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

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Date:	Tue, 27 Apr 2010 14:56:24 +0200
From:	Pavel Machek <pavel@....cz>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Avi Kivity <avi@...hat.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

Hi!

> > > Nevertheless, frontswap works great today with a bare-metal
> > > hypervisor.  I think it stands on its own merits, regardless
> > > of one's vision of future SSD/memory technologies.
> > 
> > Even when frontswapping to RAM on a bare metal hypervisor it makes
> > sense
> > to use an async API, in case you have a DMA engine on board.
> 
> When pages are 2MB, this may be true.  When pages are 4KB and 
> copied individually, it may take longer to program a DMA engine 
> than to just copy 4KB.
> 
> But in any case, frontswap works fine on all existing machines
> today.  If/when most commodity CPUs have an asynchronous RAM DMA
> engine, an asynchronous API may be appropriate.  Or the existing
> swap API might be appropriate. Or the synchronous frontswap API
> may work fine too.  Speculating further about non-existent
> hardware that might exist in the (possibly far) future is irrelevant
> to the proposed patch, which works today on all existing x86 hardware
> and on shipping software.

If we added all the apis that worked when proposed, we'd have
unmaintanable mess by about 1996.

Why can't frontswap just use existing swap api?
							Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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