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

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Date:	Sun, 25 Apr 2010 06:37:30 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	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

> My issue is with the API's synchronous nature.  Both RAM and more
> exotic
> memories can be used with DMA instead of copying.  A synchronous
> interface gives this up.
>  :
> Let's not allow the urge to merge prevent us from doing the right
> thing.
>  :
> I see.  Given that swap-to-flash will soon be way more common than
> frontswap, it needs to be solved (either in flash or in the swap code).

While I admit that I started this whole discussion by implying
that frontswap (and cleancache) might be useful for SSDs, I think
we are going far astray here.  Frontswap is synchronous for a
reason: It uses real RAM, but RAM that is not directly addressable
by a (guest) kernel.  SSD's (at least today) are still I/O devices;
even though they may be very fast, they still live on a PCI (or
slower) bus and use DMA.  Frontswap is not intended for use with
I/O devices.

Today's memory technologies are either RAM that can be addressed
by the kernel, or I/O devices that sit on an I/O bus.  The
exotic memories that I am referring to may be a hybrid:
memory that is fast enough to live on a QPI/hypertransport,
but slow enough that you wouldn't want to randomly mix and
hand out to userland apps some pages from "exotic RAM" and some
pages from "normal RAM".  Such memory makes no sense today
because OS's wouldn't know what to do with it.  But it MAY
make sense with frontswap (and cleancache).

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.
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