Re: [Xen-devel] copy on write memory

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

> Thats all pretty standard, but here comes the magic:  Any data in the
> XIP2 filesystem is not copied into the cached memory of the guest.
> XIP = eXecute In Place.  Binaries are run directly from the shared
> memory and not cached locally, so if you throw common services and
> libraries (like apache, JVM, etc from your example) into it you get the
> binaries themselves shared with basically no cost to the guests.

We've been talking about implementing something similar (though we'd talked 
about implementing it at the block level).  We'd have a shared buffer cache 
for read-only data, which domains could opt in to.  If they are sharing 
filesystem blocks (e.g. from a CoW LVM volume) the domains would get a shared 
read only mapping to the same memory.

Cheers for the info,
Mark

> Not to say an automatic memory manager to determine when it could do
> COW of ram isn't a good avenue to pursue as well, but XIP is a fairly
> good starting point for most situations that you'd want shared memory
> like this, I think.
>
> The XIP2 source code is in the IBM patches to the kernel:
> http://oss.software.ibm.com/linux390/linux-2.6.5-s390-04-april2004.shtml
>
> and by this point it's quite likely already in the bitkeeper tree as
> well, they've been pushing updates upstream.
>
> -m
>
>
>
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