is below).
> 
> 
> I'm not convinced there's that much sharing to be had between
> VMs: One side effect of implementing some test code for live
> migration was that I collected some traces containing crc
> fingerprints of the memory pages of several domains. There
> weren't many hashes in common.  Perhaps this shouldn't be
> surprising -- application text is typically small compared to the
> heap data they operate on. The actual datasets operated on by
> domains are typically distinct.
> 
> However, there might be some value in having a shared read-only
> buffer cache, if only for the saved IO ops.  Implementing a cache
> that replicates pages in memory (rather than sharing) is trivial.
> 
> Doing a proper shared cache is slightly trickier given the
> paravirtualised memory interface -- we'd have to introduce guests
> to a new kind of write fault: "A write fault has occurred, and
> you'll have to copy the page because this machine page is
> immutable as it is already shared with other domains". Modifying
> Linux to handle this wouldn't be hard.
> 
> Of course, if you're running a domain on Xen's shadow page tables
> then all of this manipulation is easy and can be done
> transparently to the domain. Of course, you pay a performance
> penalty...
> 
> Ian
> 
> 
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