Copy-on write sharing

Copy-on write sharing

[zhangzhi]

Hi, list,

xen 4.0 release has said that it had a special feature of copy-on write
sharing of identical memory pages between VMs (HVM only). 
I just wanna know the source code that illustrate this feature. 
Is anyone have an idea about the this feature as well as its specific source
code? thanks. 

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Re: Copy-on write sharing

[Tim Deegan]

Hi,

At 01:39 -0700 on 31 May (1338428371), zhangzhi wrote:
> xen 4.0 release has said that it had a special feature of copy-on write
> sharing of identical memory pages between VMs (HVM only). 
> I just wanna know the source code that illustrate this feature. 

The code inside the hypervisor lives in xen/arch/x86/mm/, mostly in the
file mem_sharing.c.

The user-space interface is in libxc, in tools/libxc/xc_memshr.c

An example program that uses it is tools/tests/mem-sharing/memshrtool.c.

This feature is not built in to the higher-level tools (libxl &c) at the
moment; originally there were extensions to blktap that could
automatically share pages that were read from disk if the VMs' disks
came from the same source image, but I think that's all bitrotted.  If
anyone wanted to try to get that working again, I'm sure it would be
welcome!

Cheers,

Tim.

Re: Copy-on write sharing

[Tim Deegan]

Hi, 

Please don't drop the list from Cc.  I have put it back.

At 09:50 +0000 on 31 May (1338457855), ???? wrote:
> Hi, I just read about the source code (mainly the mem_sharing.c) that
> you mentioned before, but it seemed not like what I was looking for,
> and it doesn't make use of the rbtree for page sharing.

mem_sharing.c is just the mechanism for having two VMs share a page.
The choices of which pages to share are left to the tools.  As I said
the in-tree Xen tools don't currently have a good way to select pages to
share (though there are some out-of-tree users).

> I felt that the functionality of this mem_sharing.c file is not like
> the KSM(kernel samepage mechanism), which operates in the KVM(kernel
> virtual machine).

That's right.  KVM reuses linux's existing in-kernel mechanism for
finding and sharing suitable pages.

Tim.