linux-kernel - Re: [PATCH v2 2/2] xen/setup: Remap Xen Identity Mapped RAM

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Message-ID: <53D64D99.3000101@citrix.com>
Date:	Mon, 28 Jul 2014 14:18:17 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Matt Rushton <mvrushton@...il.com>, <konrad.wilk@...cle.com>,
	<boris.ostrovsky@...cle.com>
CC:	<msw@...zon.com>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xensource.com>, Matt Rushton <mrushton@...zon.com>
Subject: Re: [PATCH v2 2/2] xen/setup: Remap Xen Identity Mapped RAM

On 20/07/14 01:01, Matt Rushton wrote:
> Instead of ballooning up and down dom0 memory this remaps the existing mfns
> that were replaced by the identity map. The reason for this is that the
> existing implementation ballooned memory up and and down which caused dom0
> to have discontiguous pages. In some cases this resulted in the use of bounce
> buffers which reduced network I/O performance significantly. This change will
> honor the existing order of the pages with the exception of some boundary
> conditions.
> 
> To do this we need to update both the Linux p2m table and the Xen m2p table.
> Particular care must be taken when updating the p2m table since it's important
> to limit table memory consumption and reuse the existing leaf pages which get
> freed when an entire leaf page is set to the identity map. To implement this,
> mapping updates are grouped into blocks with table entries getting cached
> temporarily and then released.
> 
> On my test system before:
> Total pages: 2105014
> Total contiguous: 1640635
> 
> After:
> Total pages: 2105014
> Total contiguous: 2098904

This looks like a worthwhile improvement but...

>  arch/x86/xen/setup.c |  370 +++++++++++++++++++++++++++++++++++++++-----------

... Ew :(

It will take me a bit longer before I can give this a proper review.

David
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