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xen-devel
Re: [Xen-devel] copy on write memory
Sorry, I had a copy-and-pasto (variant of typo) in my previous message -
The total long-term state of a client domain can be characterised by adding
'SRx' which identifies blocks in read-only storage that are shared with
similar clients
SWx which is the set of blocks in read-write storage that belong to
this client domain.
Peri Hankey wrote:
Certainly, UML is best known for copy on write filesystems. But UML's
SKAS mode is a different way of managing memory, and that was the
starting point for this proposal, which is about using the copy on
write semantics of Linux memory management to share memory pages
between Xen domains. I see now that one person's starting point may
prove to be another's red herring.
The notion is that there are applications of Xen where there would be
very many virtual computers running the same set of applications for
much of the time (eg standard web hosting, honeypots).
In the standard case of a single OS on a single machine, all processes
are loaded from a common filesystem, so the OS knows which page sets
start out with shared information. It can use this information to
share pages between processes so long as those pages are not written
to, allocating distinct pages to distinct processes when those pages
are written to - the copy on write semantics.
In the Xen case, you don't want to reinvent and reimplement existing
mechanisms, especially as these may differ in subtle ways from one
guest operating system to another. So I suggest it would make sense to
create mechanisms that allow some Xen domains to operate as memory
management servers to groups of related domains.
In effect we create a memory manager privilege. Suppose we have a
memory manager domain M1 with a collection of memory client domains
M1-x, where each memory client domain has its own kernel address space
KASx, a set of modified pages Wx and a set of shared pages Rx. Then
the situation we want to see is this:
M1 doesn't actually execute any application code, just manages
memory for its clients
M1-a executes application code in (KASa, Ra, Wa) calling on M1 for
memory management
M1-b executes application code in (KASb, Rb, Wb) calling on M1 for
memory management
M1-c executes application code in (KASc, Rc, Wc) calling on M1 for
memory management
...
There is a connection with copy-on-write storage. The execution state
of a client domain x can be frozen as:
'Rx' which identifies a set of pages that are shared read-only with
similar clients
KASx which is the kernel address space page set for this domain
Wx which is the set of user address space pages that have been
written to by this domain
The total long-term state of a client domain can be characterised by
adding
'SRx' which identifies blocks in read-only storage that are shared
with similar clients
SWx which identifies which files in read-write storage that belong
to this client domain.
It seems to me that a memory manager domain, which pretty much has to
serve pages initially drawn from a filesystem that is shared read-only
between its clients, is also in a position to manage copy-on-write use
of that file system for its clients, as it already knows which blocks
and clean and which are dirty.
Peri
On Mon, 15 Nov 2004, Peri Hankey wrote:
It occurred to me that the equivalent in the Xen world would be to
use one Linux xenU domain purely as a page-table manager for a
collection of separate xenU domains that are expected or known have
similar process populations.
UML copy on write is only for filesystems, isn't it ?
The Xen equivalent would be cloning the xenU root filesystem
as an LVM snapshot, from a read-only LVM snapshot. Then each
xenU virtual system would only use the disk space it writes
to and no more.
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