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

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Message-ID: <4BD44E74.2020506@redhat.com>
Date:	Sun, 25 Apr 2010 17:15:16 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.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

On 04/25/2010 04:37 PM, Dan Magenheimer wrote:
>> 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.
>    

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.

-- 
error compiling committee.c: too many arguments to function

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