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[Xen-devel] Re: [patch 0/6] Guest page hinting version 7.

To: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Subject: [Xen-devel] Re: [patch 0/6] Guest page hinting version 7.
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 06 Apr 2009 12:23:35 -0700
Cc: akpm@xxxxxxxx, Nick Piggin <nickpiggin@xxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, linux-mm@xxxxxxxxx, frankeh@xxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, hugh@xxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
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Martin Schwidefsky wrote:
Why should the guest want to do preswapping? It is as expensive for
the host to swap a page and get it back as it is for the guest (= one
write + one read).

Yes, perhaps for swapping, but in general it makes sense for the guest to write the pages to backing store to prevent host swapping. For swap pages there's no big benefit, but for file-backed pages its better for the guest to do it.

The only thing you can gain by
putting memory pressure on the guest is to free some of the memory that
is used by the kernel for dentries, inodes, etc.

Well, that's also significant. My point is that the guest has multiple ways in which it can relieve its own memory pressure in response to overall system memory pressure; its just that I happened to pick the example where its much of a muchness.


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