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Re: [Xen-devel] copy on write memory

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] copy on write memory
From: Jacob Gorm Hansen <jacobg@xxxxxxx>
Date: Mon, 22 Nov 2004 13:42:37 +0100
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Keir Fraser wrote:

Well, you also can over-commit on stuff that is read-only and fault in
on demand, just as you can demand-CoW writable stuff e.g., no need to
have all of kernel or glibc in memory all the time -- only hot parts
of both will be in use by the system at any time.

i.e., 1. There is fault in from no page -> shareable page on read accesses.
 2. There is fault from shareable page -> shareable page + exclusive
page on write accesses. Both of these require extra allocation of memory.

But I will need some external service to give them to me right when I need them, or I may run into the 'paged-the-pager' problem and die?

Perhaps this should just be a one-way street, you give up pages to be nice to others (and get cheaper hosting or whatever kind of reward you can think of in return), and then you lose the right to write to them for good. Should you need more writable pages, you will have to re-grow your reservation, and if that fails you will need to flush some slabs or buffer caches or or page stuff to disk or whatever you do in Linux when you have memory pressure. Ultimately you may want to migrate to a less loaded machine.

It's another way of looking at the problem (end-to-end style I
suppose). Potetntially worth investigating. :-)

Perhaps I will have a go at some point. If going in this direction perhaps it will make sense to do this in Xen anyway.


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