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Re: [Xen-devel] copy on write memory



I tried applying  it in a spirit of pure optimism (naivety). You never know.

To return for one moment to the thing I was chasing. Let's assume that in its standard configuration the Linux (or indeed some other kernel) makes a very good job of sharing memory across processes by means of the copy-on-write fork semantics and by exploiting the overall view that it has of how pages/blocks are read from the filesystem and modified by processes. Let's hope that it (or they) get even better. We would like separate xenU domains to benefit from the excellence that exists and the improvements that may be.

My idea is to use a memory-privileged xenU domain as a page-table manager for a group of client domains. It would have to know about the memory/page/block usage of every process in each of its clients as if they were all running within it. But it would never operate as the actual kernel for any of these processes. For those purposes each process would operate under the supervision of a memory-client domain that has its own kernel address space.

I haven't at present got the intimate knowledge of memory management traffic between xen, xen0 and xenU domains that I would need to see whether or not this idea has any legs. But I am very interested to know what anyone thinks. No doubt the best answer is to go and study the code.

Anyway, thanks for your help, and for your quick repsonse. Much quicker than mine.

Regards
Peri

urmk@xxxxxxxxxxxxxxxxx wrote:

It's true, you did mention it before, but I was looking for something else at the time. What I have in mind doesn't require so much configuration. On the other hand it doesn't exist, and this does.

Ah.  I couldn't remember if I'd sent it or not (or if I'd even tried to send
it from an address that was on the list, a few go to the same mailbox at the moment)

But the patch is against quite an old source, and it doesn't compile straight out of the box. Do you know if there are updated patches against 2.6.9?

I can check.

I get this error (which I haven't yet examined in detail):

CC [M]  fs/xip2fs/file.o
fs/xip2fs/file.c: In function `xip2_do_file_read':
fs/xip2fs/file.c:69: error: structure has no member named `buf'
fs/xip2fs/file.c: In function `__xip2_file_aio_read':
fs/xip2fs/file.c:119: error: structure has no member named `buf'
fs/xip2fs/file.c: In function `xip2_file_sendfile':
fs/xip2fs/file.c:302: error: structure has no member named `buf'

This was against xen-2.0.1 as of today 18 Nov 2004

I highly doubt that it will be directly applicable to xen - the entire backend
mechanism is linked into the z/VM shared memory system between guests.  I was
more pointing it out as a probable jumping off point (most of the work is done,
it just needs to use the xen memory sharing instead) and as a workable
concept for a less-cpu-intensive copy-on-write mechanism.

I'll take a look for a newer patch and see if I can scrape up some time to
apply the backend to xen, but I don't know when I'll get a chance -- don't let
me hold anyone else up who was considering working on it.

-m



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