linux-kernel - Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

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Date:	Thu, 29 Apr 2010 21:53:50 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Pavel Machek <pavel@....cz>
CC:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jeremy@...p.org,
	hugh.dickins@...cali.co.uk, ngupta@...are.org, JBeulich@...ell.com,
	chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, npiggin@...e.de,
	akpm@...ux-foundation.org, riel@...hat.com
Subject: Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

On 04/28/2010 08:55 AM, Pavel Machek wrote:
>
>> That's a reasonable analogy.  Frontswap serves nicely as an
>> emergency safety valve when a guest has given up (too) much of
>> its memory via ballooning but unexpectedly has an urgent need
>> that can't be serviced quickly enough by the balloon driver.
>>      
> wtf? So lets fix the ballooning driver instead?
>    

You can't have a negative balloon size.  The two models are not equivalent.

Balloon allows you to give up a page for which you have a struct page.  
Frontswap (and swap) allows you to gain a page for which you don't have 
a struct page, but you can't access it directly.  The similarity is that 
in both cases the host may want the guest to give up a page, but cannot 
force it.

> There's no reason it could not be as fast as frontswap, right?
> Actually I'd expect it to be faster -- it can deal with big chunks.
>    

There's no reason for swapping and ballooning to behave differently when 
swap backing storage is RAM (they probably do now since swap was tuned 
for disks, not flash, but that's a bug if it's true).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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