linux-kernel - Re: [RFC] transcendent memory for Linux

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Message-ID: <20090624150420.GH1784@ucw.cz>
Date:	Wed, 24 Jun 2009 17:04:21 +0200
From:	Pavel Machek <pavel@....cz>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	npiggin@...e.de, chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, Avi Kivity <avi@...hat.com>,
	jeremy@...p.org, Rik van Riel <riel@...hat.com>,
	alan@...rguk.ukuu.org.uk, Rusty Russell <rusty@...tcorp.com.au>,
	Martin Schwidefsky <schwidefsky@...ibm.com>, akpm@...l.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	tmem-devel@....oracle.com, sunil.mushran@...cle.com,
	linux-mm@...ck.org, Himanshu Raj <rhim@...rosoft.com>
Subject: Re: [RFC] transcendent memory for Linux

Hi!

This description (whole mail) needs to go into Documentation/, somewhere. 

> Normal memory is directly addressable by the kernel,
> of a known normally-fixed size, synchronously accessible,
> and persistent (though not across a reboot).
...
> Transcendent memory, or "tmem" for short, provides a
> well-defined API to access this unusual class of memory.
> The basic operations are page-copy-based and use a flexible
> object-oriented addressing mechanism.  Tmem assumes

Should this API be documented, somewhere? Is it in-kernel API or does
userland see it?

> "Preswap" IS persistent, but for various reasons may not always
> be available for use, again due to factors that may not be
> visible to the kernel (but, briefly, if the kernel is being
> "good" and has shared its resources nicely, then it will be
> able to use preswap, else it will not).  Once a page is put,
> a get on the page will always succeed.  So when the kernel
> finds itself in a situation where it needs to swap out a page,
> it first attempts to use preswap.  If the put works, a disk
> write and (usually) a disk read are avoided.  If it doesn't,
> the page is written to swap as usual.  Unlike precache, whether

Ok, how much slower this gets in the worst case? Single hypercall to
find out that preswap is unavailable? I guess that compared to disk
access that's lost in the noise?
								Pavel

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