linux-kernel - Re: [patch 0/6] Guest page hinting version 7.

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Message-ID: <49CD69EB.6000000@redhat.com>
Date:	Fri, 27 Mar 2009 20:06:03 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	Martin Schwidefsky <schwidefsky@...ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	frankeh@...son.ibm.com, akpm@...l.org, nickpiggin@...oo.com.au,
	hugh@...itas.com
Subject: Re: [patch 0/6] Guest page hinting version 7.

Dave Hansen wrote:
> On Fri, 2009-03-27 at 16:09 +0100, Martin Schwidefsky wrote:
>> If the host picks one of the
>> pages the guest can recreate, the host can throw it away instead of writing
>> it to the paging device. Simple and elegant.
> 
> Heh, simple and elegant for the hypervisor.  But I'm not sure I'm going
> to call *anything* that requires a new CPU instruction elegant. ;)

I am convinced that it could be done with a guest-writable
"bitmap", with 2 bits per page.  That would make this scheme
useful for KVM, too.

> I don't see any description of it in there any more, but I thought this
> entire patch set was to get rid of the idiotic triple I/Os in the
> following scenario:

> I don't see that mentioned at all in the current description.
> Simplifying the hypervisor is hard to get behind, but cutting system I/O
> by 2/3 is a much nicer benefit for 1200 lines of invasive code. ;)

Cutting down on a fair bit of IO is absolutely worth
1200 lines of fairly well isolated code.

> Can we persuade the hypervisor to tell us which pages it decided to page
> out and just skip those when we're scanning the LRU?

The easiest "notification" points are in the page fault
handler and the page cache lookup code.

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